Studies of star formation in isolated small dark clouds – II. A southern ammonia survey

A study of the set of small, southern molecular clouds (globules) compiled by Bourke, Hyland&Robinson has been undertaken, through radio observations of ammonia using the Parkes 64-m telescope. The aim of the study is to determine the physical characteristics of the globules, their role in the formation of low-mass stars, and the physical mechanism that triggers the star formation process, or stabilizes the globules against collapse. With this general aim in mind, the (1, 1) and (2, 2) inversion transitions of ammonia have been surveyed in order to determine the densities, temperatures and masses of the globules. Half of the globules have been detected in ammonia, but only 6 per cent of the detections are ‘strong’ $$(T_{\text a}^{*}\ge 0.35 K)$$. Comparing the globule properties with those of Benson&Myers for cores within complexes, we find that the globules are less opaque and less dense, and are less active sites of star formation. Other properties are comparable. The Vela cometary globules are detected more readily in ammonia than the more isolated globules, and are more active star formation sites. These results suggest that the dense core's environment, in particular the presence of either a large external mass or a significant stellar wind, plays an important role in initiating the star formation process.