A Multitransition CO and CS (2–1) Comparison of a Star‐forming and a Non–Star‐forming Giant Molecular Cloud

Abstract

CO (3-2), CO (1-0), and CS (2-1) observations of clumps in the cold, low-luminosity cloud G216-2.5 discovered by Maddalena & Thaddeus are compared to the star-forming Rosette molecular cloud. The comparisons suggest that the clumps in a cloud may be characterized as being either dormant, incipient star forming, or star forming. In the Rosette molecular cloud, each set accounts for, respectively, 80%, 10%, and 10% of the total mass, but in G216-2.5, nearly 100% of the clumps are dormant. The physical conditions of the clumps in both clouds suggest a mass agglomeration evolutionary sequence from dormant to star-forming clumps. Detailed results for the clumps in both clouds are as follows. Clump excitation conditions are remarkably uniform in G216-2.5 but show wide variation in the Rosette. CO (3-2) integrated intensities and the ratio of (3-2) to (1-0) emission are significantly greater in the star-forming cloud and greatest of all in those clumps with embedded IRAS sources. The ratio of CO (3-2) to (1-0) line widths is also greater in the Rosette cloud. Peak clump CO (1-0) temperatures are greater in the Rosette than G216-2.5, implying higher gas kinetic temperatures, and are highest of all for those clumps associated with IRAS sources. The ratios of peak CO (3-2) to (1-0) temperatures, however, are comparable in the two clouds, which implies that the volume density of emitting gas in the clumps in each cloud is similar, nH2 103 cm-3. The CS observations indicate the presence of denser gas, nH2 ~ 105 cm-3, in the clumps in each cloud. CS integrated intensities are generally an order of magnitude weaker than 13CO emission in each cloud, but the ratio of the two is a factor of 2 less in G216-2.5. CS to 13CO line width ratios are also lower in G216-2.5, which suggests that there is a deficiency of dense gas relative to the Rosette. Again, the star-forming clumps in the Rosette possess the highest ratios. In addition, CO (2-1) emission was mapped over the central region of G216-2.5 and compared to a CO (1-0) map. The ratio of (2-1) to (1-0) integrated intensities increases toward the clump edges, which is opposite to the Sakamoto et al. study of the Orion molecular cloud. High-resolution 13CO (2-1) maps of one clump in each cloud are compared to 13CO (1-0) maps for evidence of further fragmentation. The (2-1) radial profile is steeper than the (1-0) profile in each clump but decreases at the same relative rate in the two clumps despite their different absolute sizes. We conclude that the differences between clouds and clumps that are forming stars are most readily apparent in the warmer, denser gas traced by the CO (3-2) and CS (2-1) observations and note that there are two starless clumps in the Rosette molecular cloud with CO properties that are more characteristic of the star-forming clumps than the other starless clumps: these are the best candidates for the sites of future star formation in the cloud.