Optical-NIR Dust Extinction Towards Galactic O Stars J

Astronomy & Astrophysics manuscript no. ms c ESO 2018 January 3, 2018

Optical-NIR dust extinction towards Galactic O stars J. Ma´ız Apellaniz´ 1 and R. H. Barba´2

1 Centro de Astrobiolog´ıa, CSIC-INTA, campus ESAC, camino bajo del castillo s/n, E-28 692 Villanueva de la Canada,˜ Spain e-mail: [email protected]

2 Departamento de F´ısica y Astronom´ıa, Universidad de La Serena, Av. Cisternas 1200 Norte, La Serena, Chile

Received 5 October 2017; accepted 25 December 2017

ABSTRACT

Context. O stars are excellent tracers of the intervening ISM because of their high luminosity, blue intrinsic SED, and relatively featureless spectra. We are currently conducting the Galactic O-Star Spectroscopic Survey (GOSSS), which is generating a large sample of O stars with accurate spectral types within several kpc of the Sun. Aims. We aim to obtain a global picture of the properties of dust extinction in the solar neighborhood based on optical-NIR photometry of O stars with accurate spectral types. Methods. We have processed a carefully selected photometric set with the CHORIZOS code to measure the amount [E(4405 − 5495)] and type [R5495] of extinction towards 562 O-type stellar systems. We have tested three different families of extinction laws and analyzed our results with the help of additional archival data. Results. The Ma´ız Apellaniz´ et al. (2014, A&A 564, A63) family of extinction laws provides a better description of Galactic dust that either the Cardelli et al. (1989, ApJ 345, 245) or Fitzpatrick (1999, PASP 111, 63) families, so it should be preferentially used when analyzing samples similar to the one in this paper. In many cases O stars and late-type stars experience similar amounts of extinction at similar distances but some O stars are located close to the molecular clouds left over from their births and have larger extinctions than the average for nearby late-type populations. In qualitative terms, O stars experience a more diverse extinction than late-type stars, as some are affected by the small-grain-size, low-R5495 effect of molecular clouds and others by the large-grain-size, high-R5495 effect of H ii regions. Late-type stars experience a narrower range of grain sizes or R5495, as their extinction is predominantly caused by the average, diffuse ISM. We propose that the reason for the existence of large-grain-size, high-R5495 regions in the ISM in the form of H ii regions and hot-gas bubbles is the selective destruction of small dust grains by EUV photons and possibly by thermal sputtering by atoms or ions. Key words. Dust, extinction — Galaxy: structure — Methods: data analysis — Methods: observational — Stars: early-type

1. Introduction ent sightlines had different types of extinction, an average extinction law could only be an approximation and one that was bound Soon after the discovery of the existence of an intervening to fail miserably under some circumstances. It was not until the interstellar medium (ISM) that obscures and reddens starlight, seminal work of Cardelli et al. (1989) (CCM hereafter), that a Baade & Minkowski (1937) realized that sightlines could dif- family (not an average) of extinction laws was produced, with a fer not only in the amount of obscuration but also in its depen- parameter (R5495, see Appendix A for its relationship with RV ) dence with wavelength. In current terms, we say that the extinc- that characterized the type of extinction and that is associated tion law is not constant so in order to determine how the ISM with the average dust grain size (small grains yield a low value changes the light we receive from the stars we need to specify of R5495, large grains a high one). CCM became the standard ref- both the amount and the type of extinction. Extinction is a com- erence in the field for the next quarter of a century, as for the first bination of absorption and scattering by the intervening particles time it allowed to simultaneously determine the amount and type and the largest contributor is dust, which produces mostly con- of dust extinction and eliminate its effect from the observed pho- tinuum extinction but also some broad features in the UV and tometry. CCM had a few problems, though, some of which were the IR. Atoms and molecules also have an effect in the form of treated by the alternative family of extinction laws of Fitzpatrick arXiv:1712.09228v2 [astro-ph.GA] 30 Dec 2017 discrete absorption lines and some absorption features (diffuse (1999) (F99 hereafter), and later on by the Ma´ız Apellaniz´ et al. interstellar bands or DIBs) of unknown origin are also detected, (2014) family, from now on MA14. MA14 showed that its family but those will be mostly ignored in this paper, which deals with of extinction laws provided a better fit to the optical-NIR extinc- dust extinction. tion in 30 Doradus than either CCM or F99, to the point that Some astronomers are interested in dust extinction because it was possible for the first time to obtain reasonable estimates of the information it contains regarding the spatial distribution of the effective temperatures of O stars from photometry alone, and properties of the ISM but to most of them it is simply a nui- something that was not possible until then due to the problems sance, an effect that complicates the calculation of luminosities, that exist in the other families. The MA14 family of extinction and all they want to know is how to eliminate it from their data. laws was derived using O stars on 30 Doradus so an obvious Throughout much of the twentieth century astronomers have follow-up question is: does it also do a better job in characteriz- tried to generate extinction laws but kept hitting a wall: as differ-

1 J. Ma´ız Apellaniz´ and R. H. Barba:´ Optical-NIR dust extinction towards Galactic O stars ing optical-NIR extinction for Galactic O stars? Answering that for dim ones). This led us to divide the final sample into four question is the first objective of this paper. subsamples: J2G, J2GT, J2GS, and J2GTS, where the naming In the last decade we have been conducting the Galactic convention refers to the photometry included, that is, J(ohnson), O-Star Spectroscopic Survey or GOSSS (Ma´ız Apellaniz´ et al. 2(MASS), G(aia), S(tromgren),¨ and T(ycho-2), respectively. 2011), which aims to obtain high S/N, R ∼ 2500 blue-violet After compiling the photometry for the initial sample we spectroscopy of all optically accessible Galactic O stars and de- carefully analyzed each object to produce a final clean sample rive their spectral types. The sample currently available is the in the following way: largest ever uniform collection of O spectral types ever published. In this paper we use this sample to answer the ques- – For the GOSSS objects with nearby companions (visual bi- tion in the previous paragraph and a second question: what naries and multiples) we analyzed which ones had separate is the relationship between dust grain size and ISM environ- Johnson, 2MASS, and Stromgren¨ magnitudes and for which ment? Previous attempts to answer this question have been ham- ones the available basic photometry was a combination of pered by, among other things, limited samples and the method- two or more components (because we had been able to ob- ology employed. Our uses of the GOSSS sample and of the tain spatially resolved spectral types). In the latter case we CHORIZOS (Ma´ız Apellaniz´ 2004) techniques applied in MA14 merged the information on the different GOSSS components and in other recent works address both objectives. (see below for Gaia and Tycho-2). We also eliminated a few This paper is divided as follows. We first present the sample cases where no reliable photometry could be found. This pro- in detail, describing how we have culled the photometry for the cess reduced the sample to 571 stars. sample and eliminated problematic objects, and the CHORIZOS – The 2MASS photometry for bright stars is of very poor qual- methodology. We then show our results in three parts: a com- ity (uncertainties larger than 0.2 mag). Whenever possible, parison of families of extinction laws, a discussion of quantita- we searched for alternative sources such as Ducati (2002) tive reddening differences between O- and late-type stars, and to be used as substitutes. In the majority of cases where we an analysis of how the properties of dust are a function of the used the 2MASS photometry we applied the 2MASS uncer- ISM environment. In the final standard section of the paper we tainties directly, with the exception being the stars with poor present our conclusions and outline future lines of work. There quality flags, where we increased the uncertainty values. are also five appendices that discuss extinction terms and tech- – For objects with NIR excesses we followed two different niques in order to better understand the uncertainties and biases strategies. Oe stars, whose circumstellar environment can involved in this type of analysis. We recommend readers who are contribute significantly to the NIR photometry, were elim- unfamiliar with the mathematical aspects of extinction effects to inated, leaving a final sample of 562 stars. For the rest, we start with those appendices. applied a correction to the JHKs photometry based on the infrared colors that also increased the photometric uncertainties in those cases. The correction was based on the facts 2. Data and methods that [a] all O stars have very similar photospheric intrinsic The sample in this paper is built starting from the 590 O stars J − H and H − Ks colors and that [b] the extinction vector in GOSSS-I (Sota et al. 2011), GOSSS-II (Sota et al. 2014), and in the color-color plane has a nearly constant direction for GOSSS-III (Ma´ız Apellaniz´ et al. 2016)1. For the initial sample the range of E(4405 − 5495) values considered here (neither we collected our basic photometric set: of those assumptions is true for most optical or optical-NIR colors, see Appendix B). There are numerous literature ex- – Ground-based Johnson UJ BJVJ photometry using Simbad amples of this effect, one is shown in Fig. 4 of Arias et al. (Wenger et al. 2000) and the GCPD (Mermilliod et al. 1997). (2006). – Near-infrared JHKs photometry from 2MASS (Skrutskie – For the values of the Johnson and Stromgren¨ uncertainties, et al. 2006). see Ma´ız Apellaniz´ (2006). In those cases where the source – Optical G photometry from the Gaia Data Release 1 or DR1 was suspected to be of poor quality, we increased the value (van Leeuwen et al. 2017). of the uncertainties. For the zero points, see Ma´ız Apellaniz´ (2007). The majority of the stars in our sample have photometry in – Tycho-2 photometry supposedly has no saturation limit all seven of the basic photometric bands, though our quality con- while G saturates around magnitude 6.0 for DR1 (van trol (see below) led us to eliminate some of them. We also com- Leeuwen et al. 2017; Ma´ız Apellaniz´ 2017). However, when piled the extended photometric set: comparing photometry from different sources we discovered a slight saturation effect for the brightest Tycho-2 stars, so we – Tycho-2 BT VT photometry (Høg et al. 2000). eliminated those magnitudes from our analysis. Regarding – Stromgren¨ uS vS bS yS photometry from Paunzen (2015). saturated G DR1 magnitudes, we used the Ma´ız Apellaniz´ (2017) correction. The reason to differentiate between basic and extended pho- – Gaia DR1 does not include variability information and G tometric sets is that in the final sample all objects include at magnitudes appear to be the average ones. This effect turned least five of the seven basic photometric points but not necessar- out to be especially important for some eclipsing binaries, for ily the extended photometric ones, whose coverage is more lim- which we also eliminated that magnitude. We used the cali- ited (complete or near-complete for bright stars but incomplete bration of Ma´ız Apellaniz´ (2017), who corrected the nominal passband of Jordi et al. (2010). 1 The GOSSS spectral types were derived from spectra gathered with five facilities: the 1.5 m Telescope at the Observatorio de Sierra Nevada – Another relevant effect when comparing ground-based (OSN), the 2.5 m du Pont Telescope at Las Campanas Observatory (Johnson, 2MASS, and Stromgren)¨ and space (Gaia and (LCO), the 3.5 m Telescope at Calar Alto Observatory (CAHA), and the Tycho-2) photometry is spatial resolution: the published 4.2 m William Herschel Telescope (WHT) and 10.4 m Gran Telescopio space photometry may include different components of a Canarias (GTC) at Observatorio del Roque de los Muchachos (ORM). multiple system that are seen as a single source from the

2 J. Ma´ız Apellaniz´ and R. H. Barba:´ Optical-NIR dust extinction towards Galactic O stars

ground. We revised all cases in turn to decide which com- 2.00 ponents were detected. This led in some cases to combining the space photometry from two components (as we did with the ground-based photometry) and in others to discarding the 1.00 Gaia or Tycho-2 photometric points. – Finally, all the photometry was revised to determine the mu- tual compatibility in an iterative process that took weeks of 0.50 work and led to a ﬁnal culling of the used magnitudes. This is necessary because published photometry contains a sig- niﬁcant number of errors 2. The reader is invited to look up

5495 0.20 in online tools such as Simbad and VizieR any of the stars R in the sample with a large number of entries there. A quick σ perusal easily reveals cases in which, for example, Johnson 0.10 photometry from diﬀerent sources is incompatible. When

comparing different photometric systems such incompatibil- Jo+2M+Ga ity may not be obvious at first, as any two magnitudes gener- 0.05 Jo+2M+Ga+Ty ate a color (even VJ −yS , something commonly ignored), but it becomes apparent when no valid single-star SED can be Jo+2M+Ga+St built with the combination independently of the parameters Jo+2M+Ga+Ty+St or extinction-law families used. We have recently started a 0.02 different project called GALANTE (see below) with which 0.01 0.03 0.10 0.30 1.00 3.00 we will obtain uniform-quality photometry for a significant E(4405−5495) fraction of the sources here but its results are still far in the future.

Fig. 1. σR5495 as a function of E(4405 − 5495) for the MA14 ex- The collected Johnson photometry is available from the web tinction family CHORIZOS runs. Colors are used to distinguish site of the Galactic O-Star Catalog (GOSC, Ma´ız Apellaniz´ et al. the subsamples: red for J2G, blue for J2GT, green for J2GS, and 2004; Sota et al. 2008) 3. The ﬁnal subsamples have 109 (J2G), magenta for J2GTS. The two dotted lines show the prediction of

152 (J2GT), 38 (J2GS), and 263 (J2GTS) stars. Of the final 562 Eq. D.2 for [a] σAV = 0.03 and R5495 = 3.0 (lower line) and [b] stars, 28, 6, 13, 13, and 111 have UJ, J, H, Ks, and G missing, σAV = 0.10 and R5495 = 5.0 (upper line). respectively. The photometry was processed using the latest version of the CHORIZOS code (Ma´ız Apellaniz´ 2004) to determine the energy distributions (SEDs) are TLUSTY (Lanz & Hubeny amount and type of extinction to each of the star. We have used 2003). this method successfully in the past (e.g., Ma´ız Apellaniz´ et al. – Each star was processed three times, once each with the 2015b), here we describe the specific details of the CHORIZOS CCM, F99, and MA14 families of extinction laws. The type runs used in this paper: of extinction is parameterized by R5495 and the amount of ex- tinction4 is parameterized by E(4405 − 5495). See Appendix – We used the Milky Way grid of Ma´ız Apellaniz´ (2013b), A for a discussion on the use of those parameters. in which the two grid parameters are effective tempera- – The Teff-spectral type conversion used is an adapted version ture (Teff) and photometric luminosity class (LC). The lat- of Martins et al. (2005) that includes the spectral subtypes ter quantity is defined in an analogous way to the spectro- and luminosity classes used in GOSSS I+II+III. scopic equivalent, but instead of being discrete it is a con- – Teff and LC were fixed while R5495, E(4405 − 5495), and tinuous variable that ranges from 0.0 (highest luminosity for log d were left as free parameters. The values of the Teff were that Teff) to 5.5 (lowest luminosity for that Teff). We note that established from the used Teff-spectral type conversion (see the range is selected in order to make objects with spectro- previous point). For LC we used the spectroscopic luminos- scopic luminosity class V (dwarfs) have LC≈5 and objects ity class available from the GOSSS spectral classification. with spectral luminosity class I (supergiants) have LC≈1. In We note that, in any case, optical-NIR colors of O stars are this paper we are only interested in O stars, whose spectral very weakly dependent on luminosity other than wind effects. 2 Variability is also an issue but a minor one. Most sources of vari- – The four subsamples were run separately. The number of ability in O stars such as the rotation of oblique magnetic fields (Wade photometric points for the stars in each sample is 7 (J2G), et al. 2012), wind, or turbulence variability significantly affect emission 9 (J2GT), 11 (J2GS), and 13 (J2GTS). As we are fitting lines but produce only small changes in the continuum. Eruptive Oe three parameters with CHORIZOS, the degrees of freedom stars such as HD 120 678 (Gamen et al. 2012) have been excluded from (d.o.f.), are four, six, eight, and ten, respectively5. The re- the sample, so they should not concern us. The most relevant source 2 2 2 of large broad-band flux variability in the sample is eclipsing binaries. duced χ of the best model, χred= χ /d.o.f. is used to evaluate Most of them, however, are accounted for in the literature and those the quality of the fit for each star. that are not can be revealed through comparison of different sources. Nevertheless, it is possible that a few hidden binaries may be lurking in our sample, especially in the J2G subsample, where there are less bands 4 Throughout this paper we refer to E(4405 − 5495) as the amount to compare among. of extinction, though it should be more properly called the amount of 3 We have recently moved the main URL to reddening, because that is the choice the CCM, F99, and MA14 families https://gosc.cab.inta-csic.es from the old URL at use. http://gosc.iaa.es, which will be kept as a mirror site for 5 In those cases where some photometric point is missing, d.o.f. is the time being. reduced accordingly.

3 J. Ma´ız Apellaniz´ and R. H. Barba:´ Optical-NIR dust extinction towards Galactic O stars 2 red χ 2 red χ 2 red χ d log 0.0200.015 3.50.016 3.20.024 1.57 3.60.057 1.86 3.10.020 1.26 0.61 3.00.017 2.21 0.99 2.90.019 1.45 0.55 0.12 3.00.014 0.69 1.36 2.41 3.00.020 0.22 0.61 1.23 2.30.017 0.57 1.88 2.99 3.10.020 0.35 1.12 2.51 3.00.021 0.87 2.02 3.96 3.10.020 0.87 2.76 1.04 3.10.038 1.11 3.17 2.35 3.10.015 1.90 0.71 2.38 3.50.020 1.14 3.38 0.95 3.00.014 0.31 1.90 0.63 3.20.016 1.02 0.45 2.99 3.20.020 0.63 1.39 2.08 3.00.015 0.44 1.80 2.36 3.30.025 0.31 1.96 1.32 3.10.015 0.77 3.30 4.05 3.40.023 1.52 1.75 3.39 3.40.030 2.12 3.03 0.60 3.10.021 0.90 3.41 2.28 3.20.025 1.76 0.80 2.17 3.40.019 1.55 1.73 0.83 3.10.021 0.36 1.14 0.78 3.40.023 0.50 1.73 1.02 3.10.022 0.41 1.56 1.20 3.40.071 0.79 2.65 4.52 3.40.019 1.08 1.07 1.62 3.30.020 0.59 4.83 0.87 3.40.019 0.71 0.81 8.66 3.20.018 1.94 0.87 5.24 3.10.022 1.64 5.46 7.08 3.20.020 0.41 2.64 4.38 3.10.024 2.72 1.82 2.71 3.30.028 0.61 3.41 3.24 3.20.020 0.75 2.99 7.66 3.20.030 1.44 2.28 4.07 3.10.025 0.68 6.76 2.52 3.1 27.600.022 1.30 4.16 3.30.022 2.25 1.87 1.60 3.1 2.97 0.019 2.46 0.36 3.20.015 2.00 2.92 5.33 3.5 0.65 0.83 3.50 3.1 1.95 2.32 2.76 0.32 0.70 5.52 0.88 1.91 1.03 5.36 0.94 0.81 0 , are for the MA14 family of extinction laws. ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± J V d , and log 0 , J V J 0.0230.016 7.706 0.018 4.880 0.026 8.173 0.075 6.081 0.021 5.683 0.019 4.620 0.020 6.014 0.015 5.341 0.022 1.593 0.023 6.105 0.028 5.614 0.022 6.222 0.022 5.085 0.052 6.008 0.017 5.752 0.022 5.464 0.018 6.138 0.018 4.267 0.025 4.872 0.017 5.561 0.031 5.038 0.018 5.785 0.026 6.141 0.049 6.166 0.023 5.831 0.037 5.120 0.020 5.812 0.030 6.248 0.039 6.581 0.023 7.305 0.075 6.160 0.028 6.951 0.028 7.192 0.025 6.293 0.020 6.032 0.023 5.268 0.028 6.327 0.043 5.287 0.041 6.494 0.024 6.782 0.045 3.785 0.043 6.561 0.023 6.619 0.030 5.283 0.023 6.466 0.021 6.336 3.859 , J V ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± V A A , 5495 R 0.1060.124 1.578 0.117 1.307 0.243 1.182 0.293 1.043 0.122 4.616 0.208 1.338 0.093 1.068 0.102 1.522 0.105 0.962 0.165 1.610 0.268 1.707 0.066 1.401 0.089 1.905 0.132 1.437 0.056 4.110 0.120 2.015 0.156 1.091 0.134 1.075 0.105 1.088 0.100 1.798 0.170 1.489 0.095 1.947 0.089 1.637 0.093 1.979 0.060 5.290 0.105 2.805 0.244 4.062 0.062 0.683 0.071 3.838 0.232 3.752 0.110 0.852 0.048 3.516 0.045 3.992 0.072 4.172 0.051 2.725 0.056 2.935 0.055 2.621 0.067 4.145 0.159 6.485 0.049 2.435 0.153 3.664 0.084 3.074 0.039 3.798 0.068 3.236 0.080 3.073 0.037 1.890 4.546 5495), ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 5495 − R (4405 E 5495) − 0.0120.0060.007 3.139 0.010 4.299 0.035 3.764 0.006 4.218 0.010 5.573 0.006 4.199 0.006 4.088 0.007 3.828 0.012 3.187 0.011 4.196 0.006 4.107 0.006 4.855 0.030 3.480 0.006 3.480 0.007 3.667 0.007 3.382 0.006 3.564 0.009 4.050 0.006 4.009 0.015 3.663 0.008 4.079 0.008 3.968 0.025 3.680 0.009 3.721 0.021 3.830 0.006 3.634 0.012 3.907 0.018 4.224 0.006 3.580 0.012 3.108 0.012 4.571 0.011 3.781 0.010 3.156 0.007 3.221 0.007 3.582 0.011 3.948 0.026 3.825 0.023 3.605 0.008 3.495 0.031 3.393 0.019 3.682 0.007 3.429 0.011 3.310 0.008 3.376 0.009 3.352 3.360 3.519 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± (mag) (mag) (mag) (pc) CCM F99 MA14 (4405 E 0101 101 101 J2GT 101 J2GTS 1 0.495 01 J2GTS 0.300 101 J2GT 0.310 101 J2G 1 0.244 01 J2GTS 101 J2GT 0.315 0.825 101 J2GTS 1 0.258 01 J2GTS 0.392 101 J2GTS 0.297 101 J2GT 0.379 101 J2GS 1 0.411 01 J2GTS 1 0.286 01 J2GTS 0.540 101 J2GT 0.407 101 J2GTS 1 1.113 01 J2GTS 0.587 101 J2GTS 0.301 101 J2GTS 0.262 101 J2GTS 0.268 101 J2GTS 0.484 101 J2GT 0.360 101 J2GTS 1 0.485 01 J2GTS 0.439 101 J2GT 0.525 101 J2GT 1 1.375 01 J2GT 1 0.764 01 J2GTS 1 1.032 01 J2GTS 0.160 101 J2GT 1.063 101 J2GTS 1 1.196 01 J2GS 0.184 102 J2GS 1 0.921 01 J2GTS 1 1.254 01 J2GTS 1.285 101 J2GTS 0.752 101 J2GTS 0.736 101 J2GTS 0.678 101 J2G 1.141 101 J2GT 101 J2GTS 1.854 1 0.709 01 J2GT 0.986 101 J2GT 1 0.888 01 J2GTS 1 1.137 J2GTS 0.948 1 J2GTS 0.907 J2GTS 0.555 1.284 02.62 00.30 06.31 00.91 01.17 01.20 01.20 01.48 01.56 02.10 00.25 00.69 00.62 00.85 01.79 00.92 01.74 01.58 01.27 01.13 00.70 13.31 00.07 00.70 03.33 00.73 10.58 00.06 04.22 09.92 00.35 01.29 01.12 00.84 00.84 00.76 00.85 00.87 00.35 01.68 01.69 01.87 01.62 01.25 03.09 03.63 23.59 − + − − − − − − − − − + + − + − − − − − − + − − + − − − + − − − − + + + + + + + + + + + − + + 16 482614 501414 5040 GOS 015.26 13 4927 GOS 016.65 GOS 016.90 12 4979 GOS 016.98 GOS 018.25 − − − − − Oph GOS 006.28 Results from the CHORIZOS ﬁts. The name includes all GOSSS components (the NC column gives the number of components) while the GOSSS ID refers to the brightest component NameHD 164 019 GOS 001.91 GOSSS ID NC SS 63 OphHD 168 941HD 164 536Herschel 369 Sgr ABHD 164 816HD 165 052ζ HD 165 246 GOSHD 005.82 165 921 GOS 004.54 GOSHD 005.96 164 492 A GOSHD 005.97 163 800HD 163 892 GOSHDE GOS 006.06 313 006.01 846 GOSHD 006.12 164 438HD 166 546 GOS15 006.40 Sgr GOS16 006.94 Sgr GOS AaAb 007.00 HD 167 659 GOSHD 007.05 167 771 GOSHD 007.15 167 411 GOS 007.36 HD 157 857 GOSHD 010.35 167 633 GOSALS 010.36 19 618HD 165 319 GOS 010.76 BD GOS GOSHD 010.46 012.20 175 876 GOSALS 012.70 4923 GOSALS 012.72 4626 GOSHD 012.97 175 754 GOSBD 014.34 V479 GOS Sct 015.07 GOSBD 015.12 HD 168 075 GOSHD 015.28 168 076 ABHD 168 137 GOS AaAb 015.70 BD GOS 015.88 GOSALS 016.39 15 360HD 168 504MY Ser AaAb GOSBD 016.88 GOS GOS 016.94 GOSALS 016.97 016.94 4880HD 168 112 ABHD 168 461HD 171 589HD GOS 166 017.00 734 GOS GOS 017.03 018.25 GOS 018.44 GOS 018.32 GOS 018.57 GOS 018.65 GOS 018.92 Table 1. when there are two. The SS column gives the subsample to which the object belongs. The results for

4 J. Ma´ız Apellaniz´ and R. H. Barba:´ Optical-NIR dust extinction towards Galactic O stars 2 red χ 2 red χ 2 red χ d log 0.0210.022 3.40.018 3.30.025 7.24 3.50.019 2.07 3.60.022 1.49 3.99 3.70.022 1.39 0.66 3.70.017 1.04 4.00 1.14 3.40.018 1.18 1.70 0.82 3.30.016 1.26 1.90 2.72 3.10.019 0.18 0.11 0.68 3.40.030 0.63 2.40 1.06 3.60.018 0.57 0.69 1.54 3.30.022 2.18 0.82 1.00 3.20.017 0.88 1.45 0.14 3.10.020 0.72 1.34 1.13 3.10.019 0.75 0.56 1.27 3.30.015 1.24 0.34 7.50 3.40.018 0.26 0.17 1.16 3.30.017 0.90 8.10 1.21 3.40.017 1.13 0.60 1.95 3.40.017 2.64 0.70 2.38 3.20.015 0.18 1.25 1.28 3.20.017 1.03 1.46 4.74 3.30.016 0.74 1.68 3.95 3.30.016 2.25 2.19 2.10 3.20.016 1.33 0.85 3.63 3.00.023 2.26 1.28 0.73 3.20.030 0.54 2.91 3.16 3.20.012 14.25 0.79 0.59 3.40.015 1.43 3.26 1.92 3.4 4.030.015 0.79 1.84 3.60.019 1.66 3.17 4.51 3.3 1.12 0.022 4.33 2.30 3.10.014 2.10 3.54 1.78 3.50.015 2.69 3.08 5.31 3.20.017 11.70 1.58 2.23 3.20.015 1.12 3.04 2.00 3.3 2.630.015 1.43 1.97 3.20.018 1.49 2.53 6.64 3.2 0.96 0.015 1.44 4.71 3.20.016 1.59 5.92 1.09 3.40.014 0.94 2.84 1.22 3.10.016 2.52 0.71 0.81 3.20.017 1.69 0.77 2.77 2.90.018 1.06 0.87 0.45 3.10.019 0.54 2.18 4.46 3.10.015 11.40 0.54 0.47 3.4 2.57 3.37 0.68 2.9 4.91 0.29 0.87 2.26 0.64 1.89 2.28 0.57 0.83 1.29 0.49 1.39 0 , ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± J V J 0.0220.023 5.131 0.019 6.909 0.041 5.830 0.024 5.855 0.036 6.874 0.030 6.606 0.020 7.731 0.022 5.290 0.017 6.106 0.022 5.115 0.047 7.507 0.029 7.722 0.034 6.546 0.019 6.206 0.030 6.576 0.022 6.027 0.017 7.463 0.032 6.212 0.025 7.727 0.024 5.255 0.025 6.679 0.017 6.684 0.021 6.138 0.021 5.071 0.020 5.900 0.026 5.867 0.030 4.246 0.048 5.825 0.015 7.652 0.018 5.076 0.021 6.316 0.022 5.904 0.030 5.185 0.018 5.361 0.022 6.543 0.019 6.665 0.018 5.000 0.020 5.523 0.028 6.566 0.017 6.166 0.025 6.738 0.019 6.149 0.018 6.712 0.025 4.666 0.033 5.968 0.023 6.270 0.020 7.236 5.106 V ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± A 0.0310.048 4.273 0.046 2.718 0.086 2.865 0.080 3.290 0.108 2.440 0.065 2.816 0.220 3.067 0.061 0.852 0.144 2.490 0.084 1.109 0.147 2.256 0.098 2.699 0.090 2.795 0.025 3.222 0.098 3.762 0.056 3.095 0.091 2.698 0.123 1.258 0.063 2.520 0.079 3.616 0.065 2.334 0.072 2.992 0.092 1.621 0.134 1.501 0.084 1.329 0.037 1.918 0.084 4.612 0.130 2.786 0.053 4.541 0.054 2.033 0.079 2.824 0.057 2.484 0.043 2.043 0.054 4.292 0.094 2.403 0.046 2.073 0.056 2.396 0.351 2.006 0.066 0.760 0.139 3.325 0.074 1.064 0.117 2.745 0.079 1.312 0.044 1.154 0.070 3.517 0.314 4.064 0.043 0.507 3.892 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 5495 R 5495) − 0.0070.0070.007 3.418 0.020 3.471 0.013 3.604 0.023 3.163 0.016 3.290 0.008 3.107 0.012 2.949 0.006 4.077 0.012 2.895 0.020 4.269 0.018 3.374 0.023 3.515 0.007 3.324 0.022 3.050 0.013 2.867 0.006 3.226 0.019 2.964 0.014 3.469 0.010 3.375 0.011 3.441 0.006 3.373 0.008 3.371 0.011 3.471 0.008 3.304 0.012 3.267 0.019 3.553 0.047 2.951 0.006 2.985 0.012 3.328 0.014 3.381 0.008 3.156 0.011 3.168 0.012 2.996 0.013 3.143 0.006 2.843 0.008 3.192 0.012 3.315 0.019 2.972 0.007 4.116 0.016 2.883 0.011 3.997 0.006 2.948 0.010 3.096 0.022 2.972 0.009 2.929 0.013 3.089 3.357 3.062 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± (mag) (mag) (mag) (pc) CCM F99 MA14 (4405 E 0101 101 101 J2GTS 101 J2GTS 1.241 101 J2GTS 0.774 101 J2G 0.786 101 J2GT 101 J2G 1.030 1 0.732 01 J2GT 101 J2GTS 0.896 1 1.028 01 J2GT 0.206 101 J2GTS 1 0.848 01 J2GT 0.257 101 J2G 1 0.660 01 J2GT 101 J2GT 0.759 1 0.831 01 J2GTS 1 1.045 01 J2GT 1.300 101 J2GT 1 0.949 01 J2GTS 1 0.898 01 J2GT 0.357 101 J2GT 1 0.738 01 J2GTS 1 1.042 01 J2GTS 0.683 201 J2GTS 0.878 201 J2GTS 0.460 101 J2GT 0.447 101 J2GTS 1 0.401 01 J2GS 0.532 101 J2GT 1 1.555 01 J2G 1 0.921 01 J2GTS 101 J2GT 0.593 1.357 101 J2GT 1 0.884 01 J2GTS 1 0.774 01 J2GTS 0.671 101 J2GT 1.357 101 J2GT 1 0.833 01 J2GTS 1 0.640 01 J2GTS 0.714 101 J2GT 0.664 101 J2GT 1 0.183 01 J2GTS 1 1.141 01 J2GT 0.263 101 J2GT 1 0.919 01 J2GTS 1 0.417 01 J2GTS 0.381 1 J2G 1.188 1 J2GTS J2GT 0.149 1.305 1.260 02.14 01.01 01.20 02.49 03.34 00.85 01.34 11.29 03.38 04.33 00.15 00.14 00.15 00.11 00.63 00.14 00.38 03.66 01.21 03.07 02.87 02.62 02.02 02.61 01.78 01.43 00.51 00.20 00.62 01.48 00.96 00.82 01.28 00.34 01.68 01.62 01.80 03.40 06.23 00.93 06.17 02.02 04.25 02.78 01.63 00.78 09.54 01.35 + + + − − − + + + − − − − + + − + + + + + + + + + + − + + + + + + − + + + + − + + − + + + + − + 11 4586 GOS 019.08 10 468204 4503 GOS 020.24 GOS 026.85 36 4063 GOS 076.17 36 4145 GOS 077.45 − − − + + continued. NameBD GOSSS ID NC SS BD HD 169 582HD 173 010HD 173 783HD 172 175BD HD 165 174HD 175 5149 Sge GOSHDE 021.33 344 783 GOSHDE 023.73 344 782 GOSHDE 024.18 344 784 A GOSHDE 024.53 344 777HDE 344 758 GOSHDE 029.27 338 931 GOSHDE 041.71 338 916HD 186 980 GOS 059.37 HDE 227 465 GOS 059.40 GOSCyg 059.40 GOS X-1 056.48 HDE 227 018 GOS 059.41 HDE 227 245 GOS 060.17 HD 190 864 GOS 061.19 HD 190 429 GOS AB 061.47 HD 191 201 AB GOSHD 067.39 191 612 GOS 070.73 HDE 228 779HDE 228 854 GOS 071.58 ALS GOS 18 071.34 707 GOS 072.17 HD 192 639 GOS 072.59 GOSHDE 072.47 228 766 GOS 072.75 HD 193 682HD 193 443 AB GOSBD 072.99 GOS 073.18 HDE 228 841 GOS 074.54 HD 193 595HD GOS 193 074.76 514 GOSHD 074.90 192 281 GOS 075.19 Y Cyg GOS 076.15 GOSHDE 075.92 229 232 ABHD 189 957BD GOS 076.60 HD 191 978 GOSHD 076.86 193 322 AaAb GOSHDE 077.00 229 202 GOSALS GOS 077.12 11 355 077.40 HD 201 345HD 194 649 AB GOS GOS 077.25 077.43 GOS 078.10 GOS 077.87 GOS 078.19 GOS 078.29 GOS 078.46 GOS 078.44 Table 1.

5 J. Ma´ız Apellaniz´ and R. H. Barba:´ Optical-NIR dust extinction towards Galactic O stars 2 red χ 2 red χ 2 red χ d log 0.0200.022 3.20.015 3.50.018 1.32 3.10.017 0.66 3.00.016 1.23 8.92 3.20.037 0.40 0.81 3.20.027 0.94 5.65 0.78 3.20.020 0.26 3.05 1.23 3.20.024 1.07 6.12 1.50 3.20.023 0.96 1.51 1.59 3.30.025 0.85 5.00 1.63 2.90.019 1.13 2.52 1.19 2.90.043 0.88 2.19 3.63 3.1 10.070.022 3.87 0.04 3.30.026 3.60 2.58 1.59 3.2 2.04 11.600.020 1.32 3.20.017 2.89 2.04 0.77 3.3 0.46 0.020 0.23 1.81 3.10.017 2.72 3.16 1.12 3.00.021 1.00 0.48 1.37 3.10.017 2.19 1.32 0.82 3.20.021 1.80 2.77 0.32 3.20.020 1.30 1.28 1.13 3.20.018 1.92 8.20 1.42 3.00.019 1.13 2.92 2.70 3.20.022 1.35 3.13 0.38 3.30.018 2.12 2.56 0.67 3.10.030 2.77 1.38 0.99 3.20.018 2.67 0.80 2.96 3.00.019 1.03 1.88 2.35 3.10.029 1.40 4.46 3.42 3.20.021 1.61 2.08 0.94 3.00.020 2.98 3.97 0.75 3.00.020 2.48 0.94 1.42 3.00.021 3.03 0.73 0.51 3.20.031 3.08 0.71 1.01 3.30.019 1.06 1.26 1.39 2.90.020 1.67 0.44 0.89 3.10.015 3.43 2.70 3.33 3.20.014 0.93 0.91 1.63 3.40.013 1.39 3.20 1.42 3.70.026 2.47 0.54 0.96 3.10.017 3.08 1.35 2.67 3.20.020 2.97 1.17 6.01 3.30.022 2.35 3.19 2.65 3.30.021 0.74 6.85 0.65 3.40.014 1.43 6.05 0.39 2.9 1.40 1.67 0.77 3.6 2.13 1.92 2.53 0.27 2.48 3.93 0.67 1.85 1.28 3.49 1.03 1.61 0 , ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± J V J 0.0250.026 6.591 0.020 6.705 0.030 4.527 0.032 5.543 0.022 7.387 0.085 7.008 0.066 4.515 0.039 5.605 0.045 6.615 0.051 4.761 0.052 4.234 0.038 2.673 0.083 6.429 0.053 6.129 0.067 6.949 0.045 4.488 0.036 6.447 0.041 3.809 0.031 6.002 0.040 3.986 0.033 4.990 0.050 5.626 0.040 6.324 0.037 5.016 0.036 6.670 0.050 5.014 0.035 6.458 0.063 6.245 0.046 5.949 0.038 6.524 0.064 6.766 0.041 6.352 0.040 5.878 0.038 5.917 0.040 4.107 0.073 7.061 0.038 5.016 0.036 6.302 0.016 4.396 0.022 4.911 0.015 6.486 0.058 3.419 0.024 4.449 0.037 6.855 0.045 6.002 0.022 5.356 0.018 4.778 6.034 V ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± A 0.0940.139 1.662 0.040 1.328 0.109 3.944 0.131 3.909 0.064 3.573 0.078 2.633 0.098 8.555 0.064 7.321 0.068 6.572 0.054 8.310 0.076 7.584 0.044 6.445 0.094 6.586 0.051 6.709 0.062 6.654 0.049 7.025 0.034 7.044 0.058 7.053 0.052 5.925 0.070 5.061 0.060 5.385 0.081 4.569 0.069 6.091 0.063 5.175 0.044 5.342 0.074 5.546 0.042 5.081 0.078 4.603 0.083 4.712 0.050 5.630 0.075 5.182 0.063 5.787 0.044 6.646 0.060 5.699 0.077 5.677 0.091 5.757 0.066 6.939 0.063 5.603 0.260 5.690 0.062 0.712 0.030 3.024 0.113 3.670 0.092 5.352 0.065 2.213 0.055 4.285 0.101 5.582 0.082 1.172 1.690 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 5495 R 5495) − 0.0120.0110.009 2.885 0.023 3.280 0.022 3.377 0.013 3.871 0.061 4.129 0.067 3.053 0.050 3.001 0.059 3.387 0.042 2.869 0.038 3.153 0.024 2.833 0.049 3.161 0.030 3.051 0.043 2.961 0.038 2.758 0.017 2.832 0.033 2.908 0.017 3.088 0.030 2.983 0.022 3.297 0.053 3.205 0.032 3.065 0.032 2.860 0.018 3.056 0.029 3.008 0.011 3.029 0.024 2.941 0.062 3.030 0.019 2.885 0.040 2.834 0.047 2.999 0.021 2.653 0.026 2.916 0.047 2.873 0.083 3.093 0.032 2.989 0.032 2.630 0.006 3.132 0.010 3.134 0.006 4.659 0.070 3.562 0.014 3.389 0.018 2.886 0.019 3.184 0.006 3.215 0.008 3.226 3.369 3.245 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± (mag) (mag) (mag) (pc) CCM F99 MA14 (4405 E 0101 101 101 J2GT 101 J2GT 1 0.566 01 J2GTS 1 0.399 01 J2GT 1.158 101 J2G 1 1.002 01 J2GT 101 J2G 0.858 1 0.851 01 J2G 101 J2G 2.883 101 J2G 2.168 201 J2G 2.297 102 J2G 2.658 101 J2G 2.698 101 J2G 2.042 201 J2G 2.162 101 J2G 2.273 101 J2G 2.422 102 J2GT 2.494 101 J2G 2.435 1 2.293 01 J2G 101 J2G 1.985 101 J2GT 1.528 101 J2G 1.675 1 1.481 01 J2G 101 J2G 2.133 101 J2GT 1.687 101 J2GT 1.770 1 1.827 01 J2GT 1 1.721 01 J2GT 1 1.512 01 J2G 1 1.625 01 J2G 101 J2G 1.985 101 J2G 1.721 101 J2G 2.185 101 J2GT 2.286 101 J2G 1.981 1 1.833 01 J2GT 101 J2G 1.925 1 2.659 01 J2G 101 J2GTS 1.784 101 J2GTS 0.151 1.813 101 J2GTS 0.840 101 J2G 1.074 101 J2G 101 J2GT 1.852 1 J2GT 0.685 1 1.323 J2GTS 1.726 J2GTS 0.342 0.513 04.66 05.37 02.07 03.62 01.21 01.16 00.30 00.85 00.67 00.91 00.68 00.74 00.74 00.75 00.79 00.76 00.76 00.79 00.79 01.02 00.71 00.78 00.79 00.80 00.90 00.94 00.93 00.81 00.88 00.66 00.60 00.91 00.85 00.80 00.73 00.80 00.83 10.09 06.61 02.96 02.33 03.29 00.30 02.66 03.63 00.94 03.78 03.81 + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + − − + + + + 4114504 GOS 080.03 + 41 14 GOS 079.01 46 1246 11 GOS 084.88 GOS 084.88 + + + 40 4179 GOS 079.03 43 365445 3216 A GOS 082.41 GOS 083.78 + + + continued. NameHD 192 001 GOS 078.53 GOSSS ID NC SS HD 191 423HDE 229 196HDE 228 759LS III BD Cyg OB2-B17Cyg OB2-A242MASS J20315961 GOSCyg 078.64 OB2-A11 GOS 078.76 ALS 15 108 GOS AB 079.01 Cyg OB2-5 ABALS 15 134 GOSCyg 079.84 OB2-22 C GOSCyg 080.03 OB2-22 DCyg OB2-22 AB GOSALS 080.08 15 144 GOSCyg 080.11 OB2-9 GOSCyg 080.12 OB2-24Cyg OB2-8 A GOSCyg GOS 080.14 OB2-8 080.14 B GOSCyg 080.14 OB2-4 GOS A 080.14 ALS 15 119Cyg OB2-8 CCyg GOS OB2-8 080.15 DCyg GOS OB2-7 080.17 GOS 080.21 Cyg OB2-17 GOS 080.22 Cyg OB2-16 GOS 080.22 Cyg OB2-6 GOS 080.22 ALS 15 115ALS GOS 15 080.23 111 GOS 080.23 Cyg OB2-27 GOS AB 080.23 Cyg OB2-73Cyg GOS OB2-25 080.24 A GOS 080.24 Cyg OB2-10 GOS 080.24 ALS 15 125ALS GOS 15 080.26 114Cyg GOS OB2-29 080.27 GOS 080.29 Cyg GOS OB2-11 080.27 HD 188 209 GOS 080.32 GOSHD 080.44 191 781HD 195 592 GOS 080.47 BD BD GOS 080.53 LS GOS III 080.54 GOS 080.55 LS III GOS 080.57 HD 199 579 GOSHD 080.99 202 124 GOS 081.18 GOS 082.36 GOS 085.70 GOS 087.29 Table 1.

6 J. Ma´ız Apellaniz´ and R. H. Barba:´ Optical-NIR dust extinction towards Galactic O stars 2 red χ 2 red χ 2 red χ d log 0.0170.019 2.90.015 3.80.016 1.91 2.70.021 1.36 2.80.016 1.62 2.32 3.20.018 1.35 4.41 2.90.019 1.22 2.43 0.97 2.70.027 0.62 2.23 2.25 3.60.014 2.06 0.44 2.94 3.60.014 1.59 1.59 0.84 3.10.029 0.23 1.48 0.43 2.80.025 1.48 0.66 1.69 3.30.020 0.63 1.16 2.25 3.60.022 0.30 0.75 1.50 3.40.017 0.19 1.78 0.50 3.60.027 0.38 0.37 2.08 3.10.016 1.37 0.44 0.67 3.00.022 1.88 1.82 4.50 3.00.016 0.74 1.41 2.22 3.10.018 1.83 3.11 1.93 3.20.022 0.62 1.47 2.01 3.50.026 1.72 1.16 3.85 3.40.017 0.62 1.51 3.76 3.50.018 0.72 2.29 1.32 2.90.015 0.84 2.17 1.39 2.90.023 1.09 2.30 9.68 3.60.014 1.90 1.18 2.92 3.50.014 0.86 4.08 1.34 3.40.016 1.04 1.73 0.50 2.90.013 1.36 1.18 0.23 2.90.035 1.62 0.23 6.61 2.80.026 0.79 0.41 4.46 3.30.016 0.91 2.62 6.33 3.30.026 0.52 1.56 1.46 3.10.016 1.25 2.09 1.53 3.50.020 0.54 8.85 2.02 3.70.016 1.55 4.84 1.50 3.60.017 2.11 2.73 0.75 3.30.020 2.16 1.45 1.12 2.90.021 1.08 1.83 2.52 3.00.017 1.03 2.74 2.70 2.90.029 0.77 2.43 1.99 2.90.028 0.54 2.23 1.87 3.00.022 11.77 1.98 2.12 2.80.019 2.15 0.66 2.83 3.7 5.500.020 0.78 0.76 3.30.019 1.77 2.55 0.35 3.5 2.06 3.16 2.91 3.5 2.77 0.91 0.06 1.93 2.70 0.52 0.29 0.54 1.98 0.40 0.08 0.48 0 , ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± J V J 0.0190.035 4.089 0.017 7.805 0.018 4.555 0.023 4.506 0.022 6.183 0.021 5.093 0.020 4.104 0.050 6.394 0.016 8.207 0.015 5.704 0.039 4.434 0.048 7.506 0.031 8.351 0.038 7.136 0.019 8.476 0.027 4.228 0.019 3.436 0.023 5.605 0.018 3.928 0.022 6.482 0.037 8.172 0.043 7.532 0.025 7.762 0.026 5.291 0.018 4.985 0.042 6.251 0.019 8.474 0.016 6.589 0.017 5.425 0.015 5.485 0.057 4.736 0.049 7.690 0.017 6.333 0.036 6.136 0.020 7.723 0.022 6.606 0.020 6.218 0.020 5.442 0.021 5.526 0.035 5.644 0.027 4.849 0.041 4.957 0.048 5.457 0.036 4.368 0.024 8.971 0.036 6.657 0.032 8.483 7.746 V ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± A 0.1460.111 0.912 0.494 2.947 0.087 0.320 0.090 1.095 0.051 1.235 0.079 2.859 0.130 1.584 0.222 1.155 0.049 3.818 0.095 1.585 0.144 1.109 0.166 2.632 0.078 2.588 0.205 3.365 0.062 1.827 0.079 1.692 0.128 1.611 0.112 1.097 0.053 1.167 0.041 2.096 0.110 2.403 0.176 2.457 0.038 2.840 0.068 4.409 0.151 3.443 0.190 0.952 0.103 2.278 0.037 1.767 0.039 2.576 0.034 2.522 0.149 2.916 0.163 6.528 0.052 5.569 0.116 2.525 0.090 2.438 0.111 1.995 0.388 1.980 0.042 0.658 0.088 3.185 0.078 1.732 0.041 5.164 0.070 4.020 0.086 4.069 0.109 7.317 0.160 2.405 0.331 1.441 0.098 1.164 2.864 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 5495 R 5495) − 0.0060.0230.006 3.694 0.007 3.254 0.007 4.130 0.014 2.831 0.008 3.167 0.007 2.633 0.083 3.090 0.006 3.946 0.006 3.174 0.022 2.737 0.020 3.343 0.016 3.536 0.018 3.715 0.008 3.346 0.006 3.716 0.008 2.873 0.006 3.253 0.007 3.441 0.003 3.578 0.018 3.368 0.038 3.165 0.010 3.143 0.016 3.270 0.007 3.214 0.025 3.183 0.014 3.765 0.006 3.612 0.006 3.206 0.006 3.117 0.062 3.135 0.070 3.259 0.007 3.915 0.016 3.647 0.011 3.733 0.010 3.367 0.008 3.481 0.011 3.975 0.007 4.676 0.035 2.905 0.012 3.945 0.022 3.135 0.058 2.841 0.017 2.970 0.010 3.287 0.020 3.135 0.025 3.869 3.619 2.884 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± (mag) (mag) (mag) (pc) CCM F99 MA14 (4405 E 0101 101 101 J2GTS 101 J2GT 0.243 101 J2GTS 1 0.896 01 J2GTS 0.077 101 J2GTS 0.379 101 J2GT 0.383 101 J2GTS 1 1.072 01 J2GTS 0.504 101 J2GT 0.289 102 J2GTS 2 1.197 01 J2GTS 0.569 101 J2GT 0.326 101 J2G 1 0.736 01 J2GT 101 J2GT 0.689 1 0.996 01 J2GTS 1 0.486 01 J2GTS 0.579 101 J2GTS 0.487 101 J2GTS 0.314 101 J2GTS 0.321 101 J2GTS 0.614 101 J2GT 0.752 101 J2GT 1 0.771 01 J2GTS 1 0.860 01 J2GT 1.362 101 J2GTS 1 1.071 01 J2G 0.249 101 J2G 101 J2GTS 0.624 102 J2GTS 0.815 0.543 101 J2GTS 0.794 101 J2G 0.884 101 J2GT 101 J2GTS 1.667 1 1.524 01 J2GT 0.668 101 J2GT 1 0.715 01 J2GT 1 0.566 01 J2GTS 1 0.492 01 J2GT 0.140 101 J2GTS 1 1.083 01 J2GT 0.434 101 J2GS 1 1.641 01 J2GT 1 1.403 01 J2G 2 1.360 01 J2GT 1 J2GTS 2.233 1 0.757 J2GT 0.368 J2GT 0.318 0.981 03.84 01.12 16.98 07.99 03.40 05.55 03.74 03.13 02.47 04.67 02.18 00.67 00.67 00.76 01.41 06.99 02.61 05.87 05.39 00.90 00.95 02.92 02.87 05.02 05.20 06.89 02.74 01.79 02.68 02.39 02.72 00.82 00.82 00.22 01.16 01.24 00.14 11.09 05.22 01.25 05.09 04.99 05.01 05.01 00.12 06.24 06.25 01.11 − − − + + + + − + + + − − − − + + + + − − − − + + − − − + + + + + + − − − − + + + + + + + − − − 55 2722 AB55 2722 C GOS 102.81 GOS 102.81 55 284062 2078 GOS 107.30 GOS 107.45 60 252260 263566 1661 GOS 112.23 66 1675 GOS 115.90 66 1674 GOS 117.81 60 134 GOS 118.21 GOS 118.22 GOS 123.50 + + + + + + + + + + Cep GOS 103.83 continued. Name68 Cyg GOS 087.61 GOSSS ID NC SS ALS 11 76110 LacHD 202 214 AaAbBHD 206 183HD 204 827 AaAbHD 206 267 AaAbHD 210 809ALS 12 GOS 050 098.52 HD GOS 207 088.81 53814 Cep GOSBD 099.17 GOS GOS 096.65 GOSBD 099.29 098.89 ALS 12 320ALS 12 370 GOSHD 099.85 207 198λ GOS 101.08 GOSHD 101.60 209 33919 CepDH Cep GOS 102.01 ALS 12 619BD GOS 102.98 ALS GOS 12 103.05 688 GOSBD 103.14 HD 213 023 A GOSHD 104.58 218 915ALS 12 749HD 218 195 AB GOS 104.87 HD GOS 216 GOS 107.18 532 107.07 HD 216 898HD GOS 217 107.42 086Sh 2-158 GOS 2 107.73 Sh 2-158 1 GOSBD 108.06 GOS 109.32 BD GOS 108.54 HD 225 146 GOSHD 109.65 225 160 GOSAO Cas 109.93 GOSBD 110.22 HD 108 GOS 111.53 V747 Cep GOS 111.53 BD BD GOSTyc 4026-00424-1 117.23 GOSALS 117.44 6351HD 5005 ABHD 5005 GOS D 117.59 BD GOS GOS GOS 117.93 118.23 118.20 GOS GOS 122.57 123.12 GOS 123.12 Table 1.

7 J. Ma´ız Apellaniz´ and R. H. Barba:´ Optical-NIR dust extinction towards Galactic O stars 2 red χ 2 red χ 2 red χ d log 0.0170.028 3.30.019 3.40.020 1.80 3.50.020 0.79 3.60.030 1.21 1.13 3.40.019 0.33 2.95 3.30.026 0.96 1.71 1.09 3.40.017 0.74 0.70 0.82 3.10.021 1.57 0.88 1.39 3.50.015 1.18 0.72 3.04 3.40.015 0.73 0.70 1.91 3.20.018 0.89 0.90 1.66 3.10.015 0.69 1.21 0.58 3.50.017 2.76 1.23 1.04 3.70.014 0.82 1.05 0.93 3.30.015 0.66 0.82 5.26 3.30.015 0.25 0.25 0.63 3.20.018 1.21 1.18 4.50 3.30.016 0.41 1.29 3.67 3.20.015 1.29 1.55 5.04 3.30.021 1.16 2.51 3.98 3.50.015 1.17 4.54 4.43 3.50.020 1.22 1.38 2.92 3.20.015 1.77 2.05 1.88 3.70.019 0.66 3.33 4.28 3.60.019 0.68 1.18 0.42 3.10.019 1.41 1.52 0.72 3.10.016 0.42 0.09 3.04 3.20.017 1.48 0.31 5.66 3.60.023 0.31 1.44 4.77 3.60.023 0.63 4.61 1.75 3.40.016 0.54 2.33 4.14 3.10.015 1.22 1.44 0.81 3.40.015 1.41 2.38 5.55 3.00.011 0.96 0.54 6.33 3.20.018 0.91 2.21 5.80 3.00.015 0.90 3.84 0.84 3.50.016 2.20 1.81 1.63 3.60.015 1.01 0.75 1.23 3.70.023 0.53 0.95 3.40 3.00.027 0.62 1.05 1.37 3.40.013 0.44 2.04 5.16 3.50.024 0.70 2.18 0.15 3.10.150 1.26 3.00 0.74 3.60.015 0.56 1.42 2.84 2.70.021 2.06 1.30 0.96 3.00.014 0.36 1.92 1.34 3.6 0.95 3.05 2.03 3.8 1.53 1.30 1.53 0.76 1.27 1.02 0.98 1.80 1.12 2.17 1.14 0.47 0 , ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± J V J 0.0190.033 7.226 0.033 6.695 0.023 6.440 0.025 6.946 0.065 8.176 0.021 7.102 0.053 7.512 0.019 6.159 0.023 6.836 0.018 6.974 0.018 6.451 0.019 6.089 0.022 6.415 0.030 7.524 0.016 7.708 0.020 7.150 0.020 5.258 0.020 4.845 0.018 6.032 0.017 6.858 0.022 5.696 0.020 7.023 0.022 4.900 0.018 7.432 0.022 6.160 0.020 4.987 0.021 6.053 0.018 6.545 0.023 6.865 0.028 6.152 0.027 6.777 0.018 5.882 0.017 5.522 0.017 4.751 0.013 7.140 0.030 3.227 0.025 8.267 0.020 8.148 0.017 6.628 0.042 4.670 0.045 6.992 0.015 8.058 0.046 4.458 0.149 8.780 0.017 3.124 0.037 5.973 0.020 8.701 7.162 V ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± A 0.0610.125 1.909 0.157 1.968 0.081 1.798 0.202 1.835 0.106 0.736 0.072 3.903 0.116 1.454 0.089 4.069 0.055 1.340 0.065 2.242 0.056 2.376 0.087 2.692 0.071 1.449 0.099 2.426 0.047 2.573 0.056 2.444 0.050 2.675 0.048 3.293 0.044 2.395 0.046 2.545 0.079 2.303 0.055 1.467 0.166 2.943 0.076 1.098 0.061 2.206 0.064 2.131 0.057 2.213 0.129 1.926 0.064 1.007 0.091 2.523 0.076 2.089 0.037 2.035 0.040 2.932 0.564 2.357 0.127 0.313 0.166 1.060 0.092 1.852 0.070 1.585 0.049 2.358 0.069 2.072 0.145 4.185 0.080 2.865 0.122 1.313 0.541 3.012 0.179 0.907 0.110 0.784 0.073 2.557 2.684 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 5495 R 5495) − 0.0070.0140.018 3.183 0.008 3.357 0.009 3.361 0.026 3.324 0.007 3.033 0.037 3.028 0.006 3.081 0.008 3.073 0.011 3.454 0.011 3.242 0.007 3.270 0.010 3.182 0.016 3.638 0.007 3.241 0.008 3.201 0.009 3.401 0.007 3.537 0.007 3.494 0.006 3.355 0.007 3.321 0.009 3.299 0.010 3.261 0.010 3.456 0.007 3.532 0.007 3.467 0.007 3.305 0.007 3.711 0.009 3.248 0.012 3.504 0.009 3.356 0.007 3.170 0.006 3.150 0.007 3.294 0.006 3.044 0.020 4.402 0.011 3.990 0.011 3.371 0.006 2.753 0.018 3.259 0.024 3.234 0.007 3.183 0.020 3.505 0.007 3.261 0.007 3.472 0.018 3.215 0.014 3.730 3.203 3.316 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± (mag) (mag) (mag) (pc) CCM F99 MA14 (4405 E 0101 101 101 J2GTS 101 J2GT 0.591 101 J2G 1 0.578 01 J2GTS 101 J2GTS 0.544 0.528 101 J2GT 0.238 101 J2GTS 1 1.278 01 J2GT 0.464 101 J2GTS 1 1.315 01 J2GTS 0.382 101 J2GT 0.682 101 J2GT 1 0.717 01 J2GTS 1 0.835 01 J2GTS 0.393 101 J2GT 0.739 101 J2GTS 1 0.794 01 J2GTS 0.709 101 J2GTS 0.747 101 J2GTS 0.933 101 J2GTS 0.704 101 J2GTS 0.756 101 J2GTS 0.689 101 J2GTS 0.443 101 J2GT 0.842 201 J2GT 2 0.306 01 J2GTS 1 0.629 01 J2GS 0.636 101 J2GTS 1 0.589 01 J2GTS 0.584 101 J2GTS 0.283 101 J2GT 0.742 101 J2GTS 1 0.650 01 J2GTS 0.636 101 J2GTS 0.880 101 J2GTS 0.763 101 J2GTS 0.071 101 J2GT 0.262 101 J2GTS 1 0.542 01 J2GT 0.565 101 J2GTS 1 0.714 01 J2GT 0.632 101 J2GT 1 1.305 01 J2GTS 1 0.809 01 J2GT 0.396 101 J2GTS 1 0.858 J2GTS 0.278 1 J2GT 0.207 J2GT 0.788 0.800 00.75 01.35 01.82 02.57 05.87 01.39 03.40 01.17 04.99 01.32 02.46 01.04 04.96 00.99 01.00 00.94 00.92 00.86 01.01 00.92 01.74 03.82 01.15 04.73 03.46 00.90 00.88 01.28 07.58 02.73 02.88 01.50 02.08 01.54 18.20 03.12 03.14 02.97 00.71 00.94 01.79 03.95 02.61 12.89 01.00 03.60 14.04 13.11 + − + − − − − + − − + + − + + + + + + + − − + − + + + + − − − + − + − + + + − − + + + + + + + − 60 26161 411 A GOS 127.87 62 42460 49760 49860 GOS 499 133.84 60 501 GOS 134.53 GOS 134.58 60 513 GOS 134.63 GOS 134.64 GOS 134.71 GOS 134.90 60 586 A GOS 137.42 50 88652 80539 1328 GOS 150.60 GOS 151.26 GOS 169.11 + + + + + + + + + + + + Cam GOS 144.07 Per GOS 160.37 continued. NameHD 5689 GOS 123.86 GOSSS ID NC SS BD HD 10 125HD 13 022HD 12 323ALS 6967HD 12 993BD HD 13 268HD 14 442BD GOS 128.29 BD GOS 132.91 HD 13 745 GOS 132.91 BD GOS 132.94 BD GOS 133.11 BD HD 15 558 GOS A 133.96 HD 15 570 GOS 134.21 HD 15 629BD HD 14 947 GOS 134.58 HD 14 434HD 16 429 AHD 15 642 GOSHD 134.72 18 409HD 17 505 GOS AB 134.77 HD 17 520 GOS AB 134.77 BD HD 15 137 GOS 134.99 HD 16 691 GOS 135.08 GOSHD 135.68 16 832HD 18 326 GOS 137.09 HD GOS 17 603 137.19 GOS 137.12 CC GOS Cas 137.22 HD 14 633 AaAbα ALS 7833 GOS 137.46 MY Cam GOS 137.73 HDE 237 211 GOS 138.00 HD 24 431 GOS 138.03 BD GOS GOS 140.78 138.77 BD 1 Cam GOS A 140.12 NGC 1624-2ξ GOS 146.25 HD 41 161 GOS 147.14 GOSALS 146.27 8272BD GOS 148.84 GOS 155.36 GOS 151.91 GOS 164.97 GOS 168.75 Table 1.

8 J. Ma´ız Apellaniz´ and R. H. Barba:´ Optical-NIR dust extinction towards Galactic O stars 2 red χ 2 red χ 2 red χ d log 0.0160.021 3.30.016 2.60.016 1.75 3.10.018 4.44 3.30.015 1.49 1.34 3.20.015 2.20 2.16 3.40.017 0.96 0.70 5.14 3.40.017 2.33 1.79 2.50 3.50.016 0.78 3.55 2.16 3.50.015 1.71 1.12 2.91 3.40.014 2.37 1.62 2.94 3.30.014 0.53 1.66 3.29 3.20.016 0.56 2.18 2.01 3.40.016 1.09 3.31 2.40 3.10.024 1.06 1.69 1.20 3.20.023 1.27 1.01 2.32 3.60.024 0.38 1.21 1.66 3.50.022 0.84 1.31 2.00 3.30.018 1.16 0.68 0.43 3.10.024 0.66 0.50 3.45 2.60.016 0.69 1.34 2.46 3.30.011 1.83 2.20 3.50 2.80.017 0.58 1.29 0.55 2.60.016 1.62 2.97 2.33 3.20.019 0.78 2.09 0.74 2.80.016 2.23 2.37 1.49 3.40.016 0.25 1.05 4.67 3.00.021 2.08 0.95 2.06 3.00.016 0.70 6.47 2.64 3.10.020 0.57 1.43 2.23 3.00.014 2.80 2.46 3.17 3.20.015 0.73 1.17 3.22 3.10.024 1.63 2.96 1.43 2.70.023 0.62 2.53 2.14 2.80.018 1.38 0.88 1.09 3.10.023 1.62 1.59 1.20 3.10.020 0.29 1.46 2.90 2.50.011 0.55 1.20 2.00 2.70.016 0.94 2.18 3.33 2.60.015 1.11 0.97 2.16 3.40.021 1.27 3.94 1.56 3.10.020 0.24 2.11 1.63 2.60.022 1.39 1.47 2.18 3.30.015 1.00 2.25 1.43 4.00.018 1.27 1.58 1.77 3.30.028 1.51 1.43 0.57 2.90.021 1.03 1.50 1.61 3.2 1.32 1.27 1.31 3.2 0.87 1.47 1.11 0.44 0.74 1.50 1.10 1.01 0.32 1.51 0.91 1.36 0 , ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± J V J 0.0180.024 5.614 0.021 4.157 0.021 5.948 0.024 5.688 0.020 6.690 0.017 6.173 0.027 6.984 0.027 8.426 0.023 7.757 0.017 7.570 0.019 7.173 0.015 6.370 0.018 6.799 0.018 5.958 0.035 6.273 0.033 8.140 0.026 8.895 0.024 7.167 0.028 6.217 0.036 2.674 0.018 7.258 0.012 4.416 0.025 1.993 0.020 5.984 0.020 4.634 0.018 6.592 0.020 5.479 0.022 6.137 0.019 6.642 0.022 5.332 0.016 6.705 0.020 5.526 0.025 1.481 0.026 3.641 0.020 6.302 0.026 5.960 0.023 3.266 0.012 3.780 0.017 2.506 0.020 4.945 0.021 6.608 0.021 4.489 0.036 6.918 0.017 10.564 0.020 6.268 0.029 5.141 0.022 6.044 5.587 V ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± A 0.1210.093 1.163 0.079 1.820 0.103 2.244 0.107 1.209 0.078 1.924 0.062 1.877 0.116 2.062 0.119 1.879 0.062 1.559 0.056 2.580 0.121 2.184 0.971 1.260 0.072 0.214 0.102 1.609 0.245 1.273 0.215 1.811 0.084 1.560 0.059 1.255 0.413 2.201 0.112 0.751 0.752 4.015 0.964 0.237 0.229 0.230 0.154 0.886 0.088 1.422 0.138 1.329 0.093 0.877 0.080 1.464 0.096 1.539 0.081 1.444 0.065 1.536 1.130 1.739 1.250 0.269 0.079 0.137 0.054 1.956 0.242 1.971 0.326 1.780 0.932 1.254 0.100 0.248 0.103 1.279 1.470 1.311 0.199 0.113 0.186 0.778 0.146 1.526 0.110 0.941 0.501 1.070 0.483 0.366 0.423 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 5495 R 5495) − 0.0060.0080.012 3.907 0.009 3.675 0.010 3.275 0.009 2.791 0.007 3.590 0.011 3.268 0.012 3.626 0.009 3.432 0.007 3.046 0.009 3.226 0.004 3.509 0.007 3.402 0.006 6.641 0.022 3.416 0.024 3.661 0.007 3.937 0.007 3.275 0.011 2.877 0.036 3.285 0.006 4.221 0.005 3.234 0.011 4.431 0.008 6.380 0.007 3.428 0.006 4.248 0.008 3.340 0.007 3.547 0.008 3.283 0.007 3.279 0.006 3.363 0.007 3.230 0.006 3.423 0.008 6.254 0.007 4.352 0.009 3.331 0.008 3.167 0.005 6.170 0.006 6.220 0.008 6.016 0.005 3.725 0.007 3.117 0.018 5.418 0.007 3.963 0.006 3.231 0.006 3.805 0.006 3.451 4.150 4.662 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± (mag) (mag) (mag) (pc) CCM F99 MA14 (4405 E 0101 101 101 J2GTS 101 J2GTS 0.294 101 J2GT 0.489 101 J2GTS 1 0.676 01 J2GTS 0.425 101 J2GTS 0.529 101 J2GTS 0.566 101 J2GTS 0.561 101 J2GTS 0.540 101 J2GTS 0.503 101 J2GTS 0.789 101 J2GTS 0.614 101 J2GTS 0.365 102 J2GTS 0.032 101 J2GTS 0.464 101 J2GT 0.343 101 J2G 1 0.455 01 J2GTS 101 J2GTS 0.428 0.470 101 J2GS 0.661 201 J2G 1 0.177 01 J2GTS 101 J2GS 0.054 1.232 101 J2GS 1 0.036 01 J2GTS 1 0.254 01 J2GTS 0.331 101 J2GTS 0.392 101 J2GTS 0.243 101 J2GS 0.439 101 J2GTS 1 0.462 01 J2GTS 0.423 201 J2GTS 0.468 101 J2GS 0.500 101 J2GTS 1 0.044 01 J2GTS 0.032 101 J2GTS 0.579 101 J2GTS 0.613 101 J2GTS 0.286 101 J2GTS 0.200 101 J2GTS 0.042 101 J2GTS 0.339 101 J2GTS 0.413 101 J2GTS 0.022 101 J2G 0.194 101 J2GTS 1 J2GTS 0.244 0.466 1 J2GTS 0.305 J2GTS 0.087 0.090 00.27 02.26 01.88 00.61 00.09 03.24 01.66 01.66 01.67 01.72 01.74 00.11 62.15 05.89 00.48 03.36 03.36 03.21 01.98 12.00 01.64 02.20 17.74 00.55 00.31 02.09 00.80 02.04 02.00 02.07 02.25 02.07 01.84 02.63 02.11 01.17 00.80 04.57 00.68 00.78 02.63 00.35 16.59 17.34 19.38 19.37 19.58 20.98 + − + + − + − − − − − − + − + + + + − − + + − − − − + − − − − − − − − − + − − − + + − − − − − − 33 1025 A GOS 173.56 + Ori CaCbOri A GOS 209.01 GOS 209.05 Ori AaAbB GOS 206.82 Ori AB GOS 195.05 Ori GOS 210.44 Ori AaAbB GOS 206.45 Ori AaAb GOS 203.86 Ori GOS 209.52 1 2 continued. NameHD 34 656 GOS 170.04 GOSSS ID NC SS AE AurHD 36 483LY Aur AHD 35 619HD 37 737HDE 242 908BD HDE 242 935 ABALS 8294HDE 242 926 GOS GOS 172.08 172.29 HD 37 366 A GOS 172.76 HD 93 521 GOS 173.04 HD 36 879 GOS GOS 173.46 173.47 HD GOS 42 088 173.58 HDE 256 725 AHDE 256 725 BHD 44 811 GOS GOS 173.65 173.61 HD 41 997 GOSλ 177.63 Tyc 0737-01170-1 GOS 183.14 15 Mon GOS AaAbB 185.22 GOSδ 192.32 GOS 190.04 GOSHD 192.32 46 966 AaAbHD 47 129HD 46 106 GOS GOS 192.40 201.61 HD 48 099 GOS 194.15 HD 46 149 GOS 202.94 HD 46 202HD 46 GOS 150 205.81 HD 46 056HD 46 223ζ GOS 205.87 σ GOS 206.20 HD 46 485 GOS 206.21 HD 46 573 GOS 206.22 θ GOS 206.31 θ GOS 206.31 ι GOS 206.34 HD 47 432 GOS 206.44 HD 48 279 Aυ HD 52 533 GOS A 206.90 ALS 85 GOS 208.73 HD 52 266HD 54 662 ABHD 57 682HD 55 879 GOS 210.03 GOS 210.41 GOS 216.85 GOS GOS 224.17 GOS 218.82 219.13 GOS 224.41 GOS 224.73 Table 1.

9 J. Ma´ız Apellaniz´ and R. H. Barba:´ Optical-NIR dust extinction towards Galactic O stars 2 red χ 2 red χ 2 red χ d log 0.0200.016 3.10.022 3.00.020 1.96 3.50.017 1.61 3.60.020 1.55 0.54 3.50.016 1.39 0.54 3.20.018 1.15 1.54 0.84 2.80.023 1.07 1.12 2.82 3.20.029 0.22 2.83 0.54 3.00.016 0.76 2.19 2.79 3.70.019 0.74 0.46 1.17 3.70.022 0.77 2.69 0.20 3.40.020 0.50 1.07 2.14 3.80.019 2.00 1.45 3.03 3.60.017 0.78 2.16 0.68 3.70.021 0.21 2.70 1.68 3.40.023 1.60 1.77 1.31 3.30.059 0.88 2.62 4.49 3.50.016 0.57 2.38 0.86 2.80.015 1.13 3.69 1.56 3.50.018 0.47 0.79 0.72 2.90.019 1.31 1.30 1.64 3.40.020 0.52 0.72 2.27 2.90.027 1.06 0.68 2.75 3.00.018 0.72 3.67 1.64 3.30.023 0.41 0.91 4.01 3.50.022 2.18 1.77 4.46 3.60.017 0.27 2.45 2.26 3.30.019 1.04 2.08 0.90 3.20.031 1.08 2.39 1.23 3.20.020 0.73 2.35 2.97 3.20.027 0.74 0.83 5.55 3.50.023 0.69 2.34 0.31 3.30.017 0.48 3.65 0.96 3.20.018 0.54 0.62 1.90 3.60.041 0.58 1.84 1.45 3.40.049 0.20 2.30 0.76 3.70.030 0.88 0.52 1.74 3.50.023 1.83 1.06 0.55 3.40.024 1.22 1.53 0.85 3.40.019 0.75 5.28 1.20 3.60.017 0.73 3.60 0.30 3.40.021 0.45 1.00 0.90 3.30.019 0.25 4.53 0.72 3.30.046 0.64 3.33 2.97 3.40.052 0.92 0.88 2.30 3.20.030 0.33 2.09 1.37 3.3 0.51 1.81 0.41 3.3 1.24 1.05 0.35 1.49 0.26 0.19 0.82 0.49 0.66 0.99 0.23 0.05 0 , ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± J V J 0.0220.020 6.725 0.042 5.815 0.026 8.203 0.020 8.653 0.022 8.197 0.017 7.091 0.020 5.054 0.024 4.159 0.045 3.840 0.018 9.001 0.021 7.998 0.029 7.330 0.028 7.381 0.026 8.341 0.023 6.607 0.022 6.599 0.028 5.528 0.051 6.501 0.018 2.175 0.020 5.361 0.023 4.665 0.021 5.744 0.024 5.286 0.028 5.203 0.020 5.288 0.044 5.725 0.023 8.375 0.019 6.400 0.023 5.855 0.041 4.540 0.021 6.614 0.043 5.674 0.033 7.066 0.018 6.796 0.020 7.204 0.058 7.473 0.069 6.756 0.033 6.119 0.026 7.464 0.038 7.973 0.021 8.413 0.022 6.746 0.023 6.881 0.020 7.300 0.051 7.651 0.059 6.265 0.045 6.759 7.202 V ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± A 0.1380.223 0.911 0.146 0.658 0.136 2.640 0.065 1.920 0.077 3.143 1.510 1.654 0.449 0.088 0.450 0.783 0.219 0.550 0.094 2.178 0.072 1.388 0.122 1.898 0.171 2.261 0.125 1.625 0.072 2.204 0.165 2.493 0.244 0.913 1.730 0.629 0.053 0.059 0.102 2.058 0.060 2.588 0.194 2.524 0.075 0.698 0.061 1.932 0.077 2.218 0.172 1.820 0.123 2.564 0.049 1.134 0.038 2.336 0.087 3.857 0.105 3.635 0.115 1.405 0.105 3.941 0.191 2.881 0.081 0.775 0.145 1.590 0.147 5.703 0.133 6.529 0.082 1.398 0.139 3.834 0.142 3.588 0.143 1.030 0.163 1.345 0.140 0.864 0.237 0.770 0.362 1.950 0.326 2.390 1.845 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 5495 R 5495) − 0.0070.0080.019 3.325 0.014 3.339 0.013 3.622 0.007 3.654 0.004 3.481 0.008 3.431 0.006 5.610 0.025 5.965 0.007 5.314 0.007 3.755 0.016 3.669 0.014 3.692 0.013 3.575 0.010 3.653 0.006 3.932 0.010 3.439 0.009 3.928 0.006 2.889 0.014 4.964 0.009 3.380 0.006 3.849 0.008 3.251 0.006 3.718 0.006 3.285 0.020 3.354 0.006 3.839 0.007 3.881 0.009 3.648 0.022 3.399 0.006 3.274 0.035 3.173 0.019 3.905 0.006 3.274 0.007 3.361 0.037 3.993 0.070 3.589 0.009 4.314 0.021 3.614 0.031 3.503 0.007 3.532 0.009 3.587 0.007 3.860 0.007 3.702 0.013 3.644 0.021 3.152 0.023 4.762 5.557 4.318 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± (mag) (mag) (mag) (pc) CCM F99 MA14 (4405 E 0101 101 101 J2GTS 101 J2GTS 0.269 101 J2GT 0.194 101 J2GT 1 0.721 01 J2GT 1 0.519 01 J2GTS 1 0.893 01 J2GTS 0.475 101 J2GTS 0.016 101 J2GTS 0.131 101 J2G 0.103 101 J2GTS 101 J2GTS 0.373 0.574 101 J2GT 0.507 101 J2GT 1 0.625 01 J2GT 1 0.439 01 J2GTS 1 0.554 01 J2GTS 0.716 101 J2GS 0.229 101 J2GS 1 0.214 01 J2GTS 1 0.012 01 J2GT 0.601 101 J2GTS 1 0.665 01 J2GTS 0.766 101 J2GTS 0.185 101 J2GTS 0.579 101 J2GTS 0.653 101 J2GT 0.468 101 J2GTS 1 0.654 01 J2GTS 0.306 101 J2GTS 0.678 101 J2GT 1.168 101 J2GTS 1 1.135 01 J2GT 0.355 101 J2GT 1 1.194 01 J2GTS 1 0.847 01 J2GTS 0.192 101 J2G 0.437 101 J2G 101 J2GTS 1.318 101 J2G 0.393 1.808 101 J2G 101 J2GTS 1.076 101 J2GTS 0.263 0.992 102 J2GTS 0.359 101 J2GTS 0.233 1 J2GT 0.240 1 J2G 0.406 J2G 0.428 0.424 01.28 02.32 04.40 01.94 00.50 04.02 05.37 00.71 00.36 00.30 02.02 01.33 04.18 00.64 03.77 01.25 01.52 00.47 01.95 02.18 01.95 01.63 03.37 00.25 04.45 00.25 00.58 00.24 02.37 01.89 01.90 02.13 00.05 00.07 00.99 00.17 00.17 00.18 27.10 05.54 00.44 00.14 00.10 00.45 04.71 01.36 01.38 00.70 − − − + − − − + + − − − + + − + + − − − − − − − + − − − − − − − − + + − − − − − + + − − − − − − 26 270426 271628 256135 2105 AaAbB GOS 243.14 GOS 243.82 GOS 253.64 GOS 245.45 47 2963 AB47 296249 2322 GOS 267.98 GOS 268.00 GOS 271.65 15 1909 GOS 232.31 − − − − − − − − Col GOS 237.29 Pup GOS 255.98 CMa AaAbE GOS 238.18 continued. NameHD 54 879 GOS 225.55 GOSSS ID NC SS HD 53 975ALS 207BD ALS 458HD 57 236µ 29 CMaτ CPD GOS 225.68 HD 64 568 GOSHD 231.49 64 315 ABCPD GOSCPD 234.28 GOS 235.64 HD 69 464CPD HD 68 GOS 450 237.82 HD 69 106ζ GOS 243.16 GOS 243.14 HD 75 222NX Vel ABHD 71 304HD 75 759 GOS AB 253.61 HD 76 341HD 75 211 GOS 254.47 LM Vel GOS 254.52 V467 VelHD 74 920 GOS 258.29 HD GOS 76 260.18 556CPD GOS 262.80 GOS 261.76 CPD HD 76 968 GOS 263.53 CPD GOS 263.96 HDE 298 429HD 89 GOS 137 264.04 GOS 265.20 HD 90 273 GOS 265.29 SS 215 GOS 267.58 THA 35-II-42HD 89 6252MASS J10224377-5930182 GOS 270.22 2MASS J10224096-5930305 GOS 274.47 GOS 285.06 HD 90 087 GOS 285.06 HD 91 572 GOS 279.69 HD 91 824 GOS 284.18 HD GOS 92 284.52 504HD 92 206 A GOS 284.33 HD 92 206 GOS B 284.81 HD 92 206 C GOS 285.16 GOS 285.52 GOS 285.70 GOS 285.92 GOS 286.22 GOS 286.22 GOS 286.22 Table 1.

10 J. Ma´ız Apellaniz´ and R. H. Barba:´ Optical-NIR dust extinction towards Galactic O stars 2 red χ 2 red χ 2 red χ d log 0.0170.019 3.60.018 3.40.025 0.29 3.30.021 1.81 3.50.075 0.70 1.28 3.50.018 1.73 1.69 3.40.026 0.56 1.43 1.94 3.40.033 1.11 5.32 0.11 3.30.058 0.46 1.33 0.75 3.40.020 1.29 0.01 1.14 3.30.042 1.27 1.83 0.49 3.50.019 0.03 0.83 0.28 3.40.047 0.79 1.36 2.20 3.30.022 1.06 0.91 3.52 3.30.032 0.15 2.70 3.04 3.30.018 0.66 2.80 0.15 3.30.017 1.18 5.26 0.46 3.30.016 2.20 0.63 2.22 3.00.017 1.80 1.99 1.60 3.4 24.200.018 1.01 2.37 3.40.015 0.56 1.98 4.79 3.4 2.38 0.019 1.94 0.69 3.20.022 0.80 2.79 1.60 3.20.018 0.45 0.61 1.44 3.50.025 0.64 1.20 1.07 3.20.020 0.26 1.16 0.97 3.3 14.100.017 0.69 2.13 3.30.024 0.66 3.33 0.90 3.3 1.15 0.018 2.28 1.92 3.30.086 0.82 0.99 1.34 3.30.020 0.99 1.56 3.31 3.40.016 0.56 1.84 3.33 3.20.023 0.77 2.39 3.28 3.40.018 0.67 3.14 3.68 3.40.016 1.84 2.81 5.40 3.50.017 1.77 2.46 0.39 3.40.017 2.26 6.31 1.81 3.30.019 1.22 0.57 2.28 3.40.019 1.12 2.66 5.30 3.30.016 0.38 1.78 0.40 3.10.019 1.16 4.18 3.77 3.30.017 0.56 2.67 3.87 3.20.019 0.80 3.07 4.21 3.30.020 0.20 4.27 3.74 3.50.024 0.95 4.91 6.96 3.50.018 0.89 4.66 2.73 3.30.020 1.61 8.88 1.36 3.2 1.52 2.21 1.06 3.3 1.36 2.52 2.17 1.88 2.83 1.42 0.67 7.09 0.86 2.32 2.00 0.79 0 , ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± J V J 0.0210.023 7.798 0.022 7.731 0.049 6.785 0.022 7.727 0.095 7.399 0.021 8.409 0.040 7.237 0.048 6.532 0.061 8.004 0.027 7.342 0.043 6.908 0.025 7.390 0.049 4.825 0.024 7.450 0.102 7.259 0.026 7.746 0.026 5.897 0.018 5.776 0.019 7.882 0.024 7.929 0.017 7.448 0.026 5.515 0.032 4.300 0.019 7.284 0.032 5.393 0.022 6.827 0.019 6.203 0.030 5.981 0.023 6.547 0.086 7.171 0.022 8.205 0.023 6.423 0.026 7.682 0.023 8.195 0.020 7.666 0.019 7.603 0.020 6.999 0.021 8.071 0.021 6.636 0.019 6.493 0.025 7.217 0.023 6.844 0.023 7.452 0.022 7.561 0.033 6.466 0.022 7.184 0.024 4.206 7.601 V ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± A 0.2370.256 1.050 0.190 0.791 0.254 1.344 0.087 2.336 0.464 1.324 0.119 1.732 0.210 2.356 0.257 2.243 0.123 2.724 0.183 3.624 0.185 1.476 0.118 1.900 0.221 2.199 0.129 2.454 0.612 2.376 0.227 3.442 0.121 1.951 0.061 2.038 0.151 2.785 0.131 0.874 0.080 1.523 0.148 1.850 0.157 3.769 0.069 2.435 0.177 1.879 0.117 1.611 0.086 1.539 0.123 2.083 0.116 1.572 0.221 1.607 0.080 1.256 0.073 2.193 0.141 3.123 0.307 1.920 0.085 1.053 0.059 2.137 0.162 2.993 0.068 1.960 0.063 2.579 0.078 2.726 0.112 2.341 0.073 2.437 0.255 3.378 0.170 0.837 0.194 2.021 0.187 2.064 0.234 2.106 1.347 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 5495 R 5495) − 0.0110.0090.008 4.125 0.023 3.830 0.007 4.772 0.042 4.235 0.013 3.169 0.019 3.571 0.030 4.149 0.014 4.194 0.013 4.258 0.012 4.567 0.010 3.820 0.014 4.625 0.012 4.234 0.068 5.349 0.016 4.444 0.011 5.853 0.007 4.650 0.007 3.801 0.010 4.172 0.006 3.638 0.015 3.629 0.016 4.025 0.006 5.386 0.010 4.191 0.007 3.684 0.008 4.141 0.010 4.335 0.009 4.114 0.007 3.407 0.007 3.675 0.009 3.003 0.013 4.280 0.012 4.132 0.008 3.938 0.007 4.559 0.013 4.001 0.007 4.304 0.007 4.440 0.007 4.230 0.009 4.237 0.009 4.417 0.008 4.522 0.011 4.329 0.018 4.065 0.008 5.041 0.012 4.054 6.122 4.459 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± (mag) (mag) (mag) (pc) CCM F99 MA14 (4405 E 0101 101 101 J2GT 101 J2GTS 1 0.252 01 J2GTS 0.204 101 J2GT 0.279 101 J2GTS 1 0.547 01 J2G 0.411 102 J2G 101 J2G 0.483 101 J2G 0.562 101 J2G 0.529 101 J2GT 0.635 201 J2GT 0.787 1 0.382 01 J2GS 1 0.407 01 J2G 1 0.514 02 J2GT 101 J2G 0.455 2 0.529 01 J2GT 101 J2GS 0.591 1 0.416 01 J2GS 1 0.530 01 J2GTS 1 0.661 01 J2GTS 0.236 101 J2GTS 0.414 101 J2GT 0.454 102 J2G 1 0.695 01 J2GTS 101 J2GS 0.503 0.575 101 J2GTS 1 0.385 01 J2GTS 0.351 101 J2GTS 0.501 101 J2GTS 0.455 101 J2GS 0.431 101 J2GTS 1 0.411 01 J2GS 0.507 101 J2G 1 0.748 01 J2GT 101 J2GTS 0.482 1 0.229 01 J2GS 0.528 101 J2G 1 0.689 02 J2GTS 101 J2GTS 0.603 0.437 101 J2GTS 0.637 101 J2GS 0.524 101 J2GS 1 0.534 01 J2GTS 1 0.773 01 J2GT 0.203 1 J2GT 1 0.397 J2GTS 0.504 J2GT 0.342 0.299 01.72 01.69 01.02 00.55 01.27 00.58 00.59 00.63 00.36 00.57 00.58 00.79 00.44 00.59 00.59 00.96 00.54 00.54 00.71 00.81 00.34 00.71 00.71 00.86 00.61 00.69 00.62 01.13 00.68 01.19 00.90 00.85 00.94 01.01 00.52 00.59 00.56 00.61 00.74 00.75 00.64 00.66 00.69 00.64 00.65 00.67 00.68 00.70 − − − − + − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − − 58 262758 261158 2620 GOS 287.39 GOS 287.39 GOS 287.41 58 2625 GOS 287.44 59 260059 259159 262759 263459 2626 AB59 2644 GOS 287.60 59 2641 GOS 287.60 59 2636 AB GOS 287.61 59 2635 GOS 287.63 GOS 287.62 59 2629 GOS 287.64 GOS 287.64 GOS 287.64 GOS 287.64 GOS 287.64 − − − − − − − − − − − − − − continued. NameHD 91 651 GOS 286.55 GOSSS ID NC SS HD 91 837HD 92 607HDE 303 312HD 94 024CPD CPD HD 93 128ALS 15 207ALS 15 204 GOS 286.72 HD 93 249 GOS GOS A 287.11 287.33 CPD HD 93 129 GOS AaAbB 287.34 Trumpler 14-9HDE 303 316 ATyc 8626-02506-1 GOS 287.40 HD GOS 93 287.40 160 ABHD GOS 93 287.40 161 AB GOS GOS 287.41 CPD 287.41 HDE 305 438 GOSHDE 287.41 303 311 GOS 287.42 GOSHD 287.41 93 250 ABHD 93 162HDE GOS 305 518 287.44 HD GOS 93 403 287.44 HD 93 204HD 93 205 GOS 287.44 HD 93 130 GOS 287.48 HDE GOS 303 308 287.51 ABV572 CarV573 Car GOS GOS 287.51 287.51 CPD CPD GOS 287.54 CPD GOS 287.57 HD GOS 93 027 287.59 GOS 287.57 CPD GOS 287.57 CPD CPD GOS 287.59 CPD GOS 287.60 CPD HD 93 343CPD CPD GOS 287.61 HD 93 028HDE 305 523HDE 305 524QZ Car AaAcHDE 305 536 GOS 287.64 GOS GOS 287.64 287.66 GOS 287.67 GOS 287.67 GOS 287.67 Table 1.

11 J. Ma´ız Apellaniz´ and R. H. Barba:´ Optical-NIR dust extinction towards Galactic O stars 2 red χ 2 red χ 2 red χ d log 0.0270.021 3.30.027 3.40.032 0.86 3.60.027 0.33 3.50.023 1.88 1.55 3.40.030 1.55 0.54 3.40.019 0.79 3.47 2.09 3.20.023 0.21 0.40 2.28 3.10.029 1.07 2.69 0.49 3.4 19.880.026 0.64 1.04 3.20.029 1.04 5.62 0.41 3.4 2.34 0.020 3.77 0.27 3.40.023 0.22 6.50 0.75 3.50.030 0.32 2.85 1.65 3.20.032 0.42 2.15 0.66 3.30.053 0.41 0.15 2.26 3.50.022 0.51 1.50 1.66 3.30.022 1.59 2.24 1.06 3.50.022 0.87 4.60 0.18 3.50.018 1.08 0.96 1.21 3.40.023 1.07 0.09 1.19 3.50.043 0.40 2.59 1.41 3.70.069 0.19 3.55 1.81 3.70.052 1.50 1.40 0.60 3.70.019 1.36 2.46 1.20 3.60.024 0.75 8.20 0.28 3.50.083 1.28 6.93 1.18 3.60.048 0.42 3.27 1.56 3.80.022 1.29 3.46 2.86 3.80.021 0.23 1.56 1.69 3.4 12.400.018 1.31 1.14 3.00.019 1.01 4.53 1.41 3.4 2.94 0.018 2.65 1.43 3.40.021 2.58 1.05 2.48 3.70.021 0.63 1.61 1.77 3.30.022 1.17 1.88 1.78 3.50.020 0.91 1.49 1.21 3.40.016 1.11 1.50 2.91 3.30.022 1.23 0.78 2.63 3.40.016 0.57 1.36 1.89 3.40.020 0.32 2.34 1.07 3.40.084 0.95 0.85 1.23 3.20.098 1.67 1.22 2.63 3.50.026 0.37 0.95 3.90 3.70.037 0.55 1.71 1.79 3.60.020 0.42 2.69 1.15 3.80.018 0.57 1.73 0.65 3.3 2.02 7.30 0.89 3.4 1.80 3.15 1.32 3.22 5.83 1.55 1.05 0.93 1.66 0.94 0.54 0.56 0 , ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± J V J 0.0290.031 6.905 0.046 8.103 0.049 8.602 0.036 7.639 0.037 7.918 0.049 6.896 0.021 6.360 0.040 6.265 0.051 7.216 0.044 6.373 0.033 7.050 0.025 7.659 0.025 7.638 0.043 6.650 0.038 7.472 0.066 5.655 0.028 7.720 0.031 5.689 0.023 6.384 0.020 6.172 0.042 5.733 0.057 8.716 0.071 6.785 0.072 7.388 0.023 8.153 0.040 6.339 0.097 6.159 0.066 7.152 0.023 7.275 0.024 7.396 0.020 5.911 0.021 6.841 0.020 6.824 0.022 7.821 0.026 6.609 0.023 6.768 0.021 7.589 0.018 5.568 0.023 7.704 0.018 6.521 0.026 6.803 0.111 6.726 0.104 7.886 0.044 7.114 0.044 7.562 0.021 8.973 0.019 7.323 6.672 V ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± A 0.2450.370 1.535 0.597 1.221 0.885 1.324 0.194 1.179 0.087 2.116 0.158 5.233 0.213 4.884 0.191 1.837 0.119 2.946 0.166 3.677 0.141 3.305 0.132 2.236 0.108 1.956 0.253 1.289 0.190 2.757 0.218 2.692 0.073 3.071 0.127 3.138 0.167 3.038 0.111 1.146 0.173 1.346 0.092 4.166 0.160 6.194 0.616 5.364 0.228 4.553 0.112 0.794 0.167 4.587 0.161 5.323 0.098 4.390 0.101 1.514 0.087 1.497 0.181 1.412 0.065 0.778 0.103 1.825 0.105 1.279 0.134 1.732 0.086 1.079 0.098 1.539 0.084 1.138 0.063 1.554 0.093 1.721 0.245 1.776 0.117 5.496 0.118 5.421 0.163 5.113 0.149 4.605 0.084 0.958 1.675 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 5495 R 5495) − 0.0110.0160.020 5.141 0.035 4.546 0.012 5.325 0.018 4.929 0.034 4.666 0.010 4.379 0.018 4.307 0.020 5.538 0.024 4.768 0.015 3.744 0.013 4.173 0.007 3.905 0.023 3.839 0.015 3.392 0.021 4.883 0.009 4.963 0.014 4.706 0.006 3.936 0.006 4.396 0.048 4.580 0.031 4.024 0.057 3.761 0.091 3.665 0.008 3.557 0.022 5.293 0.041 3.742 0.036 4.443 0.008 3.986 0.008 3.857 0.007 3.522 0.006 3.321 0.007 3.483 0.007 3.783 0.009 3.395 0.007 3.589 0.006 3.504 0.007 3.723 0.007 3.585 0.007 3.313 0.011 3.531 0.068 3.356 0.024 2.987 0.033 3.903 0.044 3.954 0.007 3.914 0.006 3.858 3.790 3.811 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± (mag) (mag) (mag) (pc) CCM F99 MA14 (4405 E 0102 103 101 J2GT 101 J2G 1 0.296 01 J2G 101 J2G 0.267 101 J2GS 0.248 101 J2G 0.242 1 0.449 01 J2G 101 J2GT 1.189 101 J2G 1.128 1 0.329 01 J2G 101 J2GT 0.613 101 J2G 0.974 1 0.786 01 J2GT 102 J2GS 0.566 1 0.503 01 J2G 1 0.374 01 J2GT 101 J2G 0.561 1 0.538 01 J2GTS 101 J2GT 0.790 0.648 101 J2GTS 1 0.685 01 J2GTS 0.247 101 J2G 0.330 101 J2G 101 J2G 1.101 101 J2G 1.687 101 J2GTS 1.504 101 J2GT 0.209 0.865 101 J2G 1 1.027 01 J2G 101 J2GTS 1.331 101 J2GTS 0.424 1.132 101 J2GTS 0.444 101 J2GTS 0.399 101 J2GTS 0.203 101 J2GTS 0.530 101 J2GTS 0.351 101 J2GTS 0.487 101 J2GTS 0.286 101 J2GTS 0.423 115 J2GTS 0.338 102 J2GTS 0.434 103 J2GTS 0.505 101 J2G 0.584 101 J2G 101 J2G 1.406 1 J2G 1.365 1 J2GTS 1.300 J2GTS 0.249 1.187 0.434 01.05 01.05 00.71 00.75 01.02 00.84 00.71 00.88 00.87 00.63 00.86 00.87 00.87 00.40 00.96 00.90 03.06 01.26 01.14 01.18 01.23 01.81 01.22 01.18 01.23 00.57 00.40 00.33 00.80 02.24 00.19 00.34 01.53 00.09 01.20 00.15 00.57 00.50 00.52 00.53 00.53 00.52 01.68 01.28 01.05 01.06 00.86 00.73 − − − − − − − − − + − − − + − − + − − − − − − − − + + + − − + − + + + − − − − − − − + − − − − − 59 255159 255459 2610 GOS 287.67 GOS 287.67 59 2673 GOS 287.70 GOS 287.84 − − − − continued. NameHD 93 146 A GOS 287.67 GOSSS ID NC SS CPD HD 93 146 BCPD CPD V662 Car[ARV2008] 206HD 93 222 ABHDE 305 532HDE 305 525 GOSCPD 287.67 HDE 305 539HD 93 GOS 576 287.71 HD 94 370 A GOS 287.71 HDE GOS 305 612 287.74 HD 93 632 GOS 287.78 ALS 18 083 GOS 287.79 HDE 303 492HDE 305 619 GOS 287.94 HD 93 843HD 96 917 GOS 287.98 GOSALS 288.01 18 551 GOS 288.03 2MASS J10584671-6105512ALS 18 553 GOS 288.03 GOS 289.74 2MASS GOS J10583238-6110565 288.03 GOS 288.05 HD 94 963 GOS 288.22 GOS 289.75 ALS 2063THA 35-II-153 GOS 288.24 ALS 18 556 GOS 289.28 HD GOS 96 289.73 622HD 96 670 ABHD GOS 96 289.74 715HD 96 264HD 95 589 GOS 289.76 GOS 289.79 HD 97 166 GOS 289.77 HD 96 946HD GOS 97 289.80 848HD GOS 97 253 290.20 GOS AB 290.09 HD 97 966HD 97 434 GOS 290.27 HD 97 319 GOS 290.40 EM Car GOS 290.51 NGC 3603 GOS HST-51 290.67 NGC 3603 GOS HST-48 290.73 NGC GOS 3603 290.79 GOS HST-24 290.74 NGC 3603 MTT 25HD 99 546 GOS 290.96 HD 99 897 GOS GOS 291.62 291.04 GOS GOS 291.62 291.12 GOS 291.62 GOS GOS 291.22 291.63 GOS 292.33 GOS 293.61 Table 1.

12 J. Ma´ız Apellaniz´ and R. H. Barba:´ Optical-NIR dust extinction towards Galactic O stars 2 red χ 2 red χ 2 red χ d log 0.0190.018 3.30.018 3.10.023 1.47 3.50.016 2.71 3.30.022 1.25 0.93 3.30.019 2.02 2.50 3.30.020 0.54 1.02 0.52 3.20.021 1.16 1.89 2.26 3.30.023 0.41 0.17 2.99 3.40.021 0.82 1.46 2.37 3.30.019 0.20 3.59 1.54 3.20.024 0.88 1.92 3.37 3.50.018 1.54 0.88 1.88 3.80.018 1.33 2.89 1.46 3.50.017 0.32 1.35 1.34 3.40.027 1.82 1.72 1.16 3.20.023 0.49 0.91 1.64 3.40.017 0.71 0.67 2.98 3.80.015 1.09 0.54 0.22 3.30.023 0.68 3.59 0.88 3.20.018 0.37 0.21 1.89 3.30.019 2.18 1.49 3.64 3.40.023 0.41 1.26 2.75 3.60.017 0.82 3.44 2.01 3.30.018 0.37 1.23 2.28 3.10.036 2.69 0.98 0.95 3.60.021 0.60 2.32 1.97 3.40.022 0.34 1.34 0.33 3.40.020 1.83 3.67 3.40 3.70.020 0.30 0.42 2.66 3.70.021 0.63 2.38 1.13 3.30.016 0.32 4.36 0.91 3.10.016 2.03 2.95 1.14 3.30.018 2.98 2.03 1.39 3.00.021 0.68 0.82 2.46 3.10.017 0.81 0.41 0.75 3.50.018 0.80 1.28 1.62 3.70.017 0.48 1.53 0.81 3.50.014 1.23 0.47 0.55 3.50.020 0.48 1.78 1.02 2.80.026 0.39 2.25 3.21 2.90.020 0.22 0.88 3.04 3.40.027 0.69 1.58 2.40 3.40.032 0.27 3.04 0.47 3.40.021 1.39 2.37 9.86 3.30.021 1.77 0.58 0.42 3.30.028 1.53 4.48 0.82 3.1 0.48 0.73 0.34 3.2 1.95 0.80 0.93 0.35 1.98 1.88 3.32 1.20 0.19 2.25 0.41 1.16 0 , ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± J V J 0.0210.019 6.081 0.025 5.908 0.024 8.128 0.022 7.096 0.023 7.065 0.021 7.243 0.022 5.187 0.022 5.329 0.027 6.657 0.024 7.042 0.026 6.283 0.033 7.642 0.020 7.153 0.020 6.996 0.021 7.689 0.031 4.264 0.027 6.994 0.019 7.814 0.017 6.327 0.024 5.463 0.021 6.164 0.022 6.313 0.027 7.665 0.022 5.708 0.021 6.278 0.042 7.821 0.036 6.667 0.035 7.526 0.024 6.747 0.024 6.618 0.023 7.462 0.020 6.080 0.021 6.752 0.021 4.957 0.024 6.177 0.021 5.676 0.023 6.425 0.019 7.291 0.015 6.909 0.023 4.175 0.044 4.585 0.028 7.267 0.044 5.285 0.050 7.867 0.024 4.856 0.033 7.470 0.044 6.515 6.980 V ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± A 0.1060.112 1.230 0.212 1.228 0.083 1.155 0.163 1.592 0.110 1.323 0.125 1.245 0.146 1.271 0.107 1.023 0.123 1.410 0.125 1.268 0.187 1.302 0.133 1.587 0.118 2.086 0.077 1.148 0.191 1.638 0.268 1.097 0.518 1.036 0.077 0.666 0.089 1.667 0.078 1.400 0.090 1.794 0.076 1.663 0.149 2.011 0.081 1.712 0.168 2.916 0.251 1.082 0.150 1.871 0.198 3.149 0.094 1.833 0.104 2.210 0.101 2.003 0.116 1.543 0.134 1.414 0.086 1.691 0.073 1.750 0.052 2.602 0.146 3.089 0.051 1.247 0.175 2.346 0.232 0.898 0.086 0.848 0.028 5.176 0.065 5.619 0.088 5.022 0.078 7.079 0.074 2.940 0.098 3.488 3.012 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 5495 R 5495) − 0.0060.0060.011 3.645 0.007 3.796 0.010 3.861 0.007 3.439 0.007 3.846 0.007 3.634 0.007 3.899 0.008 3.698 0.008 3.918 0.013 3.318 0.012 3.621 0.006 3.977 0.007 3.805 0.009 3.704 0.012 3.521 0.013 4.066 0.006 4.004 0.006 4.479 0.006 3.630 0.008 3.675 0.012 3.616 0.012 3.470 0.012 3.076 0.010 3.908 0.015 3.881 0.031 3.584 0.018 4.640 0.011 3.497 0.012 3.775 0.007 3.648 0.008 3.471 0.013 3.638 0.008 3.617 0.011 3.651 0.012 3.546 0.009 3.561 0.007 3.093 0.006 3.593 0.008 3.437 0.039 4.259 0.010 4.115 0.024 3.107 0.072 2.962 0.013 3.071 0.017 2.806 0.018 3.653 3.221 3.145 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± (mag) (mag) (mag) (pc) CCM F99 MA14 (4405 E 0101 101 101 J2GTS 101 J2GTS 0.333 101 J2GS 0.319 101 J2GTS 1 0.295 01 J2GTS 0.456 101 J2GTS 0.340 101 J2GS 0.338 101 J2GTS 1 0.322 01 J2GTS 0.273 101 J2GTS 0.355 101 J2GTS 0.376 101 J2GT 0.354 101 J2GS 1 0.394 01 J2GTS 1 0.542 01 J2GTS 0.305 101 J2GTS 0.458 101 J2GT 0.267 101 J2GT 1 0.256 01 J2GTS 1 0.148 01 J2GTS 0.453 101 J2GTS 0.376 101 J2GTS 0.489 101 J2GT 0.472 101 J2GT 1 0.644 01 J2GT 1 0.433 01 J2GT 1 0.743 01 J2GT 1 0.298 01 J2GT 1 0.400 01 J2G 1 0.892 01 J2GTS 101 J2GT 0.599 0.480 101 J2GTS 2 0.569 01 J2GTS 0.418 101 J2GT 0.385 101 J2GTS 1 0.457 01 J2GT 0.487 101 J2GT 1 0.723 01 J2GTS 1 0.988 01 J2GTS 0.342 101 J2GTS 0.674 101 J2GTS 0.208 101 J2G 0.203 101 J2GTS 101 J2G 1.896 1.660 101 J2G 1 J2GT 1.628 1 J2GT 2.541 0.796 J2GT 1.072 0.947 01.49 01.62 01.61 01.49 04.14 01.68 01.65 00.81 01.69 01.71 01.71 00.45 04.02 00.10 02.20 06.03 02.26 16.13 00.82 00.83 00.15 00.22 02.80 00.87 00.04 01.66 01.22 03.42 02.80 00.54 01.00 01.03 00.43 00.03 00.14 09.08 02.77 02.91 02.64 01.16 00.74 00.77 00.62 00.77 01.58 00.20 00.08 00.83 − − − − − − − − − − − + + − + + − − − − + + + − + + − + − − − + − − + + + − − − + + − + + − − − 61 3973 GOS 309.13 59 5634 GOS 315.37 54 6791 AB GOS 327.56 − − − Cir AaAbAc GOS 319.69 continued. NameHD 101 190 AaAb GOS 294.78 GOSSS ID NC SS HD 101 131 ABHDE 308 813HD 101 223TU MusHD 101 191HD 101 205 ABHD 101 545 AaAb GOS 294.78 HD 101 298HD 101 413 GOS AB 294.79 HD 101 436 GOSHD 294.81 102 415HD 104 565 GOS GOS 294.85 GOSHD 294.88 GOS 294.84 105 627 294.81 HD 110 360HD 112 244 GOS 295.03 GOSHD 294.94 113 659HD 116 852 GOSHD 295.04 114 737 AB GOSHD 295.30 114 886 AaAbB GOSHD 296.51 115 071 GOSHD 298.15 115 455 GOSHD 301.80 116 282 GOSHD 303.55 117 856 GOSHD 304.52 117 GOS 797 305.52 GOS 305.41 GOSHD 304.88 117 490HD 118 198CPD GOSHD 305.76 120 521 GOSHD 306.06 123 008 GOSHD 307.01 122 313 GOSHD 307.77 123 590 AB GOSHD 307.86 123 056 GOSHD 307.88 124 314 AaAbBaBb GOSHD 307.96 125 206 ABHD 125 241 GOSCPD 310.73 GOS GOSHD 312.67 311.02 124 979 GOS 311.95 GOSHD 311.18 130 298δ GOSHD 312.17 135 591 GOS 313.45 ALS 17 591ALS 3386 GOSALS 313.54 18 049Muzzio III-9 GOSCPD 316.40 GOSHD 318.77 145 217HD 144 647 GOS 320.13 GOS 320.32 GOS 326.31 GOS 326.73 GOS 327.39 GOS 332.29 GOS 332.45 Table 1.

13 J. Ma´ız Apellaniz´ and R. H. Barba:´ Optical-NIR dust extinction towards Galactic O stars 2 red χ 2 red χ 2 red χ d log 0.0170.019 2.80.016 2.90.021 4.04 2.90.019 2.47 3.20.017 4.76 3.33 3.70.024 1.46 1.09 3.40.017 1.79 3.27 6.37 3.60.016 0.36 1.75 2.42 3.30.009 1.49 4.71 0.21 3.40.017 1.22 1.59 3.16 3.10.218 1.43 1.13 2.02 3.50.036 1.29 4.24 2.44 3.00.018 0.16 1.91 2.45 3.30.018 1.69 4.50 2.33 3.30.023 0.97 1.61 0.27 3.40.017 2.27 1.84 1.12 3.70.023 0.40 0.39 0.89 3.20.022 1.05 1.49 1.23 3.30.021 0.32 3.12 0.95 3.40.019 0.47 2.32 2.05 3.40.023 0.36 1.08 1.25 3.20.018 1.15 1.71 3.48 3.50.032 0.36 1.02 4.42 3.30.025 0.69 2.49 1.05 3.30.018 0.47 3.45 1.75 3.20.021 1.86 1.64 1.13 3.30.023 1.83 1.96 0.31 3.20.023 0.45 0.59 2.33 3.20.022 0.93 0.70 2.08 3.20.023 0.26 3.26 2.20 3.20.021 0.08 1.19 2.46 3.20.018 1.00 2.61 2.69 3.10.020 0.65 2.72 0.82 3.20.023 1.70 0.89 1.48 3.10.048 1.09 2.92 3.60 3.20.021 0.82 0.44 3.60 3.10.033 0.50 3.03 1.94 3.10.015 0.21 2.58 1.72 3.10.019 1.00 0.88 1.40 3.40.020 1.83 1.58 3.74 2.90.020 0.25 0.54 0.87 3.20.019 0.92 3.89 2.61 3.20.024 0.10 1.21 1.83 3.20.019 2.66 2.10 1.86 3.20.017 0.84 1.73 6.00 3.10.020 0.86 1.10 4.40 3.20.020 1.26 4.42 1.08 3.3 0.34 2.97 2.40 3.2 1.12 4.02 0.37 0.54 1.50 2.52 1.72 0.95 0.51 6.25 0.29 1.72 0 , ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± J V J 0.0190.021 4.455 0.025 5.084 0.024 3.858 0.026 6.071 0.019 6.522 0.034 6.227 0.018 7.045 0.018 4.995 0.013 6.751 0.019 3.743 0.217 5.802 0.055 3.293 0.020 7.337 0.020 4.715 0.027 5.230 0.019 6.785 0.024 5.354 0.025 5.561 0.025 5.137 0.020 4.887 0.025 3.930 0.021 5.707 0.036 6.078 0.028 7.055 0.026 7.184 0.027 4.680 0.032 6.276 0.026 7.331 0.031 4.448 0.037 6.489 0.032 4.999 0.027 5.754 0.024 5.997 0.024 5.958 0.048 5.491 0.022 3.866 0.034 5.698 0.017 3.903 0.022 5.796 0.025 5.185 0.024 5.637 0.030 6.770 0.035 6.306 0.022 5.671 0.018 5.670 0.024 5.789 0.024 5.907 4.364 V ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± A 0.0560.089 2.275 0.152 1.797 0.068 1.787 0.048 2.994 0.051 3.222 0.155 2.257 0.057 2.158 0.071 2.297 0.140 1.745 0.041 1.166 0.346 2.914 0.104 2.175 0.085 3.674 0.067 2.204 0.073 2.898 0.119 2.498 0.091 1.703 0.072 1.600 0.083 2.110 0.055 2.080 0.104 2.372 0.106 2.011 0.150 1.467 0.179 1.364 0.142 1.536 0.122 1.787 0.166 1.614 0.128 1.337 0.131 1.593 0.278 1.844 0.148 1.577 0.138 1.810 0.100 1.876 0.084 1.610 0.182 1.681 0.087 1.362 0.125 1.619 0.096 1.883 0.123 1.403 0.144 1.508 0.120 1.597 0.048 1.673 0.071 4.250 0.068 4.371 0.066 2.595 0.163 1.721 0.133 1.561 2.156 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 5495 R 5495) − 0.0060.0070.012 3.629 0.013 3.967 0.010 3.984 0.007 3.483 0.020 3.051 0.006 3.335 0.007 3.494 0.007 3.706 0.007 3.496 0.007 4.036 0.028 3.352 0.010 3.451 0.011 3.023 0.012 3.673 0.010 3.702 0.006 3.211 0.008 3.862 0.009 3.734 0.006 3.389 0.010 3.416 0.008 3.687 0.010 3.852 0.014 3.478 0.011 3.471 0.009 3.853 0.012 3.947 0.009 3.608 0.011 3.275 0.018 3.903 0.013 3.542 0.012 4.077 0.008 3.579 0.007 3.757 0.006 3.492 0.006 3.643 0.007 4.242 0.006 3.729 0.008 4.501 0.009 3.805 0.008 3.979 0.011 4.131 0.013 3.946 0.011 3.307 0.006 3.905 0.012 3.388 0.012 3.380 3.928 4.273 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± (mag) (mag) (mag) (pc) CCM F99 MA14 (4405 E 0101 102 101 J2GTS 101 J2GS 0.619 101 J2GS 1 0.447 01 J2GT 1 0.443 01 J2GTS 1 0.850 01 J2GTS 1.045 101 J2G 0.668 101 J2GTS 101 J2GTS 0.613 0.610 101 J2GTS 0.492 101 J2GTS 0.285 101 J2GTS 0.860 101 J2G 0.621 101 J2GT 101 J2GT 1.204 1 0.593 01 J2GT 1 0.775 01 J2GT 1 0.768 01 J2GTS 1 0.436 01 J2GTS 0.423 101 J2GTS 0.615 101 J2GTS 0.601 101 J2GT 0.636 101 J2GTS 1 0.516 01 J2GS 0.416 102 J2G 1 0.387 01 J2GS 101 J2GS 0.394 1 0.448 01 J2GS 1 0.441 01 J2GTS 1 0.402 01 J2GS 0.403 101 J2G 1 0.514 01 J2GS 101 J2GS 0.383 1 0.499 01 J2GTS 1 0.493 01 J2GTS 0.454 101 J2GTS 0.455 101 J2GTS 0.317 101 J2GTS 0.428 101 J2GTS 0.414 101 J2GTS 0.364 101 J2GTS 0.374 101 J2GTS 0.382 101 J2GTS 0.419 101 J2GS 1.276 101 J2GT 1 1.112 J2GTS 1 0.756 J2GT 0.502 J2GT 0.392 0.499 00.22 01.57 01.57 00.03 02.85 01.13 01.15 00.20 00.41 03.01 01.05 04.22 00.94 04.39 02.41 01.70 01.10 01.41 00.89 07.15 01.18 01.22 01.16 01.17 01.18 01.15 01.22 01.14 01.14 01.28 01.30 01.94 01.67 01.49 01.89 01.61 01.61 01.83 00.30 01.47 02.61 03.08 02.38 02.17 01.14 02.51 01.17 01.17 − − − + + − − − − + − − − − + + + + + + + + + + + + + + + + + + + + + + + + − + − − + + − + + + 46 8221 GOS 338.56 41 7721 A41 7733 GOS 343.44 GOS 343.46 − − − Nor GOS 339.38 continued. NameHD 148 937 AaAb GOS 336.37 GOSSS ID NC SS HD 150 135 AaAbHD 150 136 AaAbHD 149 452HDE 328 209 ABHDE 328 856CPD GOSHD 336.71 150 958 AB GOSHD 336.71 150 574µ HD GOS 151 018 338.49 GOSHD 337.47 149 404HDE 329 100 GOS A 338.56 HD 154 811 GOS 338.56 HD 152 386HD 155 756 GOSHD 339.00 151 003 ABHD 151 515 GOSHD 339.51 152 147 GOS GOSHD 340.86 340.54 152 003HD 152 424 GOSHD 341.06 148 546 GOSHD 341.11 152 219 GOS 342.72 GOSHD 342.57 152 200CPD GOSHD 342.81 152 249 GOSCPD 343.15 GOSHDE 343.33 326 329 GOSHD 343.36 152 248 AaAb GOSV1034 343.38 Sco GOSHD 343.39 152 233 AaAb GOSHDE 343.41 326 331HD 152 314 AaAb GOSHD 343.45 152 218 GOSHD 343.46 152 247 GOS AaAb 343.46 HD 151 804 GOSHD 343.48 152 246 AaAbHD GOS 152 408 343.48 GOSHD 343.52 152 405 GOS 343.49 HD 152 623 AaAbB GOSHD 343.61 152 723 AaAb GOSHD 343.53 152 590 GOSHDE 344.03 326 775 GOSHDE 343.62 322 417HD 155 GOS 913 344.62 GOSHD 344.08 156 292 GOS GOSHD 344.81 344.56 153 426HD 153 919 GOS 344.84 GOS 345.01 GOS 345.26 GOS 345.29 GOS 345.35 GOS 347.14 GOS 347.75 Table 1.

14 J. Ma´ız Apellaniz´ and R. H. Barba:´ Optical-NIR dust extinction towards Galactic O stars 2 red χ 2 red χ 2 red χ d log 0.0300.022 3.60.031 3.50.056 0.61 3.50.036 1.05 3.50.025 0.48 1.19 3.50.027 0.75 0.89 3.20.020 1.09 0.26 0.64 3.40.015 1.93 0.89 1.33 3.50.025 1.37 0.24 1.68 3.20.020 1.21 0.74 0.43 3.40.018 0.89 3.92 0.76 3.10.024 2.95 1.58 1.93 3.10.022 1.74 0.57 2.87 3.20.054 1.29 1.63 1.32 3.00.019 0.38 1.76 0.70 3.10.020 2.47 1.05 0.47 3.20.025 0.56 4.07 0.12 3.40.024 0.80 4.75 1.64 3.20.021 0.74 2.56 2.93 3.30.021 0.69 2.68 1.15 3.30.039 0.28 1.97 0.73 3.00.026 0.95 1.10 0.10 3.10.030 0.60 1.57 0.57 3.20.024 0.34 0.41 0.22 3.20.023 0.24 0.75 0.21 3.00.056 0.12 0.28 0.34 3.20.025 0.21 2.53 0.70 3.20.024 0.54 2.04 0.46 3.2 13.430.038 0.79 0.55 3.20.027 0.53 3.65 0.39 3.2 1.57 0.016 1.08 0.22 3.20.014 0.10 5.60 0.22 3.60.026 0.06 5.38 0.67 2.80.017 0.14 3.11 1.62 2.9 0.75 3.53 1.76 3.1 0.51 3.25 0.66 1.40 1.71 3.05 1.61 0.50 0.40 2.03 0.32 1.50 0 , ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± J V J 0.0490.039 6.787 0.075 8.034 0.071 8.680 0.054 8.202 0.045 6.798 0.070 7.527 0.040 5.304 0.018 5.807 0.057 5.811 0.021 5.040 0.021 3.616 0.048 5.456 0.041 6.993 0.093 5.458 0.021 5.852 0.022 5.337 0.037 5.088 0.042 5.906 0.023 5.951 0.023 6.335 0.099 5.582 0.043 5.941 0.047 6.189 0.043 6.712 0.041 4.799 0.077 7.109 0.042 6.813 0.045 6.025 0.053 4.193 0.046 5.276 0.018 6.133 0.016 6.119 0.028 4.451 0.020 5.830 6.067 V ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± A 0.0620.056 6.356 0.091 5.309 0.079 5.236 0.075 5.678 0.066 5.678 0.098 5.475 0.053 5.841 0.198 5.847 0.095 0.889 0.052 4.599 0.092 2.517 0.117 1.703 0.090 5.731 0.141 5.174 0.047 5.399 0.042 4.022 0.099 2.958 0.080 3.670 0.333 4.178 0.234 0.673 0.139 0.970 0.079 5.683 0.084 5.723 0.052 5.585 0.071 7.030 0.133 5.866 0.075 5.960 0.055 5.920 0.069 6.250 0.064 6.290 0.196 6.634 0.119 1.191 0.195 1.241 0.107 1.161 1.852 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 5495 R 5495) − 0.0340.0250.043 3.026 0.029 2.967 0.034 2.738 0.035 3.120 0.044 3.078 0.024 2.920 0.010 3.165 0.030 3.118 0.007 3.609 0.010 3.105 0.054 3.393 0.032 3.309 0.038 3.511 0.013 3.491 0.006 3.459 0.025 3.394 0.018 3.448 0.010 3.312 0.010 3.438 0.098 3.942 0.038 4.050 0.034 2.811 0.026 3.215 0.033 3.362 0.058 3.164 0.033 3.245 0.019 3.452 0.029 3.389 0.024 3.442 0.010 3.289 0.007 3.619 0.010 4.252 0.011 3.905 3.936 3.682 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± (mag) (mag) (mag) (pc) CCM F99 MA14 (4405 E 0101 101 102 J2G 101 J2G 102 J2G 2.103 101 J2G 1.783 101 J2G 1.909 101 J2G 1.815 101 J2GT 1.842 101 J2G 1.871 1 1.844 01 J2GT 101 J2G 1.874 1 0.243 01 J2GTS 101 J2GT 0.733 1.474 101 J2G 1 0.507 01 J2G 101 J2G 1.629 101 J2GT 1.476 101 J2GTS 1.555 1 1.175 01 J2GT 0.849 101 J2GT 1 1.098 01 J2GT 1 1.206 01 J2GT 1 0.169 01 J2GT 1 0.237 01 J2G 1 2.024 01 J2G 101 J2G 1.776 101 J2G 1.656 102 J2G 2.227 101 J2G 1.804 101 J2G 1.724 101 J2G 1.743 101 J2G 1.814 101 J2GT 1.912 1 J2GS 1.832 1 0.277 J2GT 0.314 J2GT 0.291 0.496 00.79 00.79 00.79 00.79 00.80 00.80 00.80 00.78 00.15 00.67 03.22 03.19 00.64 00.65 00.66 00.48 01.36 00.92 00.61 05.85 02.67 02.11 00.64 00.91 00.65 00.90 00.88 00.89 00.89 00.89 00.91 06.10 00.05 01.60 01.88 − − − − − − − − + − + + + + + + + + + − + + + + + + + + + + + − + + − continued. NameALS 18 769 GOS 348.67 GOSSS ID NC SS ALS 18 770ALS 18 771ALS 18 768ALS 18 748ALS 18 767ALS 4067 ABALS 18 747HD 155 775HDE GOS 323 348.71 110HD GOS 154 348.72 368HD GOS 154 348.72 643HDE GOS 319 348.72 703 DHDE GOS 319 GOS 348.72 703 348.73 AHDE 319 703 BaBbHD GOS 156 348.76 738 AB GOSHD 348.80 156 154 GOS 349.65 HDE 319 699 GOSHDE 349.97 319 702 GOS GOSHD 351.03 350.54 161 807 GOS 351.03 GOSHD 351.03 155 889 ABTyc 7370-00460-1 GOS 351.18 ALS 19 693Pismis 24-15 GOSALS 351.22 19 692 GOS 351.32 Pismis 24-10 GOS 351.35 Pismis 24-3 GOS 352.57 GOS 352.50 GOSPismis 351.78 24-2Pismis 24-1 ABPismis 24-17Pismis GOS 24-13 353.07 GOS 353.10 HD 163 758HD GOS 159 353.11 176 GOS 353.13 HD 158 186 GOSHD 353.15 161 853 GOS 353.17 GOS 353.16 GOS 353.17 GOS 353.20 GOS 355.36 GOS 355.67 GOS 355.91 GOS 358.42 Table 1.

15 J. Ma´ız Apellaniz´ and R. H. Barba:´ Optical-NIR dust extinction towards Galactic O stars

The results from the CHORIZOS runs are given in Table 1, 3. Results also available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via 3.1. Comparing extinction-law families http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/. We begin our analysis by comparing the results obtained The spectroscopic logarithmic distances (log d) are given with with the CCM, F99, and MA14 families of extinction laws 7. In only one digit after the decimal point as that is their expected Fig. 2 we plot χ2 as a function of E(4405 − 5495) for each fam- 6 red level of precision . In the next section we discuss the scientific 2 ily. Figure 3 shows two similar plots but with the χred differences results. Prior to that, we comment on some issues regarding between MA14 and CCM (left) / F99 (right) on the vertical axis. the methodology. The reader is referred to the Appendices for 2 Figure 4 shows the χred histograms for the three families com- explanations of why we have used monochromatic quantities pared with the expected combined distribution, built from the instead of filter-integrated ones to characterize extinction and sum of the distributions for each star, each one with its d.o.f. on some of the numerical consequences of using multifilter The most important result of the comparison is that the photometry to determine extinction properties. MA14 family provides much better results than either CCM or The quantities listed in Table 1 are the direct output of 2 F99. For MA14 all stars have χred < 4.0 while for the other CHORIZOS and the uncertainties should be taken as the random two families there is a long tail that extends to values above ones. They do not include systematic uncertainties, of which ten. The MA14 results show no strong trends as a function of there are three possible origins: 2 E(4405 − 5495) or differences between subsamples. Its χred histogram has a cutoff at the expected location but is more heav- – Photometric. Here we have possible zero-point errors and ily populated in the 1.5-3.0 region than the expected distribu- discrepancies between different photometric sources. As tion. This could be caused either by systematic errors in the in- previously mentioned, we have minimized those prob- put photometry (for example, due to variability) or by the need lems by analyzing the zero points in different systems to fine-tune the extinction laws. However, that issue appears to (Ma´ız Apellaniz´ 2005, 2006, 2007) and by assigning qual- be a minor effect for low-intermediate extinction values, so we ity flags (and associated uncertainties) to different sources. can conclude that the MA14 family of extinction laws pro- – SED. In principle, the optical-NIR SEDs of O stars are well vides the best description of Galactic optical extinction for known and depend little on metallicity. However, one should E(4405 − 5495) < 3.0 available to date. This statement is true be careful not to include or to correct for those cases where even though the MA14 laws were derived using 30 Doradus, not the SED is anomalous. We have done this by excluding Oe Galactic, data; such validity was already noted in the MA14 pa- stars from our sample and by correcting for NIR excesses per itself for a very limited sample of Galactic stars. We also note when present. that we had already reached this conclusion when we presented – Extinction laws. One can obtain results assuming any extinc- the preliminary results of this paper at two scientific conferences tion law. However, that does not mean that the used extinc- (Ma´ız Apellaniz´ 2015b; Ma´ız Apellaniz´ et al. 2017b). tion law is the correct one. If it is not, a systematic error is Why does the MA14 family work better than either CCM likely to be introduced in the process. In the next section we or F99? In the case of CCM there are two culprits. the use of discuss how we have checked this. a seventh-degree polynomial in 1/λ for wavelength interpola- tion (Ma´ız Apellaniz´ 2013a) and the behavior for the U band (see MA14). The first effect is clearly seen in the left panels Regarding systematic uncertainties, we specifically point out of Figs. 2 and 3: for a given value of E(4405 − 5495) the typ- the differences in the uncertainties in VJ,0 as calculated by two 2 ical χred is worse when Stromgren¨ photometry (which has fil- possible methods described in Appendix C. The uncertainties in ters intercalated between U and B and between B and V) is Table 1 are calculated using the whole CHORIZOS likelihood present than when is not. The F99 results are significantly worse grid (second method in Appendix C). As a consequence, the un- than for MA14. This is especially true for large values of R , certainties in V are smaller than those in A . For log d we 5495 J,0 VJ which are the worst offenders for E(4405 − 5495) < 1.5 (as ex- only give two significant figures and no random uncertainties, as plained below, our sample does not contain objects with both there are two large sources of systematic uncertainties: the intrin- large E(4405 − 5495) and R5495). sic width of the luminosity class-luminosity relationship and the It is interesting to compare our results with those of Schlafly non-inclusion of the existence of multiple systems (astrometric et al. (2016). Those authors find that the MA14 family provides or spectroscopic) in the photometric data. The reader is referred a good description of the mean optical extinction curve and of to Ma´ız Apellaniz´ et al. (2015a,b) for an example of how those its variation with R5495. However, they find that the quality of issues complicate the use of spectroscopic parallaxes for O stars. the MA14 fit becomes poorer in the NIR. The explanation for We will revisit this issue in future papers once accurate Gaia par- the latter effect is that their photometry has a better coverage for allaxes are available for a large fraction of the sample. wavelengths longer than 6000 Å (our optical photometry only Figure 1 shows the dependence of the R5495 uncertainties has the redwards portion of Gaia G in that range) and an average from the CHORIZOS runs on E(4405 − 5495). For the J2 sam- reddening larger than the one in our sample: the MA14 deriva- ple the values agree reasonably well with the predictions of tion assumed the Rieke & Lebofsky (1985) power-law form in Eq. D.2. The results for the other subsamples indicate that it is the NIR with α = −1.61 because of the impossibility of using the possible to reduce the random uncertainty of R5495 by including relatively low-extinction 30 Dor stars to measure α. This is an il- additional filters in the analysis, that is, the more (good) data, the lustration of a chronic problem in extinction studies: a strong better, as expected. extinction effect in the UV becomes a weak one in the optical and an undetectable one in the IR while a star with a strong ef- 6 Gaia is expected to provide more accurate trigonometric distances to most of the sample in this paper in a short time scale but we note that 7 We refer the reader to Appendix E for an explanation of why we the extinction parameters derived in this paper such as E(4405 − 5495) did not include Fitzpatrick & Massa (2009) in our list of comparison and R5495 are only very weakly dependent on distance. families of extinction laws.

16 J. Ma´ız Apellaniz´ and R. H. Barba:´ Optical-NIR dust extinction towards Galactic O stars

Jo+2M+Ga Jo+2M+Ga

25 Jo+2M+Ga+Ty 25 Jo+2M+Ga+Ty

Jo+2M+Ga+St Jo+2M+Ga+St

Jo+2M+Ga+Ty+St Jo+2M+Ga+Ty+St

R ≥ 4 R ≥ 4 20 5495 20 5495

15 15 (F99) (CCM) red red 2 2 χ χ

10 10

5 5

0 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 E(4405−5495) E(4405−5495)

Jo+2M+Ga

25 Jo+2M+Ga+Ty

Jo+2M+Ga+St

Jo+2M+Ga+Ty+St

R ≥ 4 20 5495

15 (MA14) red 2 χ

0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 E(4405−5495)

2 Fig. 2. χred as a function of E(4405 − 5495) for the CHORIZOS runs in this paper using the CCM (upper left), F99 (upper right), and MA14 (bottom) families of extinction laws. The color coding is the same as in Fig. 1. In addition, objects with R5495 ≥ 4 are indicated with a black star symbol. In all cases, the E(4405 − 5495) and R5495 values are the MA14 results. Both axes scales are the same in the three plots to allow for a better comparison. fect in the IR is hard to observe in the optical and impossible 3.2. Quantitative reddening differences between O-type and in the UV. As a result, one is forced to build UV-optical-IR ex- late-type stars tinction laws by stitching together results for stars of different amount of extinction. In the specific case of MA14, that paper Older studies of the spatial distribution of extinction (e.g., specifically mentions this is an expected issue and recommends Fitzgerald 1968; Neckel et al. 1980) were based on the analy- not using its results for large NIR extinctions: that is also why sis of early-type stars. Such studies had systematic uncertainties we indicate above that the results of this paper apply to the op- due to some of the issues discussed in this paper (photometric bi- tical for E(4405 − 5495) < 3.0. For a more detailed discussion ases, extinction law errors, spectral type mismatches) and were of the different wavelength regimes, see Ma´ız Apellaniz´ (2015a) limited by small samples, as early-type stars are relatively scarce and Ma´ız Apellaniz´ et al. (2017b) and the last subsection below. and fewer of them had accurate spectral types at the time of those studies compared to today. More recent studies have taken ad-

17 J. Ma´ız Apellaniz´ and R. H. Barba:´ Optical-NIR dust extinction towards Galactic O stars

25 25

Jo+2M+Ga Jo+2M+Ga

Jo+2M+Ga+Ty Jo+2M+Ga+Ty

20 Jo+2M+Ga+St 20 Jo+2M+Ga+St

Jo+2M+Ga+Ty+St Jo+2M+Ga+Ty+St

R ≥ 4 R ≥ 4 5495 5495 15 15 (MA14) (MA14) red red 2 2 χ χ 10 10 (F99) − (CCM) − red red 2 2 χ χ 5 5

0 0

0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 E(4405−5495) E(4405−5495)

2 Fig. 3. χred diﬀerence between CCM (left) / F99 (right) and MA14 as a function of E(4405 − 5495) for the CHORIZOS runs in this paper. The color coding and symbols are the same as in Fig. 2. In all cases, the E(4405 − 5495) and R5495 values are the MA14 results. Both axes scales are the same in the two plots to allow for a better comparison.

1.0 200.0 MA14 100.0 0.5 CCM 50.0 F99 0.0 20.0 expected −0.5 10.0 E(4405−5495) −1.0 5.0 Number

(G15) − −1.5 2.0

T = 3500 K E(B−V) eff 1.0 −2.0 T = 7000 K 0.5 eff −2.5 T = 30 000 K eff 0.2 −3.0 0.1 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.0 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0 22.5 25.0 E(4405−5495) 2 χred

2 Fig. 4. χred histograms for the CHORIZOS runs in this paper Fig. 5. Comparison between the reddenings measured by G15 using the MA14, CCM, and F99 families of extinction laws. We and those in this paper. The error bars include the uncertainty in also plot the expected combined distribution. We note that the log d (G15 gives E(B − V) as a function of distance), assumed orange region represents the superposition of the yellow (MA14) to be 0.1 (corresponding to an uncertainty of 0.5 in the distance and red (CCM) histograms. modulus). The three colored lines show the expected difference between E(B − V) and E(4405 − 5495) for three different temperatures assuming an MA14 extinction law with R5495 = 3.0. vantage of larger photometric databases and concentrate on the more numerous late-type stars (Marshall et al. 2006; Lallement et al. 2014; Sale et al. 2014; Green et al. 2015). The use of late- for early-type studies because extinction may not affect different type stars allows for a much better spatial sampling that is al- spectral types in the same manner (statistically speaking). Given lowed with early-type stars but that does not eliminate the need the young age of O stars, they are expected to be located prefer-

18 J. Ma´ız Apellaniz´ and R. H. Barba:´ Optical-NIR dust extinction towards Galactic O stars

5 1.50 −2.3 0.1 4 1.25 3

2 1.00

1 b

b 0.75 0

−1 0.50

−2 0.25 −3

−4 0.00 87 86 85 84 83 82 81 80 79 78 77 76 75 74 73 81.00 80.75 80.50 80.25 80.00 79.75 79.50 l l

Fig. 6. Difference between the G15 E(B−V) values and E(4405 − 5495) for the O stars with E(4405 − 5495) > 1.0 in the Cygnus-X region plotted on top of the CO J = 1 ← 0 map of Leung & Thaddeus (1992). The value is encoded in the color of each symbol (see scale on left panel and compare with the vertical axis of Fig. 5). The left panel shows the whole region while the right panel zooms in the blue rectangular region, which corresponds to Cyg OB2. The beam FWHM is 8.07 (∼ 4 pc at the distance of Cyg OB2), so the CO clouds are likely to have substructure not seen in the map. The angular resolution of the G15 values is similar to the beam size. The axes are in Galactic coordinates and the units are degrees. entially close to their natal molecular clouds. Also, their strong known to have sub-pc structures (see e.g., Alves et al. 2001; UV fluxes ionize the surrounding ISM, in some cases produc- Scappini et al. 2002; Ma´ız Apellaniz´ et al. 2015a). Therefore, ing H ii regions. In either circumstance, either the quantity or the O stars located at ∼1 pc projected distances to the clouds will type of extinction they experience should be different from the experience the additional extinction only in a statistical sense, typical extinction experienced by late-type stars, which should as some sightlines will intercept the dense cores and some will be dominated by the diffuse Galactic ISM. In other words, the not. Most late-type stars will be located farther away from the light we receive from O stars is expected to have crossed clouds and only a few of their sightlines will be affected by them. a more local, clumpy, and diverse ISM than the light from This interpretation is consistent with the scatter seen in Fig. 5 for late-type stars. This idea is what prompted us to study O-star E(4405 − 5495) > 1.0. extinction taking advantage of modern capabilities and data. In The differences between the G15 E(B − V) values and this subsection we analyze the quantitative aspects (amount of E(4405 − 5495) are clearly seen for the Cygnus-X region in extinction) and in the following subsection we analyze the qual- Fig. 6, where the most negative values take place at the loca- itative ones (type of extinction). tion of the CO clouds with the highest column depths. However, To test the differences between the extinction experienced the beam size is too large to visualize the correlation between the by O-type and cool stars we use the results from Green et al. CO gas and the clumpy extinction at sub-pc scales. To do this, it (2015), G15 from now on. G15 combined a large sample of Pan- is better to look at the relationship between the foreground dust STARRS 1 (Kaiser et al. 2010) and 2MASS photometry from lanes that obscure H ii regions in the optical and the amount of mostly cool stars to derive a 3-D reddening map of three quarters extinction affecting O stars. We have selected four Galactic H ii of the sky with an angular resolution of 3.04-13.07 and a maximum regions that have three or more O stars where the correlation be- distance resolution of 25%. Their database contains the E(B−V) tween dust lanes and extinction is clear, and we discuss them for 262 of the stars in our sample assuming the values of log d with reference to Fig. 7: from Table 1. We compared them with our E(4405 − 5495) results in Fig. 5, where we also plot the expected relationship be- – The top left panel shows a section of the Carina Nebula, tween E(B−V) and E(4405 − 5495) for three different Teff using whose optical structure is dominated by a V-shaped dust lane an MA14 extinction law with R5495 = 3.0 (see Ma´ız Apellaniz´ several tens of pc in size (see e.g., Smith et al. 2000). The 2013a and Appendix A). For most low-extinction stars our re- section shown corresponds to the southern tip of the dust sults are consistent with those of G15: in those cases there ap- lane (in the top half of the image), where the two segments of pears to be nothing special regarding the ISM that surrounds the the V meet, and where part of the dust lane is seen as a CO O stars, making their reddenings similar to those of late-type cloud (Rebolledo et al. 2016). Two stars, V662 Car (a.k.a. stars in their neighborhood. FO 15) and [ARV2008] 206 are located inside the dust lane On the other hand, for the majority of objects with and those two objects are the ones with the highest redden- E(4405 − 5495) > 1.0, G15 systematically and growingly un- ing [E(4405 − 5495)∼1.1]. On the other hand, the two ob- derestimates O-star reddenings by approximately a factor of 2. jects located farther away from the dust lane, HDE 305 523 This indicates that such O stars have an additional reddening and QZ Car AaAc (a.k.a. HD 93 206 AaAc) have the low- component that is not present in the sightline towards the sur- est reddening [E(4405 − 5495)<0.4]. The rest of the stars, rounding late-type stars. That additional component is likely to located closer to the dust lane, have intermediate values of be the dense gas clumps left over from the natal cloud that are E(4405 − 5495). Therefore, in this case it is clear that the

19 J. Ma´ız Apellaniz´ and R. H. Barba:´ Optical-NIR dust extinction towards Galactic O stars

CPD −59 2600 E(4405−5495) = 0.507 R = 4.28 CPD −59 2629 5495 E(4405−5495) = 0.773 R = 4.33 ALS 15 360 5495 CPD −59 2591 E(4405−5495) = 1.854 E(4405−5495) = 0.748 R = 3.50 V662 Car R = 4.13 5495 E(4405−5495) = 1.189 5495 R = 4.38 BD −13 4927 5495 E(4405−5495) = 1.141 R = 3.60 HDE 305 525 [ARV2008] 206 5495 E(4405−5495) = 0.974 E(4405−5495) = 1.128 R = 4.31 HD 168 075 R = 3.74 5495 5495 HD 168 137 AaAb E(4405−5495) = 0.752 HDE 305 524 E(4405−5495) = 0.678 R = 3.58 E(4405−5495) = 0.504 5495 R = 3.83 R = 4.05 5495 5495 HD 168 076 AB E(4405−5495) = 0.736 R = 3.95 5495 CPD −59 2610 E(4405−5495) = 0.449 R = 4.67 HDE 305 532 5495 HDE 305 523 E(4405−5495) = 0.613 E(4405−5495) = 0.397 R = 4.77 R = 5.04 1 pc 5495 5495

QZ Car AaAc 1 pc E(4405−5495) = 0.342 R = 6.12 5495

V747 Cep E(4405−5495) = 1.641 R = 3.13 5495

ALS 12 320 BD +66 1674 E(4405−5495) = 0.996 BD +55 2722 AB E(4405−5495) = 1.360 R = 3.35 E(4405−5495) = 0.736 5495 R = 3.54 R = 2.97 5495 5495 Tyc 4026−00424−1 E(4405−5495) = 2.233 R = 3.29 BD +66 1675 BD +55 2722 C 5495 E(4405−5495) = 1.403 E(4405−5495) = 0.689 R = 3.71 R = 2.84 5495 5495

1 pc 1 pc

Fig. 7. Extinction measurements for O stars in four H ii regions: (top left) the Carina Nebula, (top right) M16/NGC 6611, (bottom left) Berkeley 59/NGC 7822, and (bottom right) Sharpless 2-132. The top two images are from ESO press releases 1250 (BgrHα) and 0926 (BVR), respectively. The bottom two images are RGB combinations of J2MASS+RDSS2+BDSS2. North is towards the top and east towards the left and the approximate physical scale is indicated in all cases. Only a section of each H ii region is shown to better visualize the dust lanes.

dust lane is a large factor in the diﬀerential extinction in the their surface brightness in emission lines, but a bigger dark Carina Nebula8 cloud towards the north that includes a thicker pillar (see Hill – A section of M16, including the iconic “pillars of creation” et al. 2012) is likely to be in the foreground and is partially (Hester et al. 1996) is shown in the top right panel. The fa- responsible for giving the nebula its eagle shape. There are mous pillars themselves (in the bottom left quadrant) are at three objects from our sample near the center of the nebula least partially immersed in the H ii region, as indicated by (HD 168 137 AaAb, HD 168 076 AB, and HD 168 075) with E(4405 − 5495)∼0.7. The two objects closer to the thick 8 We note that such an association may not be present in some cases, northern pillar have signiﬁcantly higher reddenings, indicat- as it is possible for a star to be in the foreground with respect to the dust ing that the dark cloud is responsible for the additional ex- lane. tinction experienced by them (see Belikov et al. 1999).

20 J. Ma´ız Apellaniz´ and R. H. Barba:´ Optical-NIR dust extinction towards Galactic O stars

expect the sample to be complete to ∼1 kpc and mostly complete 20.0 to ∼1.5 kpc, though for some directions incompleteness does not Upper uncertainty by octant Median by octant become important until we reach several kpc. A median value Lower uncertainty by octant ∼ 10.0 of 1 mag/kpc is a reasonable approximation for the extinction Objects closer than 1 kpc experienced by O stars, in line with previous studies of early- Objects beyond 1 kpc type stars (Fitzgerald 1968; Neckel et al. 1980; Forbes 1985), 5.0 but should be used with care for the following reasons:

– There is a considerable dispersion in AV /d in all of the oc- 2.0 tants, an indication that some of the extinction is caused by a clumpy medium. (mag/kpc) d

/ 1.0 – The lower uncertainty, probably a better measurement of the V

A diffuse (non-clumpy) part of extinction, is ∼0.5 mag/kpc. 0.5 – There are differences among octants - the first, second, and eighth (three of the four inner octants) having significantly higher extinctions than the other five. As expected, there is more dust towards the inner Milky Way than in the opposite 0.2 direction. – The two regions that contribute the largest number of objects 0.1 to our sample, Cygnus in the second octant and Carina in the 0 45 90 135 180 225 270 315 360 fifth one, have an effect on the histogram. Cygnus is a high- l (degrees) extinction region (e.g., Fig. 6) that produces the largest median and dispersion in any octant and is the direction along which sample incompleteness becomes important at shorter Fig. 8. AV /d for the sample in this paper divided into stars closer distances. Extinction is lower than average towards Carina, than and beyond 1 kpc. No error bars are plotted but they are es- in part because many sightlines cross the interarm space be- timated at ∼20%, with the larger part of the error budget coming tween the Sagittarius and Scutum-Centaurus arms. Hence, from the distance uncertainty. The underlying histograms show the extinction distribution in the fifth octant resembles the the 16th, 50th, and 84th percentiles (median plus lower and up- four outer octants, not the other three inner ones 9. per 1σ equivalents) per Galactic octant. We note that only 19 – Stars within 1 kpc of the Sun are concentrated towards the stars have |b| > 10o, of which seven are in Orion (see Fig. 12). outer octants and show a large scatter than the most dis- tant sample. This is consistent with an ISM where extinction is produced by a slowly varying diffuse component and a – NGC 7822 is an H ii region ionized by the Berkeley 59 clumpy component: scatter in A /d is expected to increase at cluster, as shown in the bottom left panel. Three O stars, V shorter distances, where some stars will be dominated by the BD +66 1674, BD +66 1675, and Tyc 4026-00424-1 are lo- clumpy component (Lallement et al. 2014). Indeed, the ob- cated near the center of the image and of the Hα nebulosity. ject with the largest value of A /d in our sample is Tyc 4026- The first two have very similar extinctions but the third one V 00424-1 (see previous discussion about the bottom left panel is located at the same position in the sky as a dust lane and in Fig. 7). that increases its reddening by one magnitude. A fourth star, ii V747 Cep, is inside the H region but close to a dust wall One important characteristic of the solar neighborhood rele- (the right third of the image is peppered with red stars visible vant for our analysis is that we are located inside a cavity filled only in J) at the location of a CO cloud (Yang & Fukui 1992) with hot gas called the Local Bubble (Snowden et al. 1998; Sfeir and has an intermediate extinction between the extremes of et al. 1999; Ma´ız Apellaniz´ 2001; Lallement et al. 2014). Typical the other three. reddenings measured to stars located at the edge of the Local – The bottom right panel shows a section of Sharpless 2-132. Bubble (∼ 80 pc) at the Galactic plane are E(b − y ) = 0.02 The region contains BD +55 2722, a system with three O S S (Reis et al. 2011), with maxima around E(bS − yS ) = 0.04 stars and similar extinction values (AB is unresolved in our (which corresponds to E(4405 − 5495) ∼ 0.05). We note that for photometry and is analyzed as a single source). A fourth O R5495 = 3.1 the first of those values yields AV /d of ∼ 1.0 mag/kpc, star, ALS 12 320 sits at the location of a dust lane that co- which is the typical value measured above for O stars. Therefore, incides with a CO cloud (Vasquez´ et al. 2010) and has a we do not need a dense medium to produce the diffuse compo- − significantly larger E(4405 5495). nent of Galactic extinction: an ISM as thin as that inside the −3 These examples show that the explanation behind the differ- Local Bubble (∼0.01 atoms cm ) appears to be sufficient. ential extinction seen within young stellar clusters and associa- The Local Bubble has a complex shape and, in particular, tions lies in the presence of parsec-scale clouds containing dust it has a finger that extends towards the Orion OB1 association and, in many cases, dense enough to be detected in CO. This is, (Lallement et al. 2014), which is located at a distance of ∼400 pc of course, not a new idea, but clearly showing it in these cases and is separated from the Galactic plane (a fact that also con- tributes to the reduction of material in its sightlines). Some of the is a required preliminary step for the relationship with R5495 that will be analyzed in the next subsection. BA stars in the foreground part of the association have near-zero In Fig. 8 we use the values of AV and log d from Table 1 to 9 Another consequence of the existence of the interarm space in some plot AV /d, the extinction per unit distance for our sample. We fifth-octant sightlines is that we are able to include in our sample stars have also divided the sample by Galactic octant and calculated from NGC 3603, located approximately three times farther away than the median, and lower and upper 1σ equivalents in each one of the Carina Nebula, at a distance similar to that of the Galactic Center them. Given our understanding of how GOSSS is proceeding, we but with much lower extinctions.

21 J. Ma´ız Apellaniz´ and R. H. Barba:´ Optical-NIR dust extinction towards Galactic O stars

Table 2. Number of objects and ranges of E(4405 − 5495) and 8 R5495 for some of the clusters and associations in this paper. Jo+2M+Ga

Cluster/association NE(4405 − 5495) R5495 Jo+2M+Ga+Ty 7 M8 4 0.26-0.83 3.8-5.6 Jo+2M+Ga+St M16 5 0.68-1.85 3.5-3.9 NGC 6604 5 0.89-1.14 3.3-3.7 Jo+2M+Ga+Ty+St Cygnus OB2 33 1.48-2.88 2.6-3.4 6 Berkeley 59 4 1.36-2.23 2.8-3.3 Heart and Soul Nebulae 11 0.58-0.93 3.2-3.7 NGC 1893 5 0.50-0.79 3.0-3.6

NGC 2244 6 0.39-0.50 3.2-3.4 5495 5 Orion OB1 9 0.02-0.29 4.2-6.4 R Orion Nebula 2 0.20-0.29 6.2-6.2 Carina Nebula 64 0.20-1.19 3.0-6.1 NGC 3603 4 1.19-1.41 3.9-4.0 4 IC 2944 10 0.27-0.46 3.3-3.9 NGC 6231 12 0.38-0.51 3.3-4.1 Havlen-Moﬀat 1 8 1.78-2.10 2.7-3.2 3 Pismis 24 7 1.66-1.91 3.2-3.6

2 extinction (Alves & Bouy 2012). We confirm this by measuring 0.0 0.5 1.0 1.5 2.0 2.5 3.0 very small reddenings towards some of the O stars in Orion, with E(4405−5495) E(4405 − 5495) values between 0.022 and 0.044 for δ Ori AaAb, ζ Ori AaAbB, σ Ori AaAbB, ι Ori, and υ Ori. µ Col, an O star Fig. 9. R as a function of E(4405 − 5495) for the CHORIZOS ejected from Orion OB1, has an even lower reddening. The other 5495 1 2 runs in this paper using the MA14 family of extinction laws. O-type objects in Orion, λ Ori A, θ Ori CaCb, and θ Ori A, The color coding is the same as in Fig. 1. See Fig. 10 for the have significantly higher extinctions due to their surrounding histograms derived from this plot by dividing the horizontal axis material, something which will we come back to in the next sub- in five different ranges. section. The irregularity of the ISM around the Local Bubble is confirmed by the ζ Oph sightline. That object is the closest O 1000 3.426 4.036 star and is located at a high latitude (it is another runaway star 3.290 3.701 like µ Col) in a direction nearly opposite to Orion OB1 but it is 3.073 500 E(4405−5495) = 0.00−0.25, N = 50 the star with the second largest AV /d in Fig. 8: a good example of the large scatter in the extinction experienced by nearby ob- E(4405−5495) = 0.25−0.50, N = 180 jects. We note that the ζ Oph sightline is used as a reference for E(4405−5495) = 0.50−1.00, N = 197 the study of the elemental composition of dust in the intervening 200 E(4405−5495) = 1.00−1.50, N = 63 ISM (Draine 2011). Also, ζ Oph is the prototype ζ sightline for DIBs (σ Sco is the prototype σ sightline, see Krełowski et al. 100 E(4405−5495) = 1.50−3.00, N = 67 1997; Cox et al. 2005; Ma´ız Apellaniz´ 2015b), thought to represent an ISM shielded from exposure to UV radiation. 50 Number 3.3. The E(4405 − 5495)-R plane and the properties of 5495 20 dust

3.3.1. What our results show 10 To study the relationship between amount and type of extinction we present three figures and one table. Figure 9 5 shows the distribution of objects in the E(4405 − 5495)-R5495 plane. Figure 10 shows the R5495 histograms for five different E(4405 − 5495) ranges, from 0.00-0.25 (very low extinction, 2 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 blue) to 1.50-3.00 (highest extinction in our sample, dark gray). R Figure 11 shows R5495 as a function of AV /d, a measurement 5495 of average dust density along the sightline (E(4405 − 5495) is a measurement of column density). Table 2 gives the ranges of Fig. 10. R5495 cumulative histograms for different values of E(4405 − 5495) (see legend for ranges and number of objects E(4405 − 5495) and R5495 for the more relevant clusters and associations. in each one) for the CHORIZOS runs in this paper using the The most obvious feature in Figs. 9 and 10 is the exis- MA14 family of extinction laws. Each object is represented by a normalized Gaussian with a width of σ . We note that the ver- tence of a clear relationship between E(4405 − 5495) and R . R5495 5495 tical scale is logarithmic. The color crosses near the top mark the High-reddening stars have low values of R5495 (median ∼3 for E(4405 − 5495) > 1.50) and a narrow distribution. At lower red- median (also printed as the numerical value) and 1σ equivalent denings, both the median and the width of the distribution are in- points for each of the E(4405 − 5495) ranges. creased. Part of the increase of the width of the distribution is due to the increase in the individual values of σR5495 (Fig. 1) but that

22 J. Ma´ız Apellaniz´ and R. H. Barba:´ Optical-NIR dust extinction towards Galactic O stars

– The decomposition between two components usually as- 9 sumes that the foreground extinction is constant in space but Jo+2M+Ga that may also be false. See the interesting case of 30 Doradus Jo+2M+Ga+Ty (van Loon et al. 2013), where the velocity separation be- 8 Jo+2M+Ga+St tween the Galactic (“foreground”) and LMC (“cluster”) ISM Jo+2M+Ga+Ty+St lines allows for maps of both components. See also Bellini

Objects closer than 1 kpc et al. (2017) for a detailed study of the variations in the fore- 7 ground extinction to a globular cluster. θ2 Ori A θ1 Ori CaCb – The assumption of a constant R5495 for the cluster component is wrong in some cases (see Fig 7 and discussion below). 6 Herschel 36 5495 R

3.3.2. Consistency with other results: R5495 of different ISM 5 phases

There are previous studies of the distribution of R5495 in the 4 Galaxy, but most of them refer to single clusters or associations, not to samples that cover the whole Galactic plane or a signiﬁ-

ζ Oph cant part of it. We start by looking at the ones that refer to objects Tyc 4026−00424−1 3 listed in Table 2: Belikov et al. (1999) find a variable RV between 2.8 and 3.9 in M16, Pang et al. (2016) a variable RV between 2.48 and 4.06 in NGC 3603, Lim et al. (2014) a canonical extinction 0.1 0.3 1.0 3.0 10.0 law (R5495= 3.1) towards NGC 1893, and Vazquez´ et al. (1996) A /d (mag/kpc) V RV = 4.70 ± 0.65 in the Carina Nebula. Going to older studies, the high value of R5495 in the Orion Nebula has been known Fig. 11. R5495 as a function of AV /d for the CHORIZOS runs in for a long time, as it has been the prototype for a non-canonical this paper using the MA14 family of extinction laws. The color extinction law since the 1930s (Baade & Minkowski 1937). All coding is the same as in Fig. 1. We note that the horizontal scale of these results are consistent with ours, with some small dif- is logarithmic. ferences that can be explained by the sample and method differences. In a second group, we look at modern, large-area studies of is a minor effect: at high reddenings there are no examples with extinction type. Schlafly et al. (2016), a paper we already dis- R large values of 5495 while for low reddenings there are cases cussed in the previous subsection, obtain RV results different with both high and low values of R5495 with low uncertainties. from ours: they find an average value of 3.32, a dispersion of When we reach the E(4405 − 5495) < 0.25 range, the median 0.18, and no significant variation with the amount of extinction. R5495 value is ∼4. Figure 11 is relatively similar to Fig. 9 but There are no tails in their distribution, so few stars have values five objects are conspicuously placed: the previously discussed under 3.0 and even less values over 4.0. Another study is that Tyc 4026-00424-1 and ζ Oph at low R5495 values and three more of Fitzpatrick & Massa (2007), who find a mean value for R 2 1 V at high R5495 values, Herschel 36, θ Ori A, and θ Ori CaCb. We of 2.99 and a dispersion of 0.27, with a long tail that extends to analyze these later in this paper. large values beyond 4.0. How can we reconcile the different re- In the next subsubsection we combine these results with sults from the three papers? We consider the following points in those of other authors to build a picture of how R5495 varies the discusion below: with ISM phases. Before doing that, we address a preliminary point: when a sightline has two or more clouds of different – Fitzpatrick & Massa (2007) do not distinguish between R5495 R5495 and extinction is described by the MA14 (or CCM) fami- and RV . They claim to give RV but we suspect it is R5495 lies, the resulting extinction law is also a member of the family they are working with. However, their sample consists of with the equivalent R5495 being an average of the individual val- OB stars with typical values of E(B − V) around 0.45, so ues weighted by the individul reddenings (see Appendix C of the differences between the two should be small (see Fig. 3 MA14). This can be seen as an advantage or an inconvenience: in Ma´ız Apellaniz´ 2013a). A similar argument can be made it is the first from the point of view of correcting for extinction about E(B − V) and E(4405 − 5495). Schlafly et al. (2016) because it provides a universality to extinction laws but it is the use the approximation of defining monochromatic wave- second from the point of view of studying dust because it par- lengths for each filter, therefore ignoring non-linear effects tially erases the information contained in the individual clouds. in the reddening trajectories. This could bias their measure- Some papers (e.g., Valencic et al. 2003) use the above ments when comparing high and low extinctions. property to measure extinction decomposing between a fore- – Fitzpatrick & Massa (2007) use UJ BJVJ JHKs photometry, ground and a cluster component. The foreground component has so their wavelength coverage is similar to ours. Schlafly et al. R5495 = 3.1 and the R5495 of the cluster extinction is measured by (2016), on the other hand, use a filter set weighted towards the paper. We have decided not to do that in this paper for the longer wavelengths. following three reasons: – Most importantly, the three samples are different in terms of targets, amount of extinction, and average distances. – While R5495 = 3.1 may be close to the average Galactic ex- Fitzpatrick & Massa (2007) sample is relatively similar to tinction, it is not necessarily the case for every sightline. For ours but it is biased towards B stars, which tend to be older example, some objects with low extinction have large values and to be located farther away from H ii regions. The amount of R5495, so the foreground (likely the only extinction) cannot of extinction range it covers is also smaller (only low values) have R5495 = 3.1. and their sample is less concentrated towards the Galactic

23 J. Ma´ız Apellaniz´ and R. H. Barba:´ Optical-NIR dust extinction towards Galactic O stars

plane and is located at shorter distances (on average) com- fraction of the ISM volume but they have the highest dust pared to ours. Schlafly et al. (2016) sample is fundamentally densities, so their R5495-reducing signature is harder to erase 10 different from the other two, as it is much larger and con- than that of cavities with high values of R5495 . sists of red giants and red clump stars, with very few of them – Regions with intermediate levels of UV radiation have in- (if any) located close to H ii regions. The distribution of the termediate values of R5495 (between 3 and 4). These re- amount of extinction is similar to ours but they lack a signif- gions represent a typical warm to cold ISM (excluding H ii icant fraction of close objects, as their typical distances are regions, cavities, and molecular clouds) that fills the majority between 1 and 5 kpc. of the volume in the Galactic disk.

Another example of the second group of studies is He et al. That model can explain all of the results previously de- (1995), who analyzed the extinction experienced by a sample of scribed. OB stars in the Local Bubble (and adjacent bubbles) southern OB stars with E(4405 − 5495) < 2. He et al. (1995) detected in the samples in this paper and in the Fitzpatrick & is affected by some of the issues discussed in the appendices in Massa (2007) sample will have low values of E(4405 − 5495) this paper and it also has the additional problem that most of its and high values of R5495 with a relatively large scatter caused by “spectral types” are actually photometric classifications (i.e., not large uncertainties and the presence of small clouds that provide true spectral types), which are known to have frequent large er- partial shielding from UV radiation. This is what is observed for rors. Furthermore, many of their true spectral types are obtained the off-plane stars in Fig. 12, which are relatively nearby and from the Michigan Catalog (whose last volume was published for which the Local Bubble contribution to extinction should be as Houk & Swift 1999), which also contains many errors due to generally larger than for objects closer to the Galactic plane. We the imprecisions associated with objective-prism spectroscopy note that no Schlafly et al. (2016) objects are present in the Local (poor spectral resolution, source confusion, and nebular contam- Bubble. Moving to longer distances (and ignoring H ii regions ination), something that can be checked by comparing the spec- for the moment) we see mostly the effect of the typical ISM in trograms published in GOSSS I+II+III with the Michigan spec- the three samples, as the signature of the Local Bubble is easily tral types. As an example of the errors, ALS 4923 is listed in erased. This region (up to 1-2 kpc) is the classical regime for He et al. (1995) as having a spectral type of O6 V as derived extinction studies and the origin of the definition of 3.1 as the from photometry and of O9.5 III as derived from spectroscopy canonical R5495 value, although some of the objects here (e.g., (Vijapurkar & Drilling 1993). The GOSSS spectral type shows those in Cyg OB2) already show the effect of molecular-cloud that it is actually an O8.5 V + O8.5 V SB2 (see GOSSS III for extinction. Once we start moving out, the Fitzpatrick & Massa a spectrogram). Therefore, He et al. (1995) is likely to include (2007) sample disappears and we are left only with the other two systematic errors and an analysis of the sample in common with samples. Cool stars will likely not be associated with molecu- the stars in this paper shows that it tends to underestimate A . V lar clouds, so their R5495 should not change much, especially if Nevertheless, the paper has an interesting result that is consis- they are in one of the outer Galactic quadrants (we note that tent with what we find here: their measured average RV decreases the Schlafly et al. (2016) does not include objects in the fourth with distance, being 3.31 for stars closer than 1 kpc and 2.98 for quadrant). O stars in the inner quadrants (excepting those whose stars farther away than 5 kpc (see Fig. 10 - we note that more dis- sightlines are dominated by an interarm space such as those in tant stars are, on average, more extinguished than closer ones). NGC 3603) will likely have a molecular cloud in their sightline In a third group, we look at studies of individual clusters and will end up having low values of R5495. and associations not included in this paper. In the case of H ii As we have seen in the previous subsection, H ii regions regions, Westerlund 2 (Zeidler et al. 2015; Hur et al. 2015), are complicated in terms of extinction because the sightlines to- NGC 1931 (Lim et al. 2015), and 30 Doradus (MA14) have wards different stars cross different environments: the H ii region values of R5495 > 4, in the case of 30 Doradus with a large itself (first type), possibly a molecular cloud (third type), and in scatter (see below). For objects without an H ii region, Straizysˇ most cases the diffuse, typical ISM (second type). Therefore, al- et al. (2014) find an R of 2.87±0.16 for NGC 6913, a cluster in V most every value of R5495 is possible. In three of the panels of Cygnus with an intermediate extinction, and Marco et al. (2014) Fig. 7 there is a clear anticorrelation between E(4405 − 5495) find eight stars in VdBH 222 with R between 2.7 and 3.0 and 5495 and R5495: when we approach a dust lane E(4405 − 5495) in- a high reddening (E(4405 − 5495) between 2.5 and 2.9). creases, as we had already discussed, but R5495 decreases. The results in this paper and in the previously listed refer- This is consistent with our model, where R5495 is correlated with ences can be explained if we model the dust in the ISM within UV flux. The fourth panel, NGC 7822/Berkeley 59, shows a dif- several kpc of the Sun as having three different regimes depend- ferent behavior with the four sightlines having R5495 < 3. The ing on the ambient UV radiation level: likely explanation is that most of the extinction common to the – Regions with high levels of UV radiation have large val- four sightlines is coming from a molecular cloud that affects all sightlines, which is consistent with the four stars having large ues of R5495 (> 4). This includes H ii regions but also cavi- values of AV /d. To understand H ii region extinction better, we ties filled with hot gas such as the Local Bubble where UV 1 ii should look at three of the extreme cases in Fig. 11: θ Ori CaCb radiation can travel relatively unimpeded. H regions can 2 have significant dust densities but, given their small sizes, and θ Ori A, the two O stars in the Orion Nebula, and Herschel only in exceptional situations they provide column densities 36 in M8 (Fig. 13): large enough to produce strong extinctions. Cavities occupy – much larger volumes but they have low dust densities so their As we have previously mentioned, the foreground extinc- signature is easily erased when the sightline includes denser tion towards the Orion OB1 association (including the Orion clouds. Nebula) is very small. The reddenings of the two O stars – Regions with low levels of UV radiation have small val- 10 Further than a few kpc from the Sun it is possible that similar con- ues of R5495 (< 3). These regions are those with significant ditions are attained in other environments due to the dearth of sources column densities of CO or, alternatively, those that are eas- of ionizing radiation but such locations are not covered by the sample ily detected as dust lanes in the optical. They occupy a small in this paper.

24 J. Ma´ız Apellaniz´ and R. H. Barba:´ Optical-NIR dust extinction towards Galactic O stars

R < 3.0 5495 3.0 < R < 3.5 5495 3.5 < R < 4.0 5495 4.0 < R < 4.5 5495 4.5 < R 5495

Fig. 12. Extinction measurements for Galactic O stars plotted on top of the full-sky Hα image of Finkbeiner (2003). Diﬀerent colors are used for ﬁve ranges of R5495 with the size of the symbol increasing with E(4405 − 5495). The background image is showed in a logarithmic intensity scale and in Galactic coordinates with a Cartesian projection centered on the Galactic anticenter.

HD 164 536 E(4405−5495) = 0.244 R = 4.22 5495 HD 164 816 θ1 Ori CaCb E(4405−5495) = 0.258 E(4405−5495) = 0.286 R = 4.09 R = 6.17 5495 5495 9 Sgr AB E(4405−5495) = 0.315 R = 4.20 5495

Herschel 36 θ2 Ori A E(4405−5495) = 0.825 E(4405−5495) = 0.200 R = 5.57 R = 6.22 5495 5495 1 pc 1 pc

Fig. 13. Extinction measurements for O stars in two H ii regions: (left) the Orion Nebula, (right) M8. The left image is from STScI press release 2006-01 (BVHαiz) and the right image is from ESO press release 1403 (ugrHαi). North is towards the top and east to the left. The approximate physical scale is indicated in both cases.

in the Orion Nebula are significantly higher, indicating that the nebula than θ2 Ori A, as it is the main ionization source, their extinction must be fundamentally local. Evidence for and correspondingly has a higher extinction. Those circum- this effect has been found by van der Werf et al. (2013), who stances explain why the Orion Nebula stars are the prototype estimate that the extinction for Trapezium stars takes place for high R5495 extinction: they are located in the (nebular) within 1-2 pc, and by O’Dell & Harris (2010), who deter- bright part of an H ii region with very little intervening mate- mine that the nebular extinction at the edges of the nebula rial in the sightline (either as nearby molecular clouds or as 1 essentially goes to zero. θ Ori CaCb is closer to the center of a typical ISM). That configuration yields R5495 > 6.

25 J. Ma´ız Apellaniz´ and R. H. Barba:´ Optical-NIR dust extinction towards Galactic O stars

– The foreground extinction towards M8 is lower than aver- Our results are similar to Cardelli (1988) in the sense that we age for its distance, as evidenced by the low E(4405 − 5495) find that stars behind CO clouds or dust lanes have low values of HD 164 536 and HD 164 816, and likely caused by the of R5495. However, an important difference is that his sample is in-between region being an interarm space. Looking at the biased towards low extinctions (only two stars have AV >3) and right panel in Fig. 13 we see that Herschel 36 has both sig- small grain sizes (only seven stars in the 4.0≤R5495<4.4 range nificantly larger E(4405 − 5495) and R5495. Compare this to and none above that). In other words, Cardelli (1988) does not the opposite relationship between those two quantities in, for include objects in H ii regions or subject to Local-Bubble-only example, the Carina Nebula. The explanation is the geom- extinction, where the physical conditions are different than the etry described by Ma´ız Apellaniz´ et al. (2015c): Herschel more stable and benign considered by him. H ii regions are short- 36 is seen through a tunnel carved in the molecular cloud lived dynamic structures where destruction processes dominate where the gas is exposed to the strong ionizing radiation of formation ones and regions inside bubbles are too thin for dust the Herschel 36 multiple O-star system (Arias et al. 2010; grains to form. Therefore, we propose that the ISM regions with 11 Sanchez-Berm´ udez´ et al. 2014) . The resulting long op- large values of R5495 are produced by the selective destruc- tical depth yields not only a large R5495 but also a large tion of smaller dust grains with respect to large grains. E(4405 − 5495). What processes could be responsible for such selective destruction? One possibility is thermal sputtering, the erosion of The model presented here implies that the Herschel 36 case grains by impacting thermal atoms or ions (Draine 2011). The (simultaneous high E(4405 − 5495) and high R5495) is difficult grain lifetime in such a high-temperature is proportional to its to find. Outside of the sample in this paper, only in Damiani size, thus reproducing the observed behavior. Thermal sputter- 4 et al. (2017) we find some more extreme examples: four stars ing is insignificant around 10 K but can become important 5 with E(4405 − 5495) > 1.3 and R5495 > 4.0 and three stars with above 10 K. That makes it an unlikely source for grain destruc- ii E(4405 − 5495) > 2.0 and R5495 > 3.5. Those stars would be tion in H regions but a possible candidate in regions like the placed in an empty region in Fig. 9 and they are all in the Carina Local Bubble. An alternative destruction mechanism is heating Nebula. That is a logical place to find such objects, as the fore- by EUV radiation (Guhathakurta & Draine 1989; Jones 2004), ground extinction is low (the sightline is mostly an interarm re- which also acts preferentially on small grains. This is the likely gion) and the Carina Nebula is the brightest and largest H ii re- cause in H ii regions, where the sources of EUV photons are the gion in the solar neighborhood, thus making it possible to have O stars. large column densities of material exposed to UV radiation. We point out that dust extinction is not the only ISM ob- A final check on our model can be performed with the MA14 servable that is affected by the strength of the UV field. Another extinction analysis of 30 Doradus, where the foreground extinc- observable is the ratio of the equivalent widths of two diffuse in- tion is low and a wide range of values of R5495 ca be found more terstellar bands (or DIBs), DIB 5797 and DIB 5780 (Cami et al. easily than for most Galactic H ii regions. We have selected the 1997; Vos et al. 2011; Ma´ız Apellaniz´ 2015b; Ma´ız Apellaniz´ ∼ three stars with E(4405 − 5495) > 0.3 and R5495 > 6.0 in the et al. 2015a). W(5797)/W(5780) can be as large as 0.7 or sample, VFTS 451, VFTS 464, and VFTS 579 (Fig. 14), that is, smaller than 0.1. Sightlines with large ratios are produced by the three stars with more H ii-like extinction. Where are they lo- ζ clouds (after the prototype sightline of ζ Oph) while small ra- cated in 30 Doradus? They are in different positions but in the tios are produced by σ clouds (after the prototype sightline of 12 same type of environment: a compact H ii region at the top of a σ Sco) . Even though the carriers themselves are not identified dust pillar created by the radiation and winds from R136. The at this point, the observed relationship indicates that DIB 5797 first two are in relatively small pillars while VFTS 579 is in a is found only in regions shielded from UV radiation while large one called knot 1 by Walborn & Blades (1987). Therefore, DIB 5780 can originate in either shielded or unshielded regions. their environment is precisely the one predicted by our model, as This led Cami et al. (1997) to hypothesize a skin effect in ISM they are immersed in a dense gas subjected to a strong UV field. clouds, where the core (ζ sightlines) includes the carriers of both DIBs and the outer layers (σ sightlines) only the DIB 5780 car- rier. The correlation between W(5797)/W(5780) and R5495 was 3.3.3. The physics behind R5495 variations detected by Cami et al. (1997) and confirmed in a much larger sample by Ma´ız Apellaniz´ (2015b). Cardelli (1988) found anticorrelations between R5495 and either log(H2/AV ) or log(CH/AV ), where H2 and CH represent the column densities of those molecules. He also found similar re- 4. Conclusions and future work sults when substituting AV by the total hydrogen column density. Cardelli (1988) concluded that the decrease in the abundance of The main conclusions of this paper are: H2 when R5495 increases was a combination of (a) a reduction in the formation rate of H due to the smaller total grain surface per 2 – unit mass for larger grains and (b) an increase in photodestruc- The MA14 family of extinction laws provide a better description than the CCM or F99 ones for the conditions de- tion via a decrease in UV dust extinction, as R5495 anticorrelates with A /A (CCM). scribed in this paper: Galactic O stars with optical-NIR pho- UV V tometry up to E(4405 − 5495)= 3 (the response to the ques-

11 tion we ask in the introduction). However, there are signs Herschel 36 has a nearby companion that dominates the flux in the that in other cases such as higher reddenings or IR-only data MIR (Goto et al. 2006) and which is already significant in the NIR. The existence of that companion complicates extinction measurements MA14 fails (as expected), so further work is required to gen- and produces discrepancies between different works depending on the erate an improved new family. photometry, extinction laws, and techniques used (see Arias et al. 2006 and Ma´ız Apellaniz´ et al. 2015c). We note, however, that all results 12 Cami et al. (1997) refer to the extreme σ sightlines as of Orion consistently yield E(4405 − 5495) values in the 0.78-0.85 range and type, since stars in the Orion Nebula have a very weak or non existent R5495 values in the 5.4-5.9 range. DIB 5797.

26 J. Ma´ız Apellaniz´ and R. H. Barba:´ Optical-NIR dust extinction towards Galactic O stars

Fig. 14. Environment around VFTS 451 (left), VFTS 464 = [P93] 702 (center), and VFTS 579 = [P93] 1201 (right), the three stars from the MA14 sample in 30 Doradus with E(4405 − 5495) > 0.3 and R5495 > 6.0. The three objects are at the top of pillars that point towards R136. The background image is a F814W + F656N (red) + F555W (green) + F336W (blue) WFC3 mosaic built from WFC3-ERS data. Each ﬁeld is 800 × 800 (2 pc × 2 pc) with north to the top and east to the left. See Walborn et al. (2002) for an alternative view of knot 1, the environment around VFTS 579.

– Many O stars have extinctions similar to those of nearby late- claim that with the exception of some extreme cases, the UV type objects but some are located close to obscuring material and IR portions of Galactic extinction curves are not corre- that increase their extinction and can change the effective lated with each other. We plan to use the results of this pa- value of R5495. per in combination with IUE data to analyze those opposing – Young stellar clusters and associations can have large varia- claims. tions in the amount and type of extinction from star to star. – The largest source of systematic errors in the quantities In those cases the use of average E(4405 − 5495) and R5495 derived in this paper is the lack of uniformity between values fails and one needs to determine the amount and type the sources used for Johnson and Stromgren¨ photometry. of extinction star by star. Some notorious examples include Addressing that issue is one of the objectives of GALANTE, H ii regions such as the Carina Nebula and M8. a seven-band photometric survey of the northern Galactic – The average dust grain size and, hence, R5495, is determined plane we are undertaking using the 80 cm telescope of the by the level of UV radiation. On the one extreme, shielded Javalambre Astrophysical Observatory in Teruel, Spain. The ISM regions such as molecular clouds have small grains (low survey includes a significant fraction of the stars in this pa- R5495) and exposed ISM regions such as H ii regions have per and is being obtained with narrow and intermediate fil- large grains (high R5495), with the average diffuse ISM in an ters specially designed to measure the properties of OB stars. intermediate position in size and R5495. Several mechanisms GALANTE is using exposure times from 0.1 s to 100 s to ob- have been proposed to explain this relationship but we ex- tain S/N > 100 from the brightest stars in our sample to AB pect that the dominant one for the existence of high R5495 magnitudes of 16-17. The large field of view of the camera regions is the selective destruction of small grains by heat- (1.4◦ × 1.4◦) combined with a pixel size of 000. 55 allows for ing by EUV radiation. alternative internal and external calibration techniques to reduce systematic errors such as zero-point offsets. The survey Our lines of future work include the following: began in 2016 and, when complete, will be used to revisit the – We plan to analyze the extinction law that affects OB stars in results in this paper. the IR using a combination of 2MASS, Spitzer, WISE, and – We will extend this work to larger samples once additional ISO data for stars for which we also have GOSSS optical GOSSS spectra are published. spectroscopy. See Ma´ız Apellaniz´ et al. (2017b) for some – The 2016 (first) Gaia data release included the TGAS par- examples. allaxes (Michalik et al. 2015) but those were of little use to – For the extinction-law families described in this paper the estimate distances to O stars (Ma´ız Apellaniz´ et al. 2017a). optical part was derived from photometric data and later in- The second Gaia data release is planned for 2018 and should terpolated in wavelength. An alternative method that yields include accurate parallaxes for the majority of the stars in more information and potentially eliminates imprecisions is this paper, significantly improving the poor-quality spectro- to use spectrophotometry. To that purpose, we have obtained scopic parallaxes in Table 1 and facilitating exciting new sci- HST/STIS 1700-10 200 Å data for several tens of stars in ence. 30 Doradus (GO program 14 104) which we are currently Acknowledgements. We would like to thank the anonymous referee for a de- analyzing. tailed and careful reading of the paper, which led to its significant improvement. – There are two opposing views on the correlation between UV We also thank the collaborators who have contributed throughout the last decade and IR extinction. On the one hand, CCM claim that they are and a half to the building of the GOSC database that has made this paper possible. In chronological order, those are H. A.´ Galue,´ L. H. Wei, R. Y. Shida, tightly correlated in the sense that large-R5495 optical-NIR A. Sota, A. T. Gallego Calvente, M. Penades´ Ordaz, A.´ Alonso Moragon,´ L. extinctions correspond to flat UV extinctions while small- Ortiz de Zarate´ Alcarazo, A. Mart´ın Gutierrez,´ and M. Oliva Rubio. All of them R5495 optical-NIR extinctions correspond to steep UV ex- heard the lead author say that this paper would be published one day and that tinctions. On the other hand, Fitzpatrick & Massa (2007) day has finally arrived, after all the literature was revised, all the archival data

27 J. Ma´ız Apellaniz´ and R. H. Barba:´ Optical-NIR dust extinction towards Galactic O stars were collected, all the spectra observed and reduced, all the calibrations checked, Ma´ız Apellaniz,´ J. 2015a, in HSA8, 402–410 all the biases eliminated and the random uncertainties correctly estimated, and Ma´ız Apellaniz,´ J. 2015b, MmSAI, 86, 553 all the software written, implemented, tested, and optimized. Sometimes sci- Ma´ız Apellaniz,´ J. 2017, A&A, 608, L8 ence takes its time to be done properly and rushing to publication is not always Ma´ız Apellaniz,´ J., Barba,´ R. H., Simon-D´ ´ıaz, S., Negueruela, I., & the best idea. It may collect more citations but “it ain’t right” if it adds more Trigueros Paez,´ E. 2017a, in IAU Symposium, Vol. 329, The Lives and Death- noise than signal to the pool of scientific knowledge. J.M.A. acknowledges sup- Throes of Massive Stars, ed. J. J. Eldridge, J. C. Bray, L. A. S. McClelland, port from the Spanish Government Ministerio de Econom´ıa y Competitividad & L. Xiao, 136–140 (MINECO) through grants AYA2013-40611-P and AYA2016-75931-C2-2-P Ma´ız Apellaniz,´ J., Barba,´ R. H., Sota, A., & Simon-D´ ´ıaz, S. 2015a, A&A, 583, (plus other long expired grants). R.H.B. acknowledges support from the ESAC A132 Faculty Council Visitor Program, which allowed the two authors to get together Ma´ız Apellaniz,´ J., Evans, C. J., Barba,´ R. H., et al. 2014, A&A, 564, A63 for one last time and wrap up the paper. This research has made use of the Ma´ız Apellaniz,´ J., Negueruela, I., Barba,´ R. H., et al. 2015b, A&A, 579, A108 SIMBAD database and the VizieR catalog access tool, both operated at CDS, Ma´ız Apellaniz,´ J. & Sota, A. 2008, in RMxAC, Vol. 33, 44–46 Strasbourg, France. Ma´ız Apellaniz,´ J., Sota, A., Arias, J. I., et al. 2016, ApJS, 224, 4 Ma´ız Apellaniz,´ J., Sota, A., Walborn, N. 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S., & Park, B.-G. 2014, MNRAS, 443, Appendix A: On the use of filter-integrated or 454 Ma´ız Apellaniz,´ J. 2001, ApJ, 560, L83 monochromatic quantities for studying Ma´ız Apellaniz,´ J. 2004, PASP, 116, 859 extinction Ma´ız Apellaniz,´ J. 2005, PASP, 117, 615 Ma´ız Apellaniz,´ J. 2006, AJ, 131, 1184 In previous works (Ma´ız Apellaniz´ 2013a; MA14) we have Ma´ız Apellaniz,´ J. 2007, in ASP Conf. Series, Vol. 364, The Future briefly discussed the problems associated with the use of filter- of Photometric, Spectrophotometric and Polarimetric Standardization, ed. C. Sterken, 227 integrated quantities for studying extinction, an issue that has Ma´ız Apellaniz,´ J. 2013a, in HSA7, 583–589 been known of for a long time (Blanco 1956, 1957) but Ma´ız Apellaniz,´ J. 2013b, in HSA7, 657–657 frequently ignored. In these Appendices we provide a more

28 J. Ma´ız Apellaniz´ and R. H. Barba:´ Optical-NIR dust extinction towards Galactic O stars thorough treatment. Observed photometric magnitudes such as E(B−V) and RV are quantities that depend in a complex way Johnson UJ BJVJ, Tycho-2 BT VT , Stromgren¨ uS vS bS yS , Gaia G on the amount and type of dust (or extinction) and also on the or 2MASS JHKs are filter-integrated quantities, for example, for type of star we are observing. This is contrary to the common Johnson VJ: use of E(B − V) to linearly characterize the amount of extinction  R  (independent of dust type and SED) and of RV to characterize  PVJ (λ) fλ(λ)λ dλ  the type of dust (independent of amount of exinction and SED). −   VJ = 2.5 log10 R  + ZPVJ , (A.1)  P (λ) f (λ)λ dλ In Ma´ız Apellaniz´ (2004) we began by using the monochro- VJ λ,Vega matic (or single-wavelength) equivalents to Eqs. A.4-A.5: where PVJ (λ) is the total-system dimensionless sensitivity function, fλ(λ) is the spectral energy distribution (SED) of the object, E(4405 − 5495) ≡ A4405 − A5495 fλ,Vega(λ) is the Vega SED, and ZPVJ is the photometric zero- point (Ma´ız Apellaniz´ 2005, 2006, 2007). If the equivalent un- = [ fλ(4405) − fλ,0(4405)] − (A.7) extinguished SED, fλ,0(λ) has a Johnson VJ magnitude of VJ,0, [ fλ(5495) − fλ,0(5495)], then the extinction in that filter, AVJ , can be expressed as A(5495)  R  R5495 ≡ , (A.8)  PVJ (λ) fλ(λ)λ dλ  E(4405 − 5495) ≡ − −   AVJ VJ VJ,0 = 2.5 log10 R  . (A.2)  P (λ) f (λ)λ dλ VJ λ,0 where wavelengths are expressed in Å. By avoiding the integrals An equivalent expression for other magnitudes can be easily in Eqs. A.2-A.3, Eq. A.7 is a direct and linear measurement of written, for example, for Johnson B : the amount of extinction (or, more properly, of reddening but J for a given extinction law one is a multiple of the other) and  R  Eq. A.8 depends only on the type of extinction (or the extinc-  PBJ (λ) fλ(λ)λ dλ  A ≡ B − B = −2.5 log   . (A.3) tion law). Examples of the differences between E(B − V) and BJ J J,0 10 R  PBJ (λ) fλ,0(λ)λ dλ E(4405 − 5495) and between RV and R5495 are shown in Fig. 3 of Ma´ız Apellaniz´ (2013a) for CCM extinction laws (the effect From the previous equations we arrive at the frequently used of switching to other families of extinction laws such as MA14 definition of (filter-integrated) color excess (or reddening): is small). One can see there that E(B − V) (or, indeed, any other filter-integrated color excess) has a non-linear dependence on the E(B − V) ≡ A − A = (B − B ) − (V − V ), (A.4) BJ VJ J J,0 J J,0 amount of extinction. where, for notation convenience13, we have dropped the J sub- The values of 4405 Å and 5495 Å were chosen by script in E(B−V). The final filter-integrated quantity of interest is Ma´ız Apellaniz´ (2004) as representative of the central wavelengths of the BJ and VJ filters, respectively, and also to approx- RV , defined as the ratio between total extinction in the Johnson’s V filter and the color excess: imately satisfy the limits: lim E(B − V) ≈ E(4405 − 5495), (A.9) AVJ R ≡ , (A.5) A(λ)→0 V E(B − V) lim RV ≈ R5495 (A.10) where we have also dropped the J subscript in RV for notation A(λ)→0 convenience. Extinction by dust alters fλ,0(λ) to yield fλ(λ) and is usually for OB-star SEDs. The choice is reflected in Fig. 3 of expressed in magnitude form as Ma´ız Apellaniz´ (2013a): in the left panel RV ≈ R5495 for ! Teff= 30 000 K and E(4405 − 5495)= 0 and in the right panel fλ(λ) E(B − V) ≈ E(4405 − 5495) for Teff= 30 000 K in the range A(λ) = −2.5 log10 . (A.6) fλ,0(λ) E(4405 − 5495)= 0-1. We note, however, that outside those values there are significant differences between the monochromatic Where A(λ) is the total monochromatic extinction and is a and filter-integrated quantities. function of the dust properties and of the amount of extinction. For the reasons above, the family of extinction laws of MA14 It is usually normalized by the amount of extinction (see below) are parameterized in terms of E(4405 − 5495) (amount of ex- and in that case it is expressed as a(λ). That quantity is referred tinction) and R5495 (type of extinction) instead of E(B − V) and to as the extinction law and is a function only of the dust prop- RV . But what about other families? The authors of the CCM and erties14. F99 papers either were unaware of the issue or did not consider We can use Eq. A.6 to express fλ(λ) as a function of fλ,0(λ) it to be important. In any case, we have used their family of ex- and insert the result in Eqs. A.1-A.3. From there, we can see tinction laws by substituting the filter-integrated quantities by that Eqs. A.4-A.5 depend not only on a(λ) but also on integrals their monochromatic equivalents, as not doing it is [a] unprac- that include fλ,0(λ) and the amount of extinction. In other words, tical in terms of calculations (the process would require several

13 iterations until convergence) and [b] unlikely to have been the We also adopt the usual convention of calling E(B − V) “the” color authors intentions. excess. In reality, one can define a color excess for any combination of We propose that this issue is so often ignored in the literature two filters e.g., E(U − B). for two likely reasons. The first is that as long as one is work- 14 Here we consider only the “pure” extinction case, where radiation originates in a single point source and the dust cloud is located far away ing with low-extinction OB stars, Eqs. A.9-A.10 tell us that sub- from both the source and the observer. If one relaxes those assumptions stituting the monochromatic quantities by their filter-integrated the relationship between emitted and observed fluxes depends on ra- equivalents can be a good approximation. The second reason is diation transport effects (e.g., scattering back into the beam) and one that filter-integrated quantities can be easily computed from the should talk about dust attenuation, not extinction. observed photometry and spectral types while monochromatic

29 J. Ma´ız Apellaniz´ and R. H. Barba:´ Optical-NIR dust extinction towards Galactic O stars quantities have to be calculated using additional information and E(4405 − 5495) 0.25 1.00 3.00 1.00 0.25 numerical techniques. Indeed, those two reasons may have been LC MS MS MS MS SG valid 40 or 50 years ago, when most targets of extinction stud- R5495 3.1 3.1 3.1 5.0 3.1 Teff (K) ies were low-extinction OB stars and computing facilities were 0.924 0.961 1.070 0.880 1.130 3500 0.794 0.819 0.914 0.759 0.699 7000 limited. However, today they are not valid excuses as massive 0.709 0.728 0.816 0.677 0.676 10 000 photometric and spectroscopic studies provid us with data for 0.764 0.781 0.861 0.731 0.767 20 000 stars of all spectral types and for extinctions that probe into much 0.780 0.797 0.874 0.748 0.780 40 000 larger Galactic distances. Confusing the two types of quantities Table B.1. Values of α as a function of T calculated from syn- can easily lead (as it is too often the case) to biases in photomet- eff thetic photometry for different values of E(4405 − 5495), lumi- ric measurements of extinction. nosity class (main sequence, MS, or supergiant, SG), and R5495.

Appendix B: Limitations of the Q approximation The Q approximation is a method used to determine extinc- type. Therefore, in an (U − B) vs. (B − V) color plane, the trajec- tions of early-type stars that dates back to Johnson & Morgan tories created by increasing extinction are not straight lines (as (1953). Q is a linear combination of (U − B) and (B − V) of the Eq. B.1 indicates) and are not parallel for the same amount of form: extinction if the spectral type is different. The reader is referred to Fig. 1 of Ma´ız Apellaniz´ (2004) for a graphical representation Q = (U − B) − α(B − V), (B.1) of the effect. Even though knowledge of those limitations should be where α is a constant for which Johnson & Morgan (1953) give widely known, unfortunately one still sees papers where the Q a value of 0.72±0.03. They claimed that Q is nearly indepen- approximation is incorrectly applied and, as a result, the pub- dent of extinction and can be used to determine the spectral type lished extinction results are biased. To show the relevance of of the target, giving values that start at −0.93 for most O stars the effects described above, we have used the SED grid of (except the later types), −0.70 for B2, −0.44 for B5, and 0.00 Ma´ız Apellaniz´ (2013b) and the MA14 family of extinction laws for A015. As Q can be used as a proxy for the spectral type, the to compute the real value of α for different amounts and types of color excess can be also calculated in a second step. Johnson & extinction as well as input SEDs (Table B.1). In that way we can Morgan (1953) give: quantify some of the effects described above:

E(B − V) = (B − V) − 0.337Q + 0.009. (B.2) – The first column in Table B.1 lists α for low canonical extinction applied to MS stars, in other words, the case that was In its original form or in adapted versions, the Q approxima- likely considered by Johnson & Morgan (1953). For Teff≥ tion is used even today due to its simplicity. Nevertheless, it has 10 000 K, the classical value of 0.72 is not a poor approxima- some known limitations: tion, but there is considerable scatter (from 0.709 to 0.780) and the average is above 0.72 (see the cases below for possi- – Unless some additional information is present, it is restricted ble explanations of the latter effect). For lower values of Teff, to OB stars. For A stars and later types, solutions become α can be significantly higher already in this regime. multiple as, for example, a low-extinction A star can be con- – The next two columns show increasingly larger values of fused with a higher-extinction late-B star (Ma´ız Apellaniz´ the canonical extinction applied to MS stars. Already for 2004). E(4405 − 5495) = 1.0 we see how non-linear effects increase – The value of α depends on the extinction law for example, the value of α to the point of making it larger than 0.72 R5495, so it is not a real constant. Johnson & Morgan (1953) for all MS OB stars (and much larger for cool stars). For did their analysis assuming an average extinction law but for E(4405 − 5495) = 3.0 α is considerably higher and the sim- Q non-canonical values of R5495 one needs to compute the cor- ple approximation fails completely. responding α (see below for examples). With such an addi- – A comparison between the second and fourth columns in tional parameter, the application of the Q approximation is Table B.1 shows the effect of changing R5495 from 3.1 to 5.0. not straightforward. The increase in R5495 reduces the value of α. It is possible – For late-B stars (and later types) two stars of the same spec- that the original Johnson & Morgan (1953) analysis included tral types but different luminosity classes can have signifi- some objects with R5495 above the canonical value that de- cantly different intrinsic Johnson colors, thus complicating creased their average α. the method. – Finally, a comparison between the first and fifth columns – Emission-line stars need to be excluded beforehand in order shows the effect of luminosity class (or gravity). Differences to avoid biased results. are negligible for O and early-B stars but become appre- – The Q approximation can be adapted to similar filter sets ciable for late-B stars and very large for cool stars. Late-B (e.g., HST/WFPC2 F336W+F439W+F555W) but the equiv- supergiants have lower values of α than their MS counter- alent values of α have to be computed. parts, which could also help explain the average 0.72 value of Johnson & Morgan (1953). To those limitations one has to add the non-linearity of colors described in the previous Appendix: (U − B) and (B − V) do We conclude that the Q approximation can only be applied not increase with the amount of extinction at a constant rate, as (and with care) to OB stars with low canonical extinction. the rate depends on the amount of extinction and on the spectral In other cases it yields biased results both for E(4405 − 5495) and Teff. On the other hand, the use of a code that combines 15 These are the values calculated by Johnson & Morgan (1953) and optical and NIR information handling extinction properly (such they can differ from those calculated here for the examples below. as CHORIZOS) can accurately determine Teff from photometric

30 J. Ma´ız Apellaniz´ and R. H. Barba:´ Optical-NIR dust extinction towards Galactic O stars data (Ma´ız Apellaniz´ & Sota 2008), as shown by MA14. It may have been reasonable to use such an approximation decades ago but with the current computing capabilities and exten- sive availability of well calibrated NIR data it has ceased to 1.015 be so in most cases. As this paper shows, it is possible to avoid biases and, at the same time, compute E(4405 − 5495) and R5495 1.010 simultaneously and detect cases with extreme anomalous SEDs caused by, for example, IR excesses. 1.005

Appendix C: Anticorrelation between 1.000 E(4405 − 5495) and R5495 (4405−5495) (mag)

E 0.995 When using optical+NIR photometry to simultaneously ﬁt E(4405 − 5495) and R5495 to a star with a known unextinguished SED, one obtains uncertainties for both quantities, that 0.990 is, σE(4405−5495) and σR5495 . However, the ﬁnal objective of the process is usually to obtain the extinction in a given band (e.g., 0.985

AVJ ) or the unextinguished magnitude (e.g., VJ,0), so the ﬁtted quantities are intermediate values. One may be tempted to obtain

AVJ by multiplying E(4405 − 5495) and R5495 and to obtain its relative uncertainty (σ , dropping the J to avoid a triple sub- AV 3.925 3.950 3.975 4.000 4.025 4.050 4.075 4.100 script) by summing in quadrature the relative uncertainties for R 5495 E(4405 − 5495) and R5495. This is wrong for two reasons. The first is that: Fig. C.1. R5495-E(4405 − 5495) plane likelihood for a simulated CHORIZOS run. The input UJ BJVJ JHKs photometry corre- A = E(4405 − 5495) R A , (C.1) 5495 5495 , VJ sponds to a SED with input Teff = 30 000 K, LC = 5, MW- metallicity, E(4405 − 5495) = 1.0, R5495 = 4.0 at a distance of A A as 5495 is a monochromatic quantity and VJ a filter-integrated 10 pc with uncertainties of 0.01 mag for each filter. The out- one. The second reason is that the two fitted quantities, put has E(4405 − 5495) = 1.0003±0.0067, R5495 = 3.999±0.039, E(4405 − 5495) and R , can be correlated so the propagation 5495 A5495 = 4.0002±0.0151, and AV = 4.0313±0.0151 (see Eq. C.1). of uncertainties yields: J We note how the relative uncertainties of A5495 and AVJ (0.38%) are smaller than either that of E(4405 − 5495) (0.67%) or R5495

!2 !2 !2 (0.98%) due to the strong anticorrelation (ρE(4405−5495),R5495 = σ σE − σ A5495 = (4405 5495) + R5495 + (C.2) −0.962) between the latter two quantities. A5495 E(4405 − 5495) R5495 ! ! σE(4405−5495) σR5495 2 ρE(4405−5495),R5495 , E(4405 − 5495) R5495 numerical code that processes multifilter photometry simultaneously can find that simpler methods such as the Q approximation cannot. where ρE(4405−5495),R5495 is the Pearson correlation coefficient between the two quantities. In principle, ρE(4405−5495),R5495 could be An additional related issue is how to obtain VJ,0 and its un- of either sign but, as we show in the next Appendix, it is al- certainty. One could simply calculate VJ −AVJ and sum the uncer- ways negative and large under reasonable assumptions, in other tainties in quadrature. Alternatively, one can use the whole like- words, E(4405 − 5495) and R5495 are strongly anticorrelated in lihood grid calculated by CHORIZOS (2-D if E(4405 − 5495) most situations. and R5495 are the only quantities being fitted or of higher dimen- Figure C.1 shows the CHORIZOS likelihood output for an sions if other quantities such as log d or Teff are included in the example built from input synthetic photometry. As expected, fit) and derive VJ,0 directly by summing over the whole grid. The

A5495 , AVJ , though the effect is small in this particular case (it second method (which is the one used in Table 1) has two advan- becomes larger for higher extinctions or for later types). On the tages: [a] it uses information from all the photometric points and other hand, the effect in σAV is very large due to the high value [b] it naturally takes into account the possible anticorrelation be- of ρE(4405−5495),R5495 . This example is representative of the sam- tween VJ and AVJ (as a dim VJ can be caused by a large AVJ ). ple in this paper, indicating that the anticorrelation between Both of those advantages tend to lower the uncertainties. On the E(4405 − 5495) and R5495 is an important effect that should other hand, the second method has the disadvantage of being be taken into account when computing uncertainties for op- model-dependent and, hence, more prone to be affected by sys- tical extinctions. tematic uncertainties (caused by e.g., stars with strong emission We now turn our attention to an explanation behind this an- lines). It is easy to set up an example where the second method ticorrelation. One way to look at it is that if, for a given unextin- improves the value of VJ,0 over the first one thanks to the use guished SED, one knows VJ and Ks, then AVJ is relatively well of information from all photometric points: for a low-extinction determined because the Ks magnitude “anchors” the total ex- SED where one has both Johnson and Tycho-2 photometry and tinction reasonably well, as that filter is little affected (in relative σVJ σVT , then the information contained on VT reduces σVJ,0 terms) by the presence of dust along the light path. How to fit the considerably16 additional photometric points is a different story, as it becomes 16 possible to vary E(4405 − 5495) and R5495 in synchrony main- But we urge the reader not make the mistake of doing VJ = VT : taining a fixed total extinction. This is the kind of effect that a the transformation from one magnitude to another always involves one

31 J. Ma´ız Apellaniz´ and R. H. Barba:´ Optical-NIR dust extinction towards Galactic O stars

Appendix D: Uncertainties in extinction as a with real-world photometry from multiple sources there are usu- function of photometric uncertainties ally hidden calibration issues (e.g., Ma´ız Apellaniz´ 2005, 2006, 2007) plus possible disparities between the real and model SEDs To test that CHORIZOS yields the correct values for the (emission lines, IR excesses. . . ) that introduce systematic errors. extinction parameters, we set up the following experiment: Therefore, one should avoid low-extinction objects when cal- culating optical-NIR extinction properties. – For a star with a known unextinguished SED, we computed We would like to end with a cautionary note on the value of UJ BJVJ JHKs for a range of E(4405 − 5495) values between Eqs. D.1-D.4. They have been calculated for the specific exper- 0.05 and 1.00 and R5495 values between 3.0 and 7.0 using the iment described here. We would expect that if the experiment MA14 familiy of extinction laws. conditions are somewhat altered (different filters, known dis- – We assigned the same photometric uncertainty to all filters tance, unknown Teff, non-uniform photometric uncertainties. . . ) (e.g., σUJ = σBJ = σVJ = σJ = σH = σKs = 0.01 mag) the qualitative behavior should be similar but the quantitative and we randomly changed each magnitude according to a one (i.e., coefficients) should change (e.g., see Fig. 1 where in- Gaussian distribution with that uncertainty. The process was creasing the number of filters decreases the value of σR ). repeated using different Montecarlo realizations and differ- 5495 ent photometric uncertainties. – We processed each of the simulated objects through Appendix E: On the FM09 family of extinction laws CHORIZOS fitting E(4405 − 5495), R , and log d simul- 5495 It was not our original intention to comment on the appli- taneously while leaving the rest of the parameters fixed (to cability of the Fitzpatrick & Massa (2009) or FM09 family of their known values). extinction laws to our data but the referee requested that we ex- The fitted E(4405 − 5495) and R and their uncertainties plain why it was not included along with CCM and F99 in our 5495 list of comparison families and so we discuss it in this Appendix. (σE(4405−5495) and σR5495 ) were compared with the input values and the results were excellent: In FM09, ACS/HST spectrophotometry was used in the 6000-9500 Å range combined with VJ and 2MASS JHKs pho- – No significant bias is present. When a large number of tometry to derive a new family of extinction laws of the form Montecarlo realizations with the same input parameters were (Eq. 5 in their paper): processed through CHORIZOS, the difference between the A(λ) − A 0.349 + 2.087R mean of the fitted values and the input was always much less VJ V − = α RV , (E.1) than the calculated uncertainties. E(B − V) 1 + (λ/0.507) – The calculated uncertainties are consistent with the disper- where λ is in microns and we have adapted their notation to the sion observed in the Montecarlo realizations. In other words, one in this paper. The FM09 family of extinction laws is different combining this point and the previous one, CHORIZOS from the CCM. F99, or MA14 ones in that it is a two-parameter correctly calculates random uncertainties without intro- family (RV and α) instead of a one-parameter family. The intro- ducing systematic errors. duction of a second parameter should ease the fitting of a larger – The calculated uncertainties follow functional forms that al- number of photometric points but, of course, it should be used low approximate analytical results to be derived: only when necessary, as Occam’s razor dictates. FM09 use as an argument for doing so their previous results of Fitzpatrick & Massa (2007), their paper V (FM09 is their paper VI). We can use Eq. A.5 to rearrange Eq. E.1 into a more conve- σ ≈ 0.70 σ (D.1) E(4405−5495) VJ nient form: σR ≈ 0.68 σV (R5495 + 2) / E(4405 − 5495) (D.2) 5495 J A(λ) 0.349 + 2.087R 1 σ ≈ σ ≈ 1.58 σ (D.3) V A5495 AV VJ = α , (E.2) AVJ RV 1 + (λ/0.507) From Eqs. C.1 and D.1-D.3 we obtain: where we have divided the right side of the equation into two 2 different terms to study their effect on the left side. The first term R + 1.942 R5495 − 0.681 ρ ≈ − 5495 , depends only on RV (the first parameter), so it is a constant for a E(4405−5495),R5495 2 (D.4) R5495 + 2 R5495 given choice of the extinction law. The second term contains the dependence on λ as well as that on α (the second parameter). which, for R5495 in the range from 3 to 7, varies between −0.943 The first issue with Eq. E.2 we have dealt with in Appendix − and 0.983 (i.e., strongly anticorrelated, as anticipated in the A: AVJ and RV are filter-integrated quantities that cannot be prop- previous Appendix). erly used to characterize an extinction law, as they depend in a One important consequence of Eq. D.2 is that there is a limit complex way on the amount and type of extinction and also on to how well can R5495 be measured that depends strongly on the SED we are observing. Therefore, we need to find the equiv-

E(4405 − 5495). For example, if σVJ = 0.03, an extinction of alent to Eq. E.2 using monochromatic quantities. We can simply − E(4405 5495) = 0.10 and R5495 = 4.0 allows one to measure substitute AVJ by A5495 but in the case of RV we make the substi- the latter with an uncertainty of 1.2, which is too large to yield tution f = (0.349 + 2.087RV )/RV for reasons that will become much information. We can qualitatively describe what is going clear later. Therefore, we end up with: on: it is easy to measure the properties of an effect when the effect is large but difficult when the effect is small. It is also A(λ) 1 = f α . (E.3) important to keep in mind that, unfortunately, when working A5495 1 + (λ/0.507) color. The color may be close to zero for some Teff and extinctions and Once we have reached this point, there is one point that is significant for others. unclear in FM09: what is the range of applicability of Eq. E.3?

32 J. Ma´ız Apellaniz´ and R. H. Barba:´ Optical-NIR dust extinction towards Galactic O stars

Does it extend to shorter wavelengths than 6000 Å (e.g., to the Table E.1. Transformation of the results from Table 4 of FM09 whole optical-NIR range) or is it constrained to λ > 6000 Å? We using the functions described in the text. consider both of those options below. α Option 1: Equation E.3 extends to the whole optical-NIR Star (0.349 + 2.087RV )/RV 1 + 1.0838 range. If this is the case, we can plug in λ = 5495 Å = 0.5495 µm BD +56 517 2.209 2.262 BD +45 973 2.219 2.222 to Eq. E.3. As the left hand side has to be equal to one, then: BD +44 1080 2.239 2.220 α α NGC 1977-885 2.150 2.149 f = 1 + (0.5495/0.507) = 1 + 1.0838 , (E.4) HD 46 106 2.213 2.218 HDE 292 167 2.202 2.213 which leads to: HD 68 633 2.186 2.227 A(λ) 1 + 1.0838α HD 70 614 2.203 2.221 Trumpler 14-6 2.160 2.169 = α . (E.5) A5495 1 + (λ/0.507) Trumpler 14-27 2.165 2.157 Herschel 36 2.145 2.138 We note that Eq. E.5 is independent of R5495 (or of RV ) that HDE 229 196 2.200 2.207 is, it is actually a single-parameter family of extinction laws, as HD 204 827 2.236 2.248 the only parameter left in the equation is α. Furthermore, the BD +61 2365 2.199 2.201 definition of f and Eq. E.4 lead to: 0.349 0.349 RV = = , (E.6) is close in wavelength to 6000 Å, then Eq. E.4 has to be approxi- f − 2.087 1.0838α − 1.087 mately valid. We show that is indeed the case in Table E.1, where we extract the results of Table 4 of FM09 and compute the first that is, RV is a function of α, not an independent parameter. We term of Eq. E.2 and the inverse of the second term of that same can also plug in λ = 4405 Å = 0.4405 µm in Eq. E.3 and use equation evaluated at 5495 Å. As it can be seen, the values are Eqs. A.8 and E.4 to arrive at: nearly identical (in most cases the differences can be adscribed 1 + 0.8688α to round-off issues in the FM09 table). The conclusion from this experiment is that, effectively, FM09 is a single-parameter fam- R5495 = α α . (E.7) 1.0838 − 0.8688 ily of extinction laws, not a two-parameter family as claimed, as Equations E.6 and E.7 are incompatible. For example, for RV is completely determined by α. Therefore, Eq. E.5 is a valid α = 1.6 the first one yields 6.92 and the second one 5.31 while parametrization for FM09 redwards of 6000 Å (we suspect this for α = 2.9 the first one yields 1.99 and the second one 2.79. We formula or a similar one is what Schlafly et al. 2016 used, see note that changing the central wavelengths of the BJVJ filters to footnote above). An interesting corollary is that for the blueward values other than 4405 Å and 5495 Å changes the coefficients in extension of FM09 to be consistent in the sense of RV ≈ R5495, Eqs. E.6 and E.7 but not the functional forms, which are incom- its functional form would have to be well constrained by the red- patible. wards portion (and different from Eq. E.5), so even then it would This reasoning leads us to the conclusion that option 1 is not be a quasi-single-parameter family. what the authors of FM09 must have intended and therefore we This does not necessarily mean that the results of FM09 discard it. We note, however, that other authors may be unaware are invalid. What those authors could have derived is a single- of this 17. parameter family of extinction laws valid for λ > 6000 Å that Option 2: Equation E.3 is applicable only to λ > 6000 Å. In behaves as a power law in the long-wavelength regime and be- this case, the authors would have implied that a different extinc- comes modified in the optical, an idea that they adopted from tion law is required blueward of that wavelength. However, that a suggestion by Karl Gordon (see their footnote 6). However, extinction law is not explicitly mentioned anywhere in their pa- before accepting the FM09 results we must end with three addi- per. Since the majority of the photometric data in our paper is in tional words of caution. the 3000-6000 Å range, we are unable to apply the FM09 family Firstly, the authors state that “We thus quadratically combine of extinction laws for our data. Nevertheless, let us explore the the standard 2MASS uncertainties with values of 0.007, 0.040, consequences of the FM09 family of extinction laws redwards and 0.017 mag for J, H, and Ks, respectively, to arrive at the total of 6000 Å. uncertainty for each magnitude”. In this paper we did not require We start by noting that if a different extinction law has to be such additional uncertainties (except for objects with bad quality flags), which leads us to think that 2MASS photometry is correct used, then Eq. E.4 is not strictly true, as A(λ)/A5495 has a different (not specified) form around that wavelength. However, extinc- and that there must be a source of uncertainties unaccounted for tion curves are continuous and differentiable and since 5495 Å in FM09. Secondly, the FM09 sample consists only of 14 objects. We 17 We do not think we are the first to notice this issue with FM09. think that number is too small to properly constraint a fam- Schlafly et al. (2016) correctly indicate at one point that “FM09 has a ily of extinction laws, as it does not provide an adequate sam- second parameter, RV , that does not change the shape of the extinction pling of the parameter space. Furthermore, some of the stars curve redwards of V” and later on mention that “we have excluded the in their sample have quite a low extinction, making the IR gP1 band, which is outside the region where the FM09 prescription was measurements suspect. For example, for HD 46 106 we de- developed” (note that Schlafly et al. (2016) does include r in their P1 rive E(4405 − 5495) = 0.392 ± 0.007 and the spectral type is FM09 comparison even though that filter extends redward of 6000 Å). If one adds those two statements together, those authors are saying that O9.7 III(n), not B1 V as stated in FM09. Also, only two stars in their Table 1 are listed as having E(B − V) > 1.0. FM09 is not applicable to the BJ and VJ bands while RV has no effect in its range of applicability. Therefore, their conclusion must be the same Finally, the object that FM09 chose as their highest-R5495 as ours: the presence of RV as an independent parameter in FM09 is representative is Herschel 36 (a.k.a. HD 164 740), a star to which spurious. they pay special attention. However, the authors are apparently

33 J. Ma´ız Apellaniz´ and R. H. Barba:´ Optical-NIR dust extinction towards Galactic O stars unaware of the work of Goto et al. (2006), who showed that an obscured companion has a signiﬁcant contribution in the NIR, or of Arias et al. (2006), whose Fig. 2 clearly showed the NIR excess in comparison with the primary SED, in both cases three years prior to the publication of their paper (see also the discussion regarding Herschel 36 in the main body of this paper). The presence of such anomalous objects in a sample is the reason why any target list used for extinction law calculations has to be carefully culled, as we have done in this paper.