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Handbook of 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 , 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 complexes, some of which are currently actively forming . Two major OB associations, Gem OB1 and Aur OB1, are found in this direction, each with numerous massive stars and a remnant. The dominant region of 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 is shown using data from the Dobashi et al. (2005) , and with the principal regions discussed in this chapter indicated.

2. The Associations

2.1. 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 , Kawamura et al. (1998) explored the overall connection between gas and various indi- cators of star formation activity. They noted that the of the most luminous IRAS source in a cloud increases with the 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 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 - 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 /IRAS sources in the molecular gas complex, and OH and H2O masers were discovered by Lada & Wooden (1979) and K¨ompe et al. (1989). The molecular cloud complex extends to the north (K¨ompe et al. 1989, see Fig- ure ??) and envelops the small HII region Sh 2-247, excited by a B0 V star (Hunter & Massey 1990). Sh 2-247 is thus likely to be located at about the same distance as NGC 2175 (Carpenter et al. 1995a). IRAS sources, masers, and millimeter sources have been detected towards Sh 2-247 (e.g., K¨ompe et al. 1989, Carpenter et al. 1995b, Minier et al. 2005).

Figure 4. An image combined from Hα and broadband exposures of the super- remnant IC 443 in the Gem OB1 association. The diffuse emission to the northeast of the SNR is the HII region Sh 2-249. The field of view is approximately 2.1 × 1.7 degrees, with north up and east left. Courtesy Johannes Schedler. 6

3.2. IC 443 IC 443 is a large with a diameter of ∼45 arcmin and located towards the Gem OB1 association (Figure ??). A has been found that is likely associated with the supernova remnant (Olbert et al. 2001). The distance to IC 443 and the adja- cent HII region Sh 2-249 is generally accepted to be around 1.5 kpc (e.g., Fesen 1984, Welsh & Sallmen 2003), and the remnant is assumed to result from the collapse of a massive member of the Gem OB1 association. IC 443 is the clearest known case of a supernova remnant interacting with a molecular cloud (e.g., DeNoyer 1979, Dickman et al. 1992). Evidence for such interaction is available as high velocity line broadening of numerous gas tracers like CO, HCN, HCO+, as well as HI (e.g., White et al. 1987, Wang & Scoville 1992, Lee et al. 2008), and is also seen in Na and Ca absorption line towards bright stars (Welsh & Sallmen 2003). Shock-excited molecular is widespread and due to a combination of C- and J-shocks (e.g. Burton et al. 1990, Rosado et al. 2007). The shocked around IC 443 pro- duces soft X-ray emission that correlates with bright optical/radio filaments and traces the SNR encounter with the atomic cloud (e.g., Troja et al. 2006, Bykov et al. 2008). The detonation of a supernova and the interaction of the SNR with a dense molecu- lar cloud may trigger star formation (e.g., Opik¨ 1953), and Odenwald & Shivanandan (1985) have searched the IC 443 region for evidence of young stars. While they found evidence for recent star formation, they concluded that such star forming events predate the supernova detonation.

3.3. The Sh 2-235 Complex This large complex is composed of four HII regions in the direction of the northern Auriga OB1 association (see Figure ??) and listed as Sh2-231, 232, 233, and 235 in the Catalogue of HII Regions by ?. In optical images, these HII regions seem to be isolated from each other. However, they all belong to a single giant molecular cloud (?). The distances to these HII regions are not well constrained, but range from 1.6 to 2.0 kpc (???). A value of 1.8 kpc (?) is frequently used for these HII regions and represents a mean spectroscopic distance to Sh 2-233 (?).

Sh 2-231 and Sh 2-232 Sh 2-231 and Sh 2-232 are two extended, low surface bright- ness HII regions (see Figure ??). The angular sizes of Sh 2-231 and Sh 2-232 are 12′ and 40′, respectively. ? found that Sh 2-231 is excited by an O9V star, while Sh 2-232 is excited by a centrally located O9.5III star (the spectroscopic binary HD 37737, Mc- Swain et al. 2007). Radio continuum observations done by ? indicated that the low densities and the absence of structure places them in the category of well evolved HII regions.

Sh 2-235 Sh 2-235 was first noted by Minkowski (1946) and listed as M1-82. It is excited by the centrally located O9.5V star BD+35◦1201 and is the brightest HII region in this complex (??), see Figure ??, with a diameter of only about 10′. ? determined −3 a mean electron density ne = 36cm , indicating that it is a well evolved HII region, which is consistent with its fairly structureless appearance. A large molecular cloud is associated with Sh2-235, with evidence for ongoing star formation (e.g., Evans & Blair 1981, Lafon et al. 1983, Nakano & Yoshida 1986, Kirsanova et al. 2008). A large-scale far-infrared map of the region is presented by Nordh et al. (1984). 7

Figure 5. (top) An overview of the Sh 2-235 complex. The dimension of this image is about 1.5 × 1 degrees, with north up and east left. HII regions are labeled with their Sharpless numbers. (bottom) Enlargement of the Sh 2-235 region. The size of the image is 20 × 20 arcmin. Both figures are made using DSS II images. 8

About an arcminute south of Sh2-235, Allen (1972) found an infrared source, which he called M1-82#1. Subsequently, Glushkov et al. (1975) and ? noted three very small HII regions, now known as Sh2-235A, B, and C (see Figure ??) just south of Sh2-235. ? studied Sh 2-235A and Sh 2-235B at optical, radio and IR wavelengths, yielding evidence of ongoing star formation. Brγ and H2 emission were also detected in Sh 2-235A and Sh 2-235B by ????. maser sources were found to be associated with the Sh 2-235A-B region (e.g., Lo et al. 1975, Blair et al. 1978, Henkel et al. 1986, Comoretto et al. 1990), and a maser (Nakano & Yoshida 1986, Haschick et al. 1990) and SiO maser (Harju et al. 1998) were also detected. Felli et al. (1997, 2004, 2006) have studied the Sh2-235A-B region in great de- tail, and found that a very deeply embedded object is located in a compact molecular core between Sh2-235A and B. A quadrupolar molecular outflow is found there, as are several masers, radio continuum sources, and an infrared cluster (see Figure ??). The region is highly complex, and represents widely different stages of early .

Figure 6. A sketch (not to scale) of the star forming region located between Sh2- 3 235A and B. The main source is an intermediate luminosity object (∼10 L⊙ in a very early evolutionary stage. A quadrupolar molecular outflow emanates from a compact molecular core, and several masers and radio continuum sources are present. From Felli et al. (2006).

Sh 2-233 and IRAS 05358+3543 In the literature, there are two regions which are both named Sh 2-233. ? pointed out the confusion and clarified that the HII region h m s ◦ ′ ′′ Sh 2-233 is at αJ2000 = 5 38 28 δ2000 = +35 51 15 , while another one is located h m s ◦ ′ ′′ at αJ2000 = 05 39 10 δJ2000 = +35 45 19 , which should be called Sh 2-233IR (?). The HII region Sh 2-233 is excited by a B1.5II star (?). The gas in the HII region was studied by ?. An IRAS source (05351+3549, identified by ?), is located in this 9

Figure 7. Far infrared map of Sh 2-233 region. The map is made of HIRES 100 µm contours overlaid on a DSS2 image. The contour levels are 2, 4, 8, 20, 40, 60, 80% of peak flux (5280 MJy sr−1). HII regions are labeled in text. From ?.

region. Studies of the CO molecular gas around this source was done by ? and ?, while ? detected high-velocity CO line wings, indicative of outflow activity. However, no H2O and OH maser detections have been made toward this source (???). Sh 2-233IR is about 10 arcmin to the south-east of Sh 2-233. ? noted that Sh 2- 233IR is identical to IRAS 05358+3543, and detected high velocity CO in this area. Two embedded young clusters, NE and SW, were identified by ? who noted that the NE cluster is highly reddened, and SW much less so. Figure ?? shows a near-infrared image of the region, NE is the clustering surrounded by numerous shocks found by ?, while SW is just below and to the right. IRAS 05358+3543 is associated with NE. Based on detailed H2 and [FeII] images and modeling, ? concluded that the shock velocities are in the range 60-80 km s−1. Water and methanol masers were detected in the IRAS 05358+3543 region by ?, ? and ?. IRAS 05358+3543 is thought to be part of a young protocluster with outflow activity detected in CO, SiO, H13CO+,HCN,C34S and CS (????). ? and ? studied the in the IRAS 05358+3543 region using K’ band . They found that the polarization is weak towards the SW cluster, and noted that the polarization towards the NE cluster is due to reflected from a deeply embedded source, whose location could be determined by the polarization pattern, H2 emission, and maser spots. ? detected two sources at mid-IR wavelengths in the central part of the NE cluster. The IRAS 05358+3543 region has also been mapped at submillimeter and mil- limeter wavelengths (???). The most recent radio observations done by ? and ? re- 10 vealed that there are several continuum cores, named mm1a-b, mm2a-d, and mm3 (see Figure ??), several of which are likely of protostellar nature. The dominant core, mm1a, is associated with a CH3OH maser, a cm wavelength hypercompact HII region and a mid-infrared source, and is at the center of a highly collimated molecular outflow; the driving source is likely a newborn B1 star. By studying the molecular spectrum emis- sion of these sources, ? suggest that these protostellar cores are at different evolutionary stages.

G173.58+2.45 G173.58+2.45, also known as IRAS 05361+3539, is an ultracompact ′ HII region located at α2000 5:39:27.7, δ2000 +35:40:43, about 1.5 SSE of Sh 2-233IR (see Figure ??), and is associated with a large molecular cloud. It was first identified as an outflow source by ?, who found the IRAS source to be located precisely between a

Figure 8. A near-infrared image of the two massive star formation regions Sh 2-233IR (IRAS 05358+3543) and G173.58+2.45. The image is from the CFHT WIRCam using J (blue), H (green) and H2 (red) filters. The image size is 10 × 10 arcminutes. North is up and east is left (Chi-Hung Yan, unpublished). 11

Figure 9. Continuummaps toward IRAS 05358+3543at 1.2 mm (grey scale with dotted contours, from PdBI) and at 875 µm (full contours, from SMA). The cold cores are labeled mm1-3. The star, triangles, and squares mark the positions of masers and mid-infrared sources. From ?. blue and a red outflow lobe in 12CO maps, and centered within a cloud core. The core was also mapped in CS (J = 2–1) by Zinchenko et al. (1998). A more detailed study of the molecular outflow was presented by ? (see Figure ??), who used continuum emission maps at 2.7 mm to identify two sources around the center of the outflow at the location of the IRAS source; their spectral types are likely to be between late B and mid A. Near-infrared images have revealed the presence of a small cluster of embedded Class I and Class II sources around the IRAS source (???). Molecular hydrogen images have revealed a rich group of infrared shocks in the region of the cluster (??), see also Figure ??.H2O maser emission was detected towards the IRAS source by ? and ?.

3.4. Sh 2-234 Sh2-234 is located in the southern part of the OB association Aur OB2 in a region dominated by larger and more distant HII regions (see Figure ??). It is also commonly known as IC 417. The young cluster Stock 8 is associated with IC 417 (Figure ??, as is the IRAS source 05247+3422 (??). Various distance estimates have been made to Sh2-234 and Stock 8, a kinematic distance of 2.30±0.7 kpc was derived from CO mea- surements (???). Using optical of stars in Stock 8, distances between 1.9 to 2.96 kpc have been reported by ?, ?, ? and ?, the most recent by ? is 2.05±0.10 kpc. The first photometric measurements of 11 bright stars in the Stock 8 cluster was done by ?. ? found that the age of Stock 8 is about ∼ 12 Myr, however the age determined by the most recent multi-wavelength observations is less than 2 Myr ?. Based on 2MASS J, H and KS images ? and ? discuss the basic properties of Stock 8, including extinction, age, YSO candidates, distance, and IMF. The Hα observation carried out by ? shows a spectrum with two well separated Hα lines. CO contour map observed by ? reveals that molecular cloud with size about 12

Figure 10. The large molecular outflow associated with G173.58+2.45 (IRAS 05361+3539) as seen in a 12CO (J = 1–0) map obtained at the OVRO interferom- eter. Thick contours is redshifted gas, thin contours is blueshifted gas. Coordinates are equinox 2000. From Shepherd & Watson (2002).

1′in radius is adjacent to the east of Sh 2-234 ?. In paper done by ?, 1.4 GHz and 8.28 µm emissions are detected. The contour plot of these emissions formed a sharp cutoff, indicating an interface between ionized gas and molecular cloud. They also suspect that the star formation in Stock 8 was triggered by the first generation OB stars distributed around this region.

3.5. AFGL 5142 AFGL 5142 is a bright infrared source first detected in the Air Force Geophysics Labo- ratory rocket survey (Price 1977), later found to be coincident with IRAS 05274+3345 3 and with a luminosity of 3.8 × 10 L⊙ (Carpenter et al. 1990). It was first studied by Snell et al. (1988), who found it to be associated with a large molecular outflow ex- tending in the northwest-southeast direction (see Figure ??). They assumed a distance of 1.8 kpc because AFGL 5142 is only 0.5◦ south of the small bright HII region Sh 2- 237 (Sharpless 1959) and has a similar CO velocity (see Section ??). In the absence of other information it has been used in all subsequent studies of AFGL 5142, but it is little more than an educated guess. Near-infrared studies of the region (Hunter et al. 1995, Chen et al. 2005) have revealed a small cluster of at least 60 embedded sources hidden behind about 20 mag- nitudes of visual extinction. The two main sources are IRS 1 and 21, which are believed to be massive young stars. They are about 25 arcsec apart, and IRAS 05247+3345 is just southwest of IRS 2, the brightest source at K in the cluster. IRS 1 has the largest near-infrared excess among the cluster stars. The overall age of the cluster is estimated at 1 × 106 yr (Chen et al. 2005).

1These are the designations of Hunter et al. (1995). IRS 1 and IRS 2 are listed as NIRS 1 and NIRS 30, respectively, in the source catalog of Chen et al. (2005) 13

Figure11. AwidefieldHα image of the surroundingsof Sh 2-234 = IC 417. The large regions IC 405 and IC 410 are apparently located behind IC 417. North is up and east is left. Courtesy John Gleason.

Figure 12. An overview of the Sh 2-234 region. The young cluster Stock 8 is marked. The nebula Sh 2-234 is also known as IC 417. The area of the figure is approximately 20 × 30 arcminutes, with north up and east to the left. This color image was made using DSS II images. 14

Figure 13. A sketch of multi-wavelength observations of the AFGL 5142 region. Thick contours indicate the NH3 core (Zhang et al. 2002), the extended contours indicate the molecular outflows (Hunter et al. 1995), and small ellipsoids indicate shocked outflow features. From Chen et al. (2005), where more details can be found.

AFGL 5142 is associated with a dense core located in a north-south oriented ridge mapped in 12CO and 13CO by Carpenter et al. (1990). The core was mapped in CS by Pastor et al. (1991), and in NH3 by Verdes-Montenegro et al. (1989), by Estalella et al. (1993), and by Zhang et al. (2002), who found a highly complex structure of the core, see Figure ??. Further high resolution maps in several molecular line transitions are presented by Hunter et al. (1995). A rich molecular line spectrum is found by Zhang et al. (2007). Torrelles et al. (1992a) used the VLA to discover a 3.6 cm radio continuum source at the center of the cloud core. Higher resolution radio continuum observations by Hunter et al. (1995) show that the source is compact. It appears that this VLA source is offset by about 30 arcsec to the east of the IRAS source, and represents a more recent event of star formation. The VLA source may be a zero-age B2 star or earlier if the continuum emission is from a single optically thin HII region. Zhang et al. (2007) present 1.3 mm continuum maps towards the VLA source and find a compact (<2′′) group of at least five dust continuum cores, indicating the onset of the formation of a small cluster. As mentioned earlier, Snell et al. (1988) detected a major molecular outflow from AFGL 5142, which was mapped in more detail by Hunter et al. (1995). In a subsequent 15

Figure14. AJHK′ composite image of the AFGL 5142 region, showing an ap- proximately 3′×3′ area. The faint circle with diameter ∼1.8′ indicates the infrared cluster. The plus-sign indicates the source NIRS 30 = IRS 2. From Chen et al. (2005). study, Hunter et al. (1999) obtained still higher resolution CO maps of the outflow, and additionally found evidence for a compact HCO+ and SiO outflow approximately perpendicular to the principal outflow (see Figure ??). Using the SMA, Zhang et al. (2007) observed the region in CO and SO and found evidence for at least three possible molecular outflows. Eiroa et al. (1994) imaged AFGL 5142 in optical narrow-band fil- ters and found a Herbig-Haro object, HH 190, see Figure ??. Hunter et al. (1995) and Chen et al. (2005) obtained near-infrared molecular hydrogen images, and found evi- dence for numerous shocked knots. Their broadband images also revealed the presence of a small cluster of infrared sources (see Figure ??). A strong H2O maser was discovered by Verdes-Montenegro et al. (1989) at the location of the NH3 core and within a few arcseconds of the VLA source. Further observations of maser emission are presented by Torrelles et al. (1992a), Tofani et al. (1995), Hunter et al. (1995, 1999), and Goddi et al. (2004). In detailed maps of the region, Goddi & Moscadelli (2006) found 29 H2O maser features located in two elongated structures (Group I and II), with proper motions along their long axes. The Group I masers emanate from the VLA source and appear to trace outflowing gas at a large angle to the main molecular outflow. The Group II masers have positions and velocities along a line close to the axis of the SiO outflow, but its source remains unidentified.

3.6. AFGL 5144 and the Cluster NGC 1931 The young nebulous cluster NGC 1931 was discovered by in 1793. Sharpless (1959) labeled the associated small, bright HII region Sh 2-237. An optical image of the region is shown in Figure ??. Some of the cluster members were studied by Moffat et al. (1979), who suggested a distance of 1.8 kpc based on zero age main sequence fitting of a small group of four B stars, and assumed it to be located in an extension of the arm. In a UBVRI photometric study of NGC 1931, Bhatt 16 et al. (1994) estimated a distance of 2.17 kpc (consistent with the earlier study by Pandey & Mahra 1986), an age of ∼10 Myr, and a general reddening of E(B–V) = 0.45 mag. Hunter & Massey (1990) reviewed the various distance estimates to Sh 2- 237 and adopted a distance of 1.9 kpc. NGC 1931 contains a trapezium system listed as AMS 28 in the catalog of trapez- ium systems by Ambartsumian (1954), and as the triple system ADS 4112 in Aitken’s catalog (Aitken & Doolittle 1932). In the modern Washington Double Star catalog it is listed as WDS 05314+3417. The brightest of the stars (V∼11.1) is BD 34◦1074, a B0 V star (Crampton, Georgelin, & Georgelin 1978). Sh 2-237 was included in the study of Hunter (1992), who determined emission line ratios, reddening, and densities within the optical gas of small HII regions. Israel (1977) presented a 6 cm map of Sh 2-237, among others, and derived physical parame- ters for the HII region. The luminous IRAS source 05281+3412, also known as AFGL 5144, is associated with Sh 2-237. Han et al. (1998) detected an H2O maser towards the IRAS source. Ev- idently, NGC 1931 is a young cluster with still ongoing star formation, and it deserves more attention than it has received so far.

3.7. AFGL 5157 Attention was first drawn to the AFGL 5157 region by Snell et al. (1988), who found a large east-west oriented bipolar molecular outflow, but with its center offset from AFGL 5157 more than an arcminute to the north-east. AFGL 5157 is associated with IRAS 05345+3157, and with a small reflection nebula known as NGC 1985, or Ced 57, first discovered by William Herschel in 1790. An optical spectrum of the brightest star in the reflection nebula shows an early F-type spectrum (Sabbadin & Hamzaoglu

Figure 15. The young cluster NGC 1931 and its associated HII region Sh 2-237 as seen in an optical image. The figure is approximately 10′ × 14′. North is up and east is left. Courtesy Adam Block. 17

1981). To the northwest of NGC 1985 there is a small compact reflection nebula, GM 39 (Gyulbudaghian & Magakian 1977), see Figure ??, it is located north of the center of the molecular outflow, and so is unrelated to the outflow source. Snell et al. (1988) adopted a distance of 1.8 kpc on the assumption that AFGL 5157 is somehow associated with the Sh 2-235 complex about three degrees to the north. This distance is widely quoted, although highly uncertain. An NH3 core was found at the center of the molecular outflow (Verdes-Montenegro et al. 1989, Torrelles et al. 1992b) and was observed also in CS by Pastor et al. (1991). A 3.6 cm radio continuum source was found by Torrelles et al. (1992a) at the center of the dense core, and they estimated that a ZAMS B3 star is required to account for the observed flux. This appears to be the driving source of the molecular outflow. The molecular outflow was further mapped in CO J=2-1 by Zhang et al. (2005). Torrelles et al. (1992b) discovered a chain of HH objects now known as HH 281 to 285, but offset from the VLA source. Molecular hydrogen shocked knots were found by Chen et al. (1999, 2003), see Figure ??, and maser sources were studied by Verdes- Montenegro et al. (1989), Torrelles et al. (1992a), and Henning et al. (1992). A cluster of near-infrared sources was found by Chen et al. (1999, 2003) using near-infrared images. Millimeter continuum observations by Klein et al. (2005) have revealed three massive cold cores surrounding the near-infrared cluster, which coincides with the IRAS source (see Figure ??).

3.8. M36 = NGC 1960 The M36 = NGC 1960, which apparently forms the center of the Aur OB1 association, has been the subject of numerous analyses, and of these the earliest stud- ies are today of historical interest only (e.g. Hopmann 1924, Boden 1951, Johnson & Morgan 1953, Barkhatova et al. 1985). The cluster is dominated by a number of bright (V∼11 mag) stars, but at fainter levels is only slightly enhanced above the background

Figure 16. An optical I-band CCD image of the AFGL 5157 region. Two small reflection nebulae, NGC 1985 and GM 39, are seen. The AFGL 5157 infrared source is located south of GM 39, approximately equidistant to the two nebulae. North is up and east is left. From Torrelles et al. (1992b). 18

Figure 17. A sketch of the AFGL 5157 region. The east-west oriented molecular outflow is indicated, as is the NH3 core (thick contours) and the HII region detected in a 3.6 cm map (small tilted contours). H2 knots are indicated by small ellipses. From Chen et al. (2003).

2 1

1 3

0 ARC MINUTES

-1

2

-2 2 1 0 -1 -2 ARC MINUTES

Figure 18. The regionaroundAFGL 5157. The contoursare millimeter dust con- tinuum superposed on a K-band 2MASS image. The IRAS 05345+3157 source is marked with a diamond surrounded by its uncertainty ellipse. The plus signs are MSX sources. The triangle is an NVSS (NRAO VLA Sky Survey, Condon et al. 1998) radio point source. Apparently the cluster has carved a cavity in the massive cold cloud complex. Three cloud cores are numbered. The units on the axes are arcminutes, (0,0) corresponds to 5:37:45.2, +31:59:25 (J2000). North is up and east is left. From Klein et al. (2005). 19

(Figure ??). Harris (1976) classified several of the bright stars as spectral type B2V. Membership of NGC 1960 based on proper motions have been determined by Meurers (1958), Chian & Zhu (1966), and Sanner et al. (2000). A detailed UBV CCD photomet- ric study of NGC 1960 has been performed by Sanner et al. (2000), who used isochrone fitting in a color- diagram of 178 astrometrically determined members to es- timate a distance of 1318±120 pc, an overall reddening of EB−V ∼0.25±0.02 mag., +10 and an age of 16−5 Myr. Hasan (2005) has used infrared J,H,K photometry to study NGC 1960, and suggests a distance of 1380 pc and an age of 125 Myr, but since only an abstract is available, it is not possible to examine these results. Most recently, Mayne & Naylor (2008) suggest an age of 20 Myr from comparison with different main sequence models. Magnier et al. (1996, 1999) found a faint star with a cometary nebula in the out- skirts of NGC 1960 (at 05:36:05.9, +34:06:12, J2000, see Figure ??). It is associated with IRAS 05327+3404, and has a molecular outflow along the axis of the reflection nebula. The star, which has spectral type K2, has brightened by >1.5 mag since the 1954 POSS plate, and shows a high velocity profile at Hα. The source was detected in the 3.6 cm radio continuum by Anglada & Rodr´ıguez (2002). Given its obvious youth, it is not likely that the object is associated with the much more evolved NGC 1960 cluster, rather it is probably an outlier linked to the slightly more distant Sh 2-235 complex two degrees to the north (see Section ??). NGC 1960 has recently attracted attention as the most likely origin of a massive OB star that exploded about 40,000 yr ago, creating the supernova remnant (Figure ??), a large (∼ 3◦ diameter), old supernova remnant listed in the catalog compiled at Simeiz2 by ?. A pulsar, PSR J0538+2817, has been found near the center of Simeis 147 (?). Using VLBA , ? have determined a pulsar distance of +0.42 1.47−0.27 kpc. With a spatial separation of only ∼220 pc from Simeis 147, NGC 1960 is the closest candidate cluster. Assuming an age of NGC 1960 of between 10 and 40 Myr, this distance would be transversed if the progenitor OB star was moving away from NGC 1960 with a velocity of between 5 and 20 km sec−1.

3.9. NGC 2129

NGC 2129 is a sparse cluster, which has been the subject of only a few studies. The cluster is dominated by two bright, common stars, HD 250289 (B2III), which is a (Mermilliod 1982), and HD 250290 (B3IB). It is surrounded by fainter members within an angular diameter of about 5 arcmin. Cuffey (1938) was the first to study this cluster using photographic photometry. Subsequent photometric studies include those of Hoag et al. (1961), Lindgren & Bern (1980), and Pe˜na & Peniche (1994), the latter suggested that the cluster might be a line-of-sight grouping of stars. In a recent study, Carraro et al. (2006) conclude that NGC 2129 is a real cluster, with an age of about 10 million years and located at a distance of 2.2±0.2 kpc.

2It has been customary in the West to spell the objects of Gaze and Shajn as Simeis, while the location in Ukraine is often spelled Simeiz 20

Figure 19. The NGC 1960 cluster appears to form the nucleus of the Aur OB1 association. Red image from the second epoch Digitized Sky Survey.

Figure 20. The supernova remnant Simeis 147 as seen in a continuum subtracted Hα image. The cross marks the geometric center of the SNR. The arrow in the center indicates the pulsar’s motion and points to its current position. Tick marks indicate birth sites if the pulsar is 2, 4, and 6×104 yr old. The direction to potential originating clusters for the progenitorstar are indicated. NGC 1960 is the most likely candidate. The image is approximately 4◦ × 4◦. From ?. 21

4. Individual Objects and Small-Scale Regions

4.1. CB 34 CB 34 is a relatively small , about 4-5 arcmin in diameter, listed in the catalog of dark clouds by ?. ? noted that CB 34 is associated with the IRAS source 05440+2059. The distance to CB 34 is not well constrained. ? and ?, based on kine- matic arguments, use 1500 pc, while the average distance of Bok globules in the CB catalog, 600 pc, is used by other studies to estimate the physical parameters of CB 34 (?). Based on IRAS and 1.5 mm continuum observations, the bolometric luminosity is ∼ 130 L⊙, assuming a distance of 1500 pc (?).

Figure21. Thecentralregionof CB 34as seen in a2.12 µm molecular hydrogen image. Submillimeter cores from ? as well as shocked knots are marked. From ?.

The first near-IR observations toward CB 34 were carried out by ?. Together with 12CO(J=1-0) observations, ? suggested that the negative (–0.67) 12/25 µm spectral index implied that IRAS 05440+2059 is deeply embedded in the cloud. Another IR 22 observation done by ? revealed 12 young stellar objects embedded in the globule. These YSOs are distributed in a 3 arcmin2 area centered close to IRAS 05440+2059 in a small cluster (see Figure ??). Using optical interference filter images, ? discovered several Herbig-Haro objects (HH 290 and HH 291), probably driven by one of the embedded YSOs found by ?. Deeper near IR observations carried out by ? revealed H2 shocks up to 1.6 pc from the center of the globule. Molecular outflow activity has been noted in CB 34 around the embedded source IRAS 05440+2059 (??). One of the brightest optically visible stars, CB 34V (also known as V1184 Tau), was noticed by ? and ? because its brightness has increased by 3.7 mag in the R-band between 1951 and 1996. ? found that V1184 Tau has a spectral type of G5. Based on photometric studies (??), ? suggested that V1184 Tau may be a UX Orionis type variable. ? considered it very unlikely that V1184 Tau could be an eruptive star of the EXor type. Millimeter and submillimeter observations of CB 34 are reported in a number of papers, e.g. ?, ?, ?, ? and ?. ? detected a water maser toward the submillimeter sources CB 34 SMM 3 and SMM 4 from ?. Most recently, ? have used Spitzer IRAC and MIPS data to identify a small cluster of 8 Class 0/I and 14 Class II young stellar objects within CB 34.

4.2. GGD 4 GGD 4 was initially assumed to be a Herbig-Haro object by ? when examining prints of Palomar Sky Survey, but it was later recognized as a reflection nebula. The distance toward this source was suggested to be 1.0 (?) or 1.7 kpc (?). One IRAS source, IRAS 05373+2349, was found to be located near the position of GGD 4 (?). (?) also found a small cluster of young stars near GGD 4. The SED of IRAS 05373+2349 corresponds to a 930 K blackbody, and its infrared luminosity was determined to be between 600 and 1900 L⊙, depending on the assumed distance, suggesting that the source is an intermediate mass YSO (?). H2O masers have been found at a position near GGD 4 by ? and ?, one of them at the position of an IR source (?). Further maser observations were done by ?. JCMT ob- servations obtained by ? have revealed two submillimeter cores, one of them coincident with the H2O maser emission. A number of molecular transitions were detected toward 18 13 this target, such as CS, NH3,C O, CO and HCN (???). Additionally, a bipolar CO outflow was found to extend approximately in a N-S direction (?).

4.3. RR Tau RR Tau (HBC 170, IRAS 05363+2620) is one of the original Herbig Ae/Be stars listed by Herbig (1960), who gives references to the early literature. It is an irregular vari- able with deep minima of around 4 magnitudes (∼10 < V < ∼14), and is classified as belonging among the UX Orionis stars (e.g., Zaitseva & Lyutyi 1979, Herbst & Shevchenko 1999). It is located in a small dark cloud, where one also finds the neb- ulous star BD+26o887 (HBC 492, HD 245906) and a few fainter Hα emission stars, including LkHα 206 and 207. There are numerous studies of RR Tau, and only a few key references can be given here. A range of spectral types have been given by different observers, Hern´andez et al. (2004) summarize these and argue that RR Tau is an A0 star. Grinin et al. (1996) and Rostopchina et al. (1997) present spectroscopic, pho- tometric and polarimetric observations and conclude that RR Tau is surrounded by a disk-like, highly flattened dusty envelope, which is responsible for the irregular obscu- ration of the star. Rodgers et al. (2002) carried out a spectroscopic monitoring program 23 of RR Tau, and their results are consistent with an occulting screen that obscures the stellar surface.

4.4. HD 250550 in CB 39 HD 250550 (V1307 Ori, HBC 192) is also one of the original Herbig Ae/Be stars identified by Herbig (1960), who gives references to the early literature, and it has since been studied by numerous observers. It is a B9 star (Herbig 1960, Hern´andez et al. 2004) and an irregularly (8.3 < V < 10.7, Herbst & Shevchenko 1999). It is located in a small cloud known as CB 39 (Clemens & Barvainis 1988) and displays a compact reflection nebula on its northern side. Spectroscopic studies show significant variability in the Ca II K line profiles of HD 250550, suggesting circumstellar streams connected to a large scale magnetic field (Catala et al. 1991). An H2O maser was detected towards the star by Schwartz & Buhl (1975). CB 39 has an angular size of about 88 arcmin2 (?) and harbors other young stars.? found 12CO(J=1-0) outflow activity in this area, and concluded that IRAS 05591+1630, located about 4′ northeast of HD 250550, is likely to be the correspond- ing driving source. In subsequent IR photometry done by ?, IRAS 05591+1630 is classified as a Class II source. Using the VLA in D-configuration, ? detected a 3.6 cm source with no IR counterpart, and speculated that it could be a Class 0 source. HCN (?) and SO (?) were also detected in this region. Observations done by ? in H2 revealed no obvious H2 emission in CB 39.

Figure 22. The surroundings of the Herbig Ae/Be star LkHα 208 as seen on the red Digitized Sky Survey. 24

4.5. LkHα 208 LkHα 208 (HBC 193, IRAS 06048+1839) is another Herbig Ae/Be star belonging to the original list of Herbig (1960). It has a characteristic appearance with a biconical reflection nebula (see Figure ??) first noted in 1874 by Jean Marie Edouard Stephan, which led to its inclusion in Dreyer’s catalog as NGC 2163. The nebula was again noted by Hubble (1922) and later listed as Ced 62 by Cederblad (1946). The nebula is illuminated by the central star, as documented by polarization measurements (Shirt et al. 1983). The spectral type of the star is somewhat in dispute, ranging from B1 (Hubble 1922) to F0 (Cohen & Kuhi 1979). Hern´andez et al. (2004) suggests that the type is A7±3 subclasses. Leinert et al. (1997) found that LkHα 208 has a companion at a separation of 0.1 arcsec; with a flux partition at 2.2 µm of 1:2 the companion is likely to be also a Herbig Ae/Be star. LkHα 208 is located along the sharp western edge of a large cloud oriented north-south, and Good et al. (1981) attempted to study the cloud structure around the star taking advantage of a lunar occultation. About 7 arcmin to the ESE is the bright Hα emission line star LkHα 209 (Herbig 1960), a G3V star according to Calvet & Cohen (1978).

4.6. IC 2144 The brightest star in the small HII region IC 2144 was discovered by Merrill & Bur- well (1949), in whose catalog of B stars with hydrogen lines in emission it is listed as MWC 778. The rich emission line spectrum and steeply rising energy distribution in the infrared were commented upon by Allen (1974). The IRAS satellite detected the source as IRAS 05471+2351, and further infrared observations were presented by Garcia-Lario et al. (1997). Spectroscopy by Vieira et al. (2003) indicate a possible spectral type of B1, and they suggest that the star could be a Herbig Ae/Be star. A detailed spectroscopic and imaging study of IC 2144 is presented by Herbig & Vacca (2008).

Acknowledgements. CHY thanks the Canada-France-Hawaii for hos- pitality in Waimea while parts of this chapter was written. We would like to thank Kazuhito Dobashi for providing the extinction data used in Fig. 1, and George Her- big for information on IC 2144. We also thank Filipe Alves, Johannes Schedler, John Gleason, and Adam Block for providing the images shown in Figures 2, 4, 11, and 15, respectively. This article has made use of the SIMBAD database, operated at CDS, Strasbourg, France, and NASA’s Bibliographic Services. We also acknowledge the use of NASA’s SkyView facility located at NASA Goddard Space Flight Center and Digitized Sky Surveys. The Digitized Sky Surveys were pro- duced at the Space Telescope Science Institute under U.S. Government grant NAG W-2166. The images of these surveys are based on photographic data obtained using the Oschin Schmidt Telescope on Palomar Mountain and the UK Schmidt Telescope at Siding Spring. BR was supported by the NASA Institute under Coopera- tive Agreement No. NNA04CC08A and by the NSF through grants AST-0507784 and AST-0407005.

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