All stars are formed in molecular clouds, but not all molecular clouds necessarily form stars. According to our best theories, gas in the Galaxy should be forming stars much faster than we actually observe. Most large clouds of gas are converting a few percent of their mass into stars, but some do not, and we'd like to know why.

Nearby isolated Bok globules are ideal for detailed study of the initial properties of individual starforming cores, their velocity structure, chemical evolution, and the physics of fragmentation and collapse. The Coalsack nebula appears to be a prime example of a nonstarforming molecular cloud. This 7 × 5 degree region, 70 light years in diameter, is approximately 550 light years away and is easily visible to the naked eye as a dark patch next to the Southern Cross. It offers an excellent opportunity to probe the physical nature of dense, dusty molecular cloud cores. We have been studying the densest parts of the Coalsack to try to understand if stars have started to be formed there and if not, why not.

The dense molecular cores comprising the dark Coalsack Nebula are of great interest in studies of early star formation. Although these cores demonstrate conditions appropriate for initial cloud collapse and protostar formation, they appear anomalously quiescent. High precision, multitransitional emission line mapping of the turbulent cores and resolution of the substructures making up the Coalsack has now been made possible with the Mopra radio telescope in the Warrumbungles near Coonabarabaran, NSW. The determination of temperature and density variations in this region afforded by the several CO (carbon monoxide) transitions and other lines will enable effective modeling of the cooling, dynamics and chemistry of these intriguing cores.

The CS emission line integrated intensity map shows a fragmenting ring in a typical starless Coalsack globule.

Martin Saul and Wilfred Walsh