A&A manuscript no. ASTRONOMY (will be inserted by hand later) AND Your thesaurus codes are: 08:10.07.2;10.15.1;09.04.1 ASTROPHYSICS Foreground and background dust in star cluster directions C.M. Dutra1 and E. Bica1 Universidade Federal do Rio Grande do Sul, IF, CP 15051, Porto Alegre 91501–970, RS, Brazil Received; accepted Abstract. This paper compares reddening values E(B-V) kpc, as well as 86 RR Lyrae from Burstein & Heiles (1978). derived from the stellar content of 103 old open clusters These two samples provided slightly lower values on the and 147 globular clusters of the Milky Way with those average as compared to Schlegel et al.’s reddening values derived from DIRBE/IRAS 100 µm dust emission in the (∆E(B-V) = -0.008 and -0.016, respectively). The redden- same directions. Star clusters at |b| > 20◦ show compara- ing comparisons above hardly exceed the limit E(B-V) ≈ ble reddening values between the two methods, in agree- 0.30, so that a more extended range should be explored. ment with the fact that most of them are located beyond Since the Galaxy is essentially transparent at 100 µm, the disk dust layer. For very low galactic latitude lines the far-infrared reddening values should represent dust of sight, differences occur in the sense that DIRBE/IRAS columns integrated throughout the whole Galaxy in a reddening values can be substantially larger, suggesting given direction. Star clusters probing distances as far as effects due to the depth distribution of the dust. The dif- possible throughout the Galaxy should be useful to study ferences appear to arise from dust in the background of the dust distribution in a given line of sight. Globular clus- the clusters consistent with a dust layer where important ters and old open clusters are ideal objects for such pur- extinction occurs up to distances from the Plane of ≈ 300 poses because they are in general distant enough to pro- pc. For 3 % of the sample a significant background dust vide a significant probe the galactic interstellar medium contribution might be explained by higher dust clouds. and have a suitable sky coverage. Clearly, star clusters We find evidence that the Milky Way dust lane and higher beyond the disk dust layer are expected to have redden- dust clouds are similar to those of several edge-on spiral ing values essentially comparable to those of galaxies in galaxies recently studied in detail by means of CCD imag- the same direction. On the other hand, clusters within ing. the dust layer should have contributions from clouds in background regions. Another issue is the thickness of the Key words: The Galaxy: globular clusters: open clusters: Milky Way dust lane and whether some dust clouds oc- arXiv:astro-ph/0005108v1 5 May 2000 interstellar medium: dust cur at higher distances from the Plane. Recently, several edge-on spiral galaxies have been studied in detail (Howk & Savage 1999) and a comparison of their dust distribu- 1. Introduction tion with that of the Milky Way is worthwhile. Full-sky surveys in the far infrared have been achieved The aim of the present study is to compare star clus- by means of the IRAS and COBE satellite observations. ter reddening values measured from direct methods, i.e. Schlegel et al. (1998) built a reddening map from the sampling the dust effects seen in the light emitted by the 100 µm IRAS dust emission distribution considering tem- cluster members, with those derived from the 100 µm dust perature effects using 100/240 µm DIRBE data. The emission. We investigate the possibility of background and transformation to E(B-V) maps was obtained from dust foreground dust contributions in star clusters directions. columns calibrated via the (B-V)-Mg2 relation for early In Sect.2 we present an overview of Schlegel et al.’s (1998) type galaxies. This far-infrared reddening (hereafter E(B- reddening values predicted in different environments in the Galaxy. In Sect.3 we gather the necessary data for glob- V)FIR) presents a good agreement at high galactic lati- tudes with that derived from H I and galaxy counts by ular clusters and old open clusters and describe the sam- Burstein & Heiles (1978, 1982) with an offset of 0.02 mag ple properties. In Sect.4 we discuss the results, especially (lower values for the latter method). Recently, Hudson the star cluster lines of sight with evidence of background dust. Finally, the concluding remarks are given in Sect.5. (1999) analysed E(B-V)FIR maps using 50 globular clus- ters with |b| > 10◦ and distance from the plane |Z| > 3 Send offprint requests to: C.M. Dutra - [email protected] 2 C.M. Dutra & E. Bica: Foreground and background dust in star cluster directions 2. Overview of dust emission reddening values and corresponding solid angles the zones responsible for E(B-V)FIR the accumulation of reddening throughout the arms ap- pear to be the cloud halos rather than the cores, possibly For a better understanding of the reddening distribution combined with diffuse galactic dust. throughout the Galaxy we extracted E(B-V)FIR values from Schlegel et al.’s maps using the software dust-getval provided by them. We discuss (i) directions along the 2.1. Comparison with reddening values from JHK galactic plane which accumulate reddening from sources photometry for nearby dark clouds in different arms and other large structures, and (ii) galac- We show in Table 1 embedded infrared star clusters and T tic latitude profiles to see the effects of relatively isolated Tauri groups which are related to nearby dust complexes. nearby (high latitude) dust clouds. They can be useful to analyse the high extinction regime We show in Fig.1 the entire Galaxy longitude profile. and to compare Schlegel et al.’s reddening values with The upper panel is in direction of the galactic centre and those measured directly from Infrared (JHK) photometry the lower one is in direction of the anticentre. Note the of the stellar content. We give E(B-V)FIR values for the enormous reddening differences between the two panels: central positions of these stellar aggregates and indicate to the lower panel has typical values of E(B-V)FIR ≈1.5, and which complexes they belong. The identifications and lo- the values in the upper panel are a factor ≈ 10 higher. We cations of these objects are from: (i) Gomez & Lada (1998) indicate a series of H I, CO and optical features which help for the T Tauri groups related to the dark clouds Barnard interpret the reddening distribution: (i) tangent regions of 30 and 35; (ii) Lada et al. (1991) for the infrared clus- the spiral arms Sagittarius-Carina, Scutum (5 kpc arm) ters embedded in the nebulae NGC2071, M78, NGC2023 and 4 kpc arm (Henderson 1977, Georgelin & Georgelin and NGC 2024 in the Orion Complex LDN 1630 Molecular 1970a, Cohen et al. 1980); (ii) the extent of the 3 kpc arm Cloud; (iii) Minchin et al. (1991) , Jones et al. (1994) and (Kerr & Hindman 1970, Bania 1980); (iii) the extent of Reipurth et al. (1999 and references therein) respectively the far side of the Sagittarius-Carina arm (Grabelsky et for the three deeply embedded clusters OMC-1, OMC- al. 1988); (iv) the Molecular Ring (MR) and the Central 2 and OMC-3 in Orion Complex Molecular clouds; (iv) Molecular Zone (CMZ), (Combes 1991, Morris & Serabyn Strom et al. (1993) for the 7 objects in the Orion complex 1996); and finally, (v) the extent of the Local (Orion) and LDN1641 Molecular Cloud; (v) Carpenter et al. (1997) Perseus arms (Georgelin & Georgelin 1970b). for the Mon R2 IR cluster; (vi) Lawson et al. (1996) for The relatively low reddening in the anticentre panel two concentrations of T Tauri stars around the reflection can be basically explained by the cumulative effect of the nebulae IC2631 and Cederblad 110/111 in the Chama- three external arms: Orion, Perseus and Outer arm (Di- leon I dark cloud; (vii) Comer´on et al. (1993 and refer- gel et al. 1990). It is worth noting that E(B-V)FIR on the ences therein) for the ρ Ophiuchi IR cluster. The larger average is higher in the second quadrant than in the third E(B-V)FIR values occur for IR star clusters, while the T quadrant, probably by the interruption of the Perseus Tauri groups tend to be associated with lower reddening arm. The steady increase of E(B-V)FIR in the first and values. This must reflect the need of higher dust densities fourth quadrants towards the direction of the Galactic for the formation of star clusters and massive stars, condi- center can be explained by the cumulative effect of in- tions which occur in the cores of Giant Molecular Clouds ner arms and especially their tangent zones. Owing to the in contrast to less massive dark clouds (e.g. Comer´on et 100 µm dust emission transparency the far side arms of al. 1993, Carpenter et al. 1997). the Galaxy will also contribute to E(B-V)FIR (see the ex- For the infrared photometric reddening comparisons tent of far side of the Sagittarius-Carina arm in the fourth with E(B-V)FIR we adopt a total to selective extinction AV quadrant). The Molecular Ring is also a major contribu- ratio R = E(B−V ) = 3.1. When the original studies do tor, leading to a plateau level E(B-V)FIR ≈ 20. Finally, not express their results in terms of AV , we adopt the the Central Molecular Zone is responsible for the central ratios AJ = 0.276, AH = 0.176 and AK = 0.112 from AV AV AV cusp.