Extinction Law Survey Based on Near IR Photometry of OB Stars W. Wegner
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ACTA ASTRONOMICA Vol. 43 (1993) pp. 209±234 Extinction Law Survey Based on Near IR Photometry of OB Stars by W. Wegner Pedagogical University, Institute of Mathematics, Chodkiewicza 30, 85-064 Bydgoszcz, Poland Received June 21, 1993 ABSTRACT The paper presents an extensive survey of interstellar extinction curves derived from the near IR photometric measurements of early type stars belonging to our Galaxy. This survey is more extensive and deeper than any other one, based on spectral data. The IR magnitudes of about 500 O and B type (B V ) stars with E 0 05 were selected from literature. The IR color excesses are determined with the aid of "arti®cial standards". The results indicate that the extinction law changes from place to place. The mean galactic extinction curve in the near IR is very similar in different directions and = changes very little from the value R 3 10 0 05 obtained in this paper. Key words: extinction ± Infrared: stars 1. Introduction The interstellar extinction is commonly believed to be caused by grains of interstellar dust. Their physical and geometrical properties are thus responsible for the wavelength dependence of the interstellar extinction (the extinction law or curve). Unfortunately, this curve is typically rather featureless (Savage and Mathis 1979) and thus the identi®cation of many, possibly different, grain parameters (chemical composition, sizes, shapes, crystalline structure etc.) is very dif®cult. Possible differences between extinction curves originating in different clouds may be very useful as probes of the physical conditions inside interstellar dark nebula. It is now rather well known that absorption spectra of single interstellar clouds (including extinction curves) differ very substantially (see e.g.,Kreøowski 1989 for review). The problem how the extinction law depends on galactic coordinates has be- came especially important since the ®rst UV spectra were obtained from IUE, ANS and TD-1 (Bless and Savage 1972, van Duinen et al. 1975, Jamar et al. 1976, Wesselius et al. 1982) and when ®rst near infrared observations were available 210 A. A. (Johnson 1966, Hackwell and Gehrz 1974, Wiemer 1974, Schulz and Wiemer 1975, Kuriliene and Straizis 1977, Whittet and van Breda 1980, Koornneef 1983, The et al. 1986, Straizis 1987). A great majority of observationally determined extinction curves (see Aiello et al. 1988, Fitzpatrick and Massa 1990) concerns,however, relatively distant, heavily reddened objects. Such objects are very likely to be obscured by several interstellar clouds situated along the same line of sight, differing in their physical parameters and/or dust content (Kreøowski and Wegner 1989, Papaj, Wegner and Kreøowski 1991). Theextinctioncurvesderivedfromtheirspectraare ill-de®ned averages over all observed clouds and therefore ± useless as a source of information concerning physical parameters of dust particles contained in any one of them. The same concerns several "mean extinction curves" (Savage and Mathis 1979, Seaton 1979) averaged usually over the available samples ± they cannot be used to determine structural details of interstellar grains. With an increasing number of observations in the near infrared (071 5 m) of objects highly obscured by interstellar extinction, a precise determination of the reddening law is important, particularly for observations ofdark clouds. Mostwork in the past has been tied to observations in the visible (Johnson 1968, Hackwell and Gehrz 1974) with correspondingly smaller reddening longward of 1 m than encountered for sources undetectable at visible wavelengths. Becklin et al. (1978a) determined the reddening law longward of 1 m but they concentrate on wave- lengths longer than 1.65 m and have a limited number of J ®lter measurements. The majority of determinations of interstellar extinction in near the IR ranges were calculated from a few sources of infrared radiation (Jones and Hyland 1980) and for several IR passbands only. For Sco and for a few stars in the galactic center Rieke and Lebofsky (1985) obtained the "universal" extinction law from measurements between 1 and 13 m. These authors received the mean value (K V ) E (B V ) of R deduced from the diagram E vs. , very similar to other determinations obtained in various directions of galactic longitude: Whittet and van Breda (1980), (except the region in Orion and Sco-Oph dark cloud), Smyth and Nandy (1978), Sneden etal. (1978), Wiemer(1974). In thepapers of Kreøowskiand Wegner(1989), Wegner, Papaj and Kreøowski (1990),Wegner, Papaj and Kreøowski (1993) the authors reported different conclusions regarding interstellar extinction derived from spectra of "normal" OB stars and from stars OB with emission lines (in particular a part of Oe, Be stars with very great value of rotational velocities): the extinction law varies from place to place. The method of dereddening with the aid of "arti®cial standards" adopted in this (B V ) work allowed to investigate stars with color excesses E 0 05. This fact, as well as using a sample of OB stars much bigger than in other papers previously publishedon IR extinction, may help in the future discussionon physical conditions of dust in interstellar or circumstellar clouds. Vol. 43 211 2. Reduction of Data and Results This paper is based on UBVRIJHKLM ± magnitudes of about 700 O and B type stars of luminosity class I±II, III and IV±V obtained from literature. The main sources are: the catalogue of infrared photometry of southern early type stars containing 229 stars (Whittet and van Breda 1980) and being a part of the catalogue of infrared observations (Gezari et al. 1984) of nearly 500 OB stars, a catalogue of photometric data of 259 stars from 0.15 to 4.8 m (based in part on observations collected at the European Southern Observatory, La Silla, Chile ± The et al. 1986), 55 Be stars were taken from catalogue of Ashok et al. (1984). The Johnson VRI data are taken from published catalogues by Johnson (1966) and by Fernie (1983). The accuracy of these data is of the order of 0 01. The Cousins red VRI measurements were transformed to Johnson`s revised photometric system with the aid of the formulae (Johnson ) = R (Cousins ) ( ) r = R 0 988 0 002 0 02 ; 0 998 (Johnson ) = I (Cousins ) ( ) r = I 0 990 0 007 0 02 ; 0 995 where r means the respective value of the correlation coef®cient. The accuracies (in parentheses) give the standard deviations of the mean values of the differences (Johnson) ± (Cousins). The effective wavelengths of R and I are 0.71 and 0.97 m respectively. We have utilized 93 commonly observed stars. Table 1 presents the distribution ofoursample ofstars according to their spectral type (O5 ± B9.5) and luminosity class (I±II, III, IV±V). There are 349 "normal" OB stars and 151 stars with emission lines. Fig. 1 shows the distribution of stars included into this survey. The "normal" OB stars are presented as circles, the stars with emission lines as dots. As one can see the stars are fairly uniformly distributed along the Milky Way. Most of the infrared photometry is available in L K and bands. For all stars of our sample color-color diagrams have been formed in different two-color combinations but for different luminosity classes separately. Altogether 63 diagrams have been examined. They are clearly linear after a few stars signi®cantly deviating from the linear relations were eliminated. In mostcases these stars are variable in one or several bands, or peculiar for their spectral and K V ) luminosity class. For illustration, the most used two-color relations ( vs. L V ) ( for three luminosity classes (I±II, III, IV±V) are shown in Figs. 2, 3, 4. They also contain 49 stars with emission lines except those deviating from linear relations, known mostly as variables in one or several infrared bands (see Feinstein 1975, Feinstein and Marraco 1981), or as extreme Be stars with infrared excesses. For some O stars and B8 and B9 stars no luminosity class assignment was found in literature. They have been assigned to the best ®tting two-color diagrams. All JHKL(M) measurements were reduced to the Glass (1974) JHKL photometric 212 A. A. Table1 The distribution of the OB stars according to spectral type and luminosity Sp/L I±II III IV±V Uncertain O5 2 2 7 6 O6 7 6 15 11 O7 4 10 16 13 O8 13 6 24 17 O9 15 10 21 1 B0 35 19 19 1 B1 23 2 24 0 B1.5 5 3 9 1 B2 12 4 44 5 B2.5 2 0 9 0 B3 9 7 22 6 B4 2 1 3 0 B5 5 2 8 4 B6 4 0 3 2 B7 3 0 6 3 B8 13 0 16 8 B9 8 3 17 9 Total 162 75 263 87 Fig. 1. Distribution of the stars, considered in this paper in the galactic coordinates. Dots ± stars with emission lines, circles ± normal OB stars. system, which is tied to the Johnson system, by comparing with standard stars of Glass (1974); transmission curves for the ®lters are presented by Glass (1973) and Vol. 43 213 K V L V ) Fig. 2. ( diagram for supergiants I±II OB type stars. 12 stars with emission lines Oe or Be are also located in this diagram. K V L V ) Fig. 3. ( diagram for giants III OB type stars. 4 stars with emission lines Oe or Be are also located in this diagram. M effective wavelengths are 1.25 ,1.65 ,2.2 and 3.5 m. The system is de®ned by effective wavelength of 4.8 m. The Johnson UBV data are used. These data are obtained from a catalogue: Blanco et al. (1970), Hof¯eit and Jaschek (1982), Nicolet (1978) (the accuracy of these data of the UBV magnitude is typically 0 01).