Ongoing Cluster Formation in Large Globules

Most of the molecular clouds of small angular extent are physically small and nearby. Such clouds, known as Bok globules, are often sites of star formation typically containing only a few forming stars. However, some clouds of small angular extent are actually physically large and distant (beyond 1 kpc). Since these ``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.

The three large globules CB 3, CB 34, and L810 were mapped at 450 um and 850 um using SCUBA on the JCMT. These globules are known to be associated with small clusters of near-infrared YSOs, representing evolved protostars that had formed in the large globules. Our maps of these large globules reveal submillimeter sources, presumably the thermal emission from the dust surrounding protostars embedded within the globules. These submillimeter sources have rather large sizes 0.07-0.2 pc and have masses 1-40 M_o, at least an order of magnitude greater than those of low-mass protostars found in Bok globules. The submillimeter sources detected toward the large globules probably represent clusters of young protostars rather than individual sources. If the large globules in this study are similar to nearby star-forming cores (e.g. NGC 1333 and Rho Oph) for which near-infrared and submillimeter observations have revealed clusters of embedded Class 0 and Class I protostars, then the submillimeter sources detected in our survey are not likely to be submillimeter counterparts to the near-infrared YSOs. Rather, the submillimeter sources may be small clusters of young, Class 0 protostars perhaps blended with a slightly more evolved Class I protostar. The detection of submillimeter sources toward the clusters of evolved near-infrared YSOs within CB 3, CB 34, and L810, the large sizes and masses derived for these submillimeter sources, and comparisons made between these distant sources and those within nearby star forming regions suggest that these large globules are currently adding members to the YSO clusters. Our results suggest that cluster formation in large globules is a process that occurs over millions of years, consistent with previous studies of cluster formation in nearby large molecular clouds such as Orion, Rho Ophiuchus, and Serpens.

The 850 micron map of the YSO cluster embedded within CB 34 is shown above. Besides showing dust emission extended across much of the region containing the evolved, near-infrared YSO cluster, this map reveals several submillimeter sources. The positions of these sources are identified by asterisks and the beamsize is represented by the circle in the upper right corner of the map. The positions of the known near-infrared sources within the field are identified by open diamonds and plus signs, where the plus signs are those sources exhibiting near-infrared excesses and therefore thought to be YSOs associated with the globule.

A preprint of this paper, accepted for publication in Astronomy & Astrophysics, is available here.