Astron. Astrophys. 338, 223-242 (1998)

6. Summary and conclusions

In search for dense, star-forming cores and molecular outflows we surveyed 35 southern Bok globules for 1.3 mm dust continuum emission as well as ¹²CO (2-1) and CS (2-1) line emission. This is the first comprehensive survey of Bok globules in the southern sky using these tracers. There is an overlap of 20 globules with the ammonia survey of Bourke et al. (1995b) which makes these two surveys a valuable database for further, more detailed investigations of individual globules. The globules were selected from the catalogues of Hartley et al. (1986) and Feitzinger & Stüwe (1984). The selection criteria were: (1) isolated location and compact and opaque appearance and (2) association with a cold IRAS point source ( 35 K). The second criterion clearly biased our sample towards globules with young star-forming cores, in accordance with the goal of this study.

Five globules (marked in Table 4.2) were studied in more detail. The results for DC 297.7-2.8 were already published (Bourke et al. 1997). The results for the other four globules will be published in a succeeding paper.

The main results and conclusions can be summarized as follows:

1. For the first time, reliable it distance estimates were obtained for most of the globules of our sample. A very efficient method of associating the globules with larger molecular cloud complexes was used. Although Bok globules seem to appear as isolated objects, they are, in most cases, still loosely connected with the molecular cloud complexes from which they were originally formed. Half of the objects is located in the local spiral arm at distances between 100 and 400 pc (average distance 300 pc). The most prominent features in the spatial distribution of these globules are the Lindblad ring and the Vela-Gum complex. Eight globules are located at the near side of the Carina arm at distances between 0.7 and 1.3 kpc, and 6 objects are in the far Carina arm at distances beyond 2 kpc. The latter 6 objects are larger and more massive than "normal" globules.

2. Out of the 35 globules observed, all globules were detected in the ¹²CO(2-1) line (detection rate 100% at a 3 detection limit of = 0.3 K), 24 globules were detected in the CS(2-1) line (detection rate 69% at a 3 detection limit of = 0.2 K), and 18 globules were detected in the 1.3 mm continuum emission (detection rate 51% at a 3 detection limit of 40 mJy/beam). In 12 globules (34%), CO line wings indicating the presence of molecular outflows have been found, of which 8 outflows were previously unkown. The colours of the embedded IRAS point sources, the mm dust continuum emission, the CS (and NH₃) line emission as well as the presence of molecular outflows are all well correlated with each other. We found that the 1.3 mm dust continuum emission and the CS line emission are equivalent tracers for the detection of dense, star-forming molecular cloud cores. Therefore, reliable statements on the nature and evolutionary stage of the individual objects could be made.

3. The objects could be divided into it two groups according to the location in the IRAS colour-colour diagram. Group 1 turned out to represent self-embedded protostars (star-forming globule cores with embedded Class 0 and Class I objects). Group 2 comprises non- and pre-star-forming globule cores. In comparison to Paper I, no "star-less" cores nor group 3 objects are contained in this southern sample.

4. The group 1 globule cores span a range in masses between 0.15 (sensitivity limit) and 2.3 with a mean mass of 0.60.25 and a slope of the mass distribution of -1.8. The beam-averaged column and number densities of these cores are = (55) 10²²cm^-2 and = (85) 10⁵cm^-3, respectively. The mean CO and CS linewidths (Gaussian HPWs) are 2.30.25 K and 1.30.2 K, respectively. Two thirds of the group 1 globules have molecular outflows , which ultimatively proofs that these globules form new stars. The momentum fluxes of these outflows compare well to the values derived for Class 0 and I sources in dark cloud complexes.

5. The group 2 globule cores are less massive, more diffuse, and more quiescent than the group 1 sources. Their SEDs are steeply rising from 60 to 100 µm and they were mostly not detected at shorter wavelengths. These globules usually don't have outflows. Their beam-averaged column and number densities are = (10.5) 10²² cm^-2 and = (63) 10⁴ cm^-3 (mostly upper limits), respectively. The mean CO and CS linewidths (Gaussian HPWs) are 1.60.1 K and 1.30.2 K, respectively. We checked that these objects are not simply cirrus clouds, but rather belong to the group of dark clouds.

6. The youngest sources within our sample which clearly resemble the properties of Class 0 protostars being in their main accretion phase are DC 297.7-2.8 and 253.3-1.6. These objects have envelope masses of the order of 2 , / ratios of 0.13 /, and drive the most powerful outflows. The sources in DC 267.4-7.5 and 303.8-14.2 have properties intermediate between Class 0 and Class I objects. The globules DC 267.7-7.4 and 275.9+1.9 harbour more evolved YSOs of Class I. The other local group 1 sources and the objects in the near Carina arm resemble the properties of Class I YSOs, although no clear statement can be made about their exact evolutionary stage.

7. Candidates for pre-protostellar cores within our sample are DC 249.4-5.1, 267.2-7.2, 268.2-9.7, 289.3-2.8, and 319.9-4.8. These group 2 objects are associated with extremely cold IRAS sources. They do not show well-condensed cores in the CS line nor in the 1.3 mm continuum emission. The other group 2 sources don't have cores which are compact and massive enough to form solar-like stars.

8. The objects in the far Carina arm ( kpc) are assumed to be cores of dark cloud complexes with embedded clusters of low-mass YSOs.

9. The (beam-averaged) kinetic gas temperatures are in the range between 10 and 15 K, while the broad-band SEDs are consistent with (mass-averaged) dust temparatures of the order of 25 to 30 K for group 1 sources and of 20 K for group 2 sources. This discrepancy can be partially explained by internal heating, optical depth effects, and thermal de-coupling of gas and dust in the envelopes.

10. We could clearly confirm our finding from Paper I that Bok globules form solar-like stars with typical masses between 0.5 and 1 . More massive dark clouds resembling the simple structure of globules are rare. In contrast to the globules, such clouds often contain multiple cores and are able to form clusters of low-mass stars.

© European Southern Observatory (ESO) 1998

Online publication: September 8, 1998
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