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Star Formation and Molecular Clouds Towards the Galactic Anti-Center Handbook of Star Forming Regions Vol. I Astronomical Society of the Pacific, c 2008 Bo Reipurth, ed. Star Formation and Molecular Clouds towards the Galactic Anti-Center Bo Reipurth Institute for Astronomy, University of Hawaii 640 N. Aohoku Place, Hilo, HI 96720, USA Chi-Hung Yan Institute of Astronomy and Astrophysics, Academia Sinica P.O. Box 23-141, Taipei 10617, Taiwan National Taiwan Normal University 88 Sec. 4, Tingzhou Road, Taipei, 11766, Taiwan Abstract. The Galactic Anticenter region hosts a a number of massive molecular cloud complexes, some of which are currently actively forming stars. Two major OB associations, Gem OB1 and Aur OB1, are found in this direction, each with numerous massive stars and a supernova remnant. The dominant region of star formation is cen- tered around the Sh 2-235 complex and the nearby regions AFGL 5142, 5144, and 5157 towards Aur OB1. Studies of these regions have long been affected by relatively poor distance determinations, although there is general consensus that most regions are lo- cated at distances between 1.5 and 2 kpc. A number of well-known, relatively isolated Herbig Ae/Be stars are found in this general direction, including RR Tau, HD 250550, and LkHα 208. 1. Overview The region towards the Galactic Anticenter is rich in molecular clouds and star forma- tion activity, although most is located at a considerable distance. In this chapter, we summarize key features of the principal regions of star formation out to approximately 2 kpc. A number of very interesting regions exist at larger distances, but those are be- yond the scope of this review. In Figure ?? the region from l = 169◦ to l = 195◦ within 10 degrees of the Galactic plane is shown using data from the Dobashi et al. (2005) extinction atlas, and with the principal regions discussed in this chapter indicated. 2. The Associations 2.1. Gemini OB1 The Gem OB1 association was first recognized by Morgan, Whitford, & Code (1953), who called it the I Geminorum aggregate. The present name originates with the study of Humphreys (1978). Distances to stars in Gem OB1 have been estimated as 1.4 kpc (Crawford et al. 1955), 1.5 kpc (Humphreys 1978), 1.6 kpc (Hardie et al. 1960), and 1.9 kpc (Georgelin et al. 1973). Haug (1970) suggested that the association consists of two groups of stars, one at 1.2 kpc and the other at ∼2 kpc. In an analysis of the various 1 2 Figure 1. An overview of the Galactic Anticenter region based on the extinction maps of ?. The x-axis and y-axis are Galactic longitude and latitude, respectively. The background represents the level of extinction, with green being little extinction, yellow is higher, and orange is highest extinction. Regions discussed in this chap- ter are labeled. The limits of the Gem OB1 and Aur OB1 associations are from Humphreys (1978). data sets, Carpenter et al. (1995a) tentatively concluded that Gem OB1 is a single entity at a distance of about 1.5 kpc, and favored a distance of about 2 kpc for the adjacent large molecular clouds. It is unclear if the difference is significant given the inherent uncertainties, and it is widely assumed in the literature that there is a direct connection between the stars and the gas. Carpenter et al. (1995a) have presented a major study of the molecular gas and star formation towards Gem OB1, based on large-scale 12CO and 13CO maps. Carpenter et al. (1995b) followed up on this with a global survey for dense gas using CS (J = 2–1) maps, and found 11 dense core regions. They noted that while the Gem OB1 complex contains several distinct sites of massive star formation in addition to the OB associa- tion, there are large parts of the molecular cloud complex that are not currently forming stars. The structure of the clouds seen in the millimeter maps suggest that they bear the imprint of past star formation events, and that expanding bubbles from HII regions and stellar winds have created high density molecular gas in which star formation is induced. Most of the dense cores found by Carpenter et al. (1995b) are associated with IRAS sources with colors characteristic of young stars. 3 In a lower-resolution but larger-scale study of 13CO gas in Gemini and Auriga, Kawamura et al. (1998) explored the overall connection between gas and various indi- cators of star formation activity. They noted that the luminosity of the most luminous IRAS source in a cloud increases with the mass of the cloud, and they found that clouds associated with IRAS sources tend to be more massive and larger in size and have higher column densities than clouds without any sign of star formation. A number of individual star forming regions towards Gem OB1 are marked in Figure ?? and discussed individually below. 2.2. Aur OB1 The Aur OB1 association covers a large region of the sky, see Figure ??. According to Humphreys (1978), Aur OB1 contains 5 O stars and is located at a distance of 1.32±0.1 kpc. The relatively young cluster NGC 1960 is considered the nucleus of the Aur OB1 association, see Section ?? There is a smaller group of OB stars, known as Aur OB2, located within the large area of the sky defined by the Aur OB1 association, and at a larger distance of 2.8±0.2 kpc (Humphreys 1978). Tovmassian et al. (1994) suggests that the OB stars in the general region of Auriga should be further divided into several smaller groupings. Many molecular clouds are found in this region (e.g. Kawamura et al. 1998), some associated with well known HII regions like the Sh 2-235 complex discussed below. 3. Clusters and HII Regions 3.1. NGC 2175 and Sh 2-252 The nebula NGC 2175 was discovered in 1857 by Carl Christian Bruhns, and this des- ignation is now also used for the cluster within. A small condensation known as NGC 2174 was found in 1877 by Jean Marie Edouard Stephan. The central star of NGC 2175, HD 42088, is of spectral type O6.5V (Conti & Alschuler 1971, Walborn 1972) and is the main source of ionization, and a probable member of the Gem OB1 association. The HII region, also known as Sh 2-252 and W 13 (Sharpless 1959, Westerhout 1958), is roughly spherical with a diameter of about 25 arcmin, and is bounded by an ion- ization front on the west side, while it is density bounded on the east side (Felli et al. 1977). The kinematics of the gas has been studied through Hα line studies (Fountain et al. 1983) and radio maps (Felli et al. 1977, Falchi et al. 1980), which support a standard blister model. Detailed 12CO and 13CO maps have been obtained by Lada & Wooden (1979) which along the western edge of the HII region show the presence of a dense molecular cloud ridge embedded in a more diffuse but extended (90 pc) molecu- lar cloud complex. Further multi-line millimeter observations are presented by K¨ompe et al. (1989) and Carpenter et al. (1995a). Luo (1992) studied the interface between the HII region and the molecular cloud using IRAS data, and inferred the presence of very small dust particles. A cluster is associated with the HII region (Collinder 1931). Further photometric and/or spectroscopic studies of the stars are given by Pismis (1970), Grasdalen & Car- rasco (1975), Chavarria et al. (1987), and Haikala (1994, 1995). Two stars, Sh 2-252a and Sh 2-252b, associated with a compact reflection nebula and marked a and b in Fig- ure ??, have drawn particular attention (Garnier & Lortet-Zuckermann 1971, Grasdalen & Carrasco 1975, Pismis 1977). They are B stars, and the fainter one is a strong Hα 4 Figure 2. The NGC 2175 cluster in the HII region Sh 2-252. The small cluster NGC 2175s is indicated at the edge of the image. The stars marked a and b are discussed in the text. Interference filter Hα, [SII], and [OIII] image. The field is approximately 35′ × 30′, with north up and east left. Courtesy Filipe Alves. Figure 3. Integrated 13CO (J = 1–0) intensity map of the Sh 2-252 region. The cloud complex stretches up to the small HII region Sh 2-247. The field is 105′ × 145′, and north is up and east is left. From K¨ompe et al. (1989). 5 emission star, probably a Herbig AeBe star. Chavarria et al. (1987, 1989) found a com- pact stellar grouping, likely T Tauri stars, around these two stars. Pismis (1970) found a second cluster, labeled NGC 2175s (s stands for ’small’), which is marked in Figure ??. However, Pismis thought it to be a background object. Subsequently, Chavarria et al. (1987) found that NGC 2175s after all is most likely associated with NGC 2175. The distance to the NGC 2175 cluster has been measured variously as 2.0 kpc (Hardie et al. 1960), 1.95 kpc (Pismis 1970), 2.2 kpc (Humphreys 1978), 2.3 kpc (Chavarria et al. 1987), and 2.2 kpc (Haikala 1995, Khachikian et al. 2005). The age of the cluster is around 1-2 Myr (e.g., Grasdalen & Carrasco 1975, Haikala 1995). The molecular cloud associated with Sh 2-252 contains a number of young stars. Felli et al. (1977) found several ultracompact HII regions in their 1415 MHz and 4995 MHz maps. Wright et al. (1981), K¨ompe et al. (1989) and Carpenter et al. (1995b) identified various infrared/IRAS sources in the molecular gas complex, and OH and H2O masers were discovered by Lada & Wooden (1979) and K¨ompe et al.
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