Astron. Astrophys. 362, 635-645 (2000)

Astron. Astrophys. 362, 635-645 (2000)

1. Introduction

Most of the molecular clouds of small angular extent (Clemens & Barvainis 1988; Bourke et al. 1995) are physically small and nearby. Such clouds, known as Bok globules, are often sites of star formation typically containing only a few forming stars (Huard et al. 1999; Yun & Clemens 1990, 1994b). However, some clouds of small angular extent are actually physically large and distant (beyond 1 kpc). Such clouds are referred to as "large globules." Because large globules have small apparent diameters, young stellar objects (YSOs) observed in the vicinity of large globules are likely associated with the globules. Since large globules are much more massive than Bok globules, star formation within such globules tends to produce small stellar clusters rather than aggregates of only a few stars.

Surveys of the YSO populations of nearby clouds demonstrate that stellar cluster formation in these clouds is a continuous process, rather than a relatively instantaneous event (e.g., Sandell & Knee 2000, Motte et al. 1998, Testi & Sargent 1998). Since the sizes and masses of large globules are comparable to those of nearby, star-forming molecular clouds, we might expect the YSO populations of large globules to be similar to those of the nearby clouds. Because large globules have small angular extents, YSO surveys of large globules can be done efficiently without mapping a large region of the sky.

YSOs of different evolutionary stages are detectable in different regions of the spectrum. Very young protostars, known as Class 0 sources, are surrounded by large amounts of cold dust and are sources of outflows. Thus, protostars are detected by the submillimeter continuum emission from the surrounding dust or by molecular line observations identifying outflow activity. Since the optical depths at near-infrared wavelengths are very large toward protostars, they are never detected in near-infrared observations. More evolved YSOs, referred to as Class I and Class II sources, are surrounded by less dust and thus are faint in the submillimeter. However, because the central stars of the Class I or Class II sources are less obscured, these sources are observed as heavily reddened near-infrared souces.

Submillimeter mapping offers the best way to identify the young, protostellar population within a star-forming region. As an initial probe of current star formation activity within star-forming large globules, we have mapped cold IRAS sources associated with the large globules CB 3, CB 34, and L 810 in the submillimeter continuum. The colors of the associated IRAS sources suggest that these large globules currently are forming stars. Furthermore, the IRAS sources are known to be in the vicinity of clusters of YSOs, seen in near-infrared surveys, which have formed within these globules.

Online publication: October 24, 2000
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