Star Formation in Bok Globules and Small Clouds

Handbook of Star Forming Regions Vol. II Astronomical Society of the Pacific, c 2008 Bo Reipurth, ed. Star Formation in Bok Globules and Small Clouds Bo Reipurth Institute for Astronomy, University of Hawaii 640 N. Aohoku Place, Hilo, HI 96720, USA Abstract. While most star formation occurs in giant molecular clouds, numerous small clouds and Bok globules are known to each harbor one or a few young stars. Studies of such isolated newborn stars offer insights into the star formation process unencumbered by the confusion that often complicates studies of richer star forming regions. In this chapter, a dozen Bok globules and small clouds have been selected for discussion as examples of small scale star formation. Particularly interesting or well studied cases include BHR 71, CG 12, B62, B93, L723, and B335. 1. Introduction Many Bok globules are known across the sky, and small clouds and cloud fragments are found even more commonly. Despite their numbers, these small objects are not an important contributor to the production of low-mass stars in our Galaxy, accounting for at most a few percent (Reipurth 1983). The importance of these regions lies mostly in their simplicity, which allows us to study the formation of one or a few stars without the confusion that often complicates observations of richer regions of star formation. Bok globules are now widely regarded as remnant cloud cores that have been ex- posed due to the presence of nearby OB stars (Reipurth 1983). The specific processes involved has been under some debate among theoreticians (e.g., Sandford, Whitaker, & Klein 1984, Bertoldi 1989, Lefloch & Lazareff 1994, Kessel-Deynet & Burkert 2003, Miao et al. 2006). While observational evidence for star formation in Bok globules was late in coming (e.g., Bok 1978, Reipurth 1983, Yun & Clemens 1990), numerous cases of star formation in globules are now known. Many surveys have been made of globules at various wavelengths, including Clemens & Barvainis (1988), Yun & Clemens (1992), Bourke, Hyland, & Robinson (1995), Bourke et al. (1995), Wang et al. (1995), Launhardt & Henning (1997), Henning & Launhardt (1998), Launhardt et al. (1998), Huard, Sandell, & Weintraub (1999), Moreira et al. (1999), Ogura, Sugitani, & Pickles (2002), and Kandori et al. (2005). The distinction between a Bok globule and a small cloud is diffuse, as are the terms themselves. The term globule originated in the papers by Bok & Reilly (1947) and Bok (1948), who already then noted that “It is not a simple matter to draw the line between true globules and minor condensations in dark lanes or in regions of variable obscuration.” Bok & Reilly distinguished between small globules, which are seen in contrast against bright HII regions, and large globules; it is these latter that are now known as Bok globules. On empirical grounds, Bok considered these objects to be “compact mostly near-circular dark nebulae” with diameters typically between 3 and 20 arcmin, corresponding to radii likely in the range 0.15 to 0.8 pc (Bok 1977). A number of very interesting star forming clouds exist which are sometimes not much 1 2 larger than Bok globules, and for lack of a better term they are here denoted as “small clouds”. Some clouds, like L810, which have the shape and angular diameter to fall in the category of Bok globules turn out to have such large distances that they do not qualify as bona fide Bok globules, and they too are considered small clouds. Many cases of star formation in globules and small clouds are presented through- out this Handbook, for example cometary globules in the Gum Nebula are discussed in the chapter on Puppis-Vela, B68 and FeSt 1-457 are presented in the chapter by Alves et al., the globules B362 and L1014 are discussed in the chapter on Cygnus, and numerous small clouds are reviewed in the chapter on Cepheus. So many cases are known across the sky that it would be impossible to discuss all here. The following is therefore a very limited selection rather than an attempt to do a more comprehensive presenta- tion (see Table 1). Inevitably the discussion focuses on globules or small clouds that have caught the interest of the author, but hopefully the selection is representative of the many known cases of star formation in globules and small clouds. The sections are ordered after right ascension. Table 1. Bok Globules and Small Clouds discussed in this Chapter Object α2000 δ2000 l b Other ID’s L1438 04:56.9 +51:31 156.2 +5.3 L1439 05:00.1 +52:05 156.1 +6.0 CB26 CB54 07:04.4 –16:23 229.0 –04.6 BHR71 12:01.7 –65:09 297.7 –2.7 Sa136,TGU1840 CG12 13:57.6 –39:59 316.5 +21.1 BHR92,TGU1970,MBM112 L43 16:34.5 –15:47 1.4 +21.0 TGU23 B62 17:15.9 –20:56 3.1 +10.0 L100 B92 18:15.5 –18:11 12.7 –0.6 L323,CB125,TGU135 B93 18:16.9 –18:04 13.0 –0.8 L327,CB131 L483 18:17.5 –04:40 24.9 +5.4 TGU259/P1 L723 19:17.9 +19:12 53.0 +3.0 B335 19:36.9 +07:34 44.9 –6.5 L663,CB199 L810 19:45.4 +27:51 63.6 +1.7 CB205 2. Individual Bok Globules and Small Clouds 2.1. V347 Aur in L1438 L1438 is the central core in a low-extinction cloud complex encompassing many small cores, including L1429, L1430, L1431, L1432, L1433, L1435, L1436, L1437, L1438, and L1439 (CB26) in the catalog of Lynds (1962). The cloud complex is known as TGU 1046 in the cloud atlas of Dobashi et al. (2005), where L1438 is listed as the core TGU 1046 P1. The region is located at the intersection between Auriga, Perseus, and Camelopardalis, see Figure 1 in the chapter by Straizys & Laugalys in Volume I of this Handbook. L1438 harbors the variable star V347 Aur = HBC 428 = IRAS 04530+5126 ◦ ◦ (α2000 4:56:57.0, δ2000 +51:30:51; l = 156.2 , b = 5.3 ), first recognized as variable by Morgenroth (1939). A more detailed photographic study of the light variations was made by Wenzel (1978), who found that V347 Aur had four maxima that were consistent with a period of about a year. V347 Aur is surrounded by a small reflection nebula 3 Figure 1. V347 Aur in its small globule L1438. From the Digitized Sky Survey. known as GM 1-3 (Gyulbudaghian & Magakian 1977), PP 24 (Parsamian & Petrosian 1979), or RNO 33 (Cohen 1980), see Figure 1. A low-dispersion spectrum was taken by Cohen, who noted a rich emission line spectrum superposed on a late-type spectrum. Herbig (priv. comm.) has obtained several high-resolution spectra of V347 Aur, which show very significant spectral variability. Magakian et al. (2008) has discovered a Herbig-Haro object, HH 715, near V347 Aur. The distance to L1438 is unknown, but its proximity to another globule, L1439 (see next section), might suggest that it is at about the same distance, ∼140 pc. 2.2. L1439 About half a degree from L1438 is another small globule, L1439, also known as CB 26 (Clemens & Barvainis 1988). As can be seen in Figure 2, the globule is about 5 ar- cminutes across, and has a faint luminous rim and tail that is facing towards the WSW. Launhardt & Henning (1997) suggested a distance of 300 pc, but based on kinematic ar- guments Launhardt & Sargent (2001) proposed that L1439 is part of the Taurus-Auriga complex at a distance of about 140 pc. The IRAS 04559+5200 source is located towards L1439. Stecklum et al. (2004) have obtained near-infrared images of the globule, and find a bipolar reflection nebula, bisected by a high extinction lane, suggesting the presence of an edge-on circumstellar disk. Using near-infrared imaging polarimetry, Stecklum et al. (2004) find that the illuminator is located towards the center of the extinction lane. About 6 arcmin to the 4 Figure 2. The globule L1439 = CB 26 as seen on the red Digitized Sky Survey. The field is 15′ × 15′, and north is up and east is left. Figure 3. The globule CB 54 as seen on the red Digitized Sky Survey. The field is 15′ × 15′, and north is up and east is left. 5 NNW of the embedded source and along an axis perpendicular to the extinction lane, Stecklum et al. (2004) noted a Herbig-Haro object, HH 494. Henning et al. (2001) used SCUBA to detect a 850 µm source towards the IRAS source. More detailed observations were done by Launhardt & Sargent (2001) with the Owens Valley Radio Observatory millimeter-wave array in the continuum at 1.3 and 2.7 mm, revealing an extended structure about 400 AU wide that precisely coincides with the extinction lane seen in the near-infrared. Additional 13CO (1-0) data display a Keplerian rotation pat- tern about an axis perpendicular to the disk. The central illuminating source has a total luminosity of only ∼0.5 L⊙ and an energy distribution consistent with a Class I source (Stecklum et al. 2004). 2.3. CB 54 CB 54 is a small globule from the catalog of Clemens & Barvainis (1988) and located a little below the Galactic plane in Canis Major. It is associated with a bright rim (see Figure 3) and is listed as LBN 1042 in the catalog of bright nebulae by Lynds (1965). Launhardt & Henning (1997) adopted a distance of 1.5 kpc, but Henning et al. (2001) recognized the probable association of CB 54 with the molecular complex containing the nebula BBW 4, for which Brand & Blitz (1993) suggest a kinematic distance of 1.1 kpc.

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