The Chemical and Physical Structure of Giant Molecular Cloud Cores

Details The Chemical and Physical Structure of Giant Molecular Cloud Cores The Chemical and Physical Structure of Giant Molecular Cloud Cores

Sep 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995phdt........20b&link_type=abstract

Thesis (PH.D.)--UNIVERSITY OF MASSACHUSETTS, 1995.Source: Dissertation Abstracts International, Volume: 56-11, Section: B, page:

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Scientific paper

We present the results of a study of the emission from 33 molecular transitions in the GMC cores Orion A, M17, and Cepheus A. We have mapped the emission over a 4' × 12' area in Orion, and a 4' × 5' area in M17 and Cepheus A. We have used these observations to probe the physical and chemical structure within each core. To derive the temperature we have used the J = 6 --> 5 (K = 0,1,2) transitions of CH3C2H and the J = 1 --> 0 transition of 12CO. A comparison of temperatures obtained from the two thermometers in Orion and M17 show significant differences, which we propose are the result of a line of sight temperature gradient. Densities were determined through a multitransitional study of HC3N. The density structure within each core is similar, showing no evidence of large scale variations, with values typically n(H2) ~106 cm-3. The temperatures and densities determined directly from CH3C2H and HC3N were used to derive abundances for 12 species in six positions in Orion A, and two positions in M17 and Cepheus A. Although abundance differences exist, in general, chemical abundances are found to be similar both within and among GMC cores. We have constructed a chemical model of GMC cores, accounting for variations with AV, to compare with measured abundances. This model was found to be unable to reproduce the abundances of many molecules for any particular time. Both the inclusion of clumps and variations in the gas-phase C/O ratio within the model were examined to obtain between agreement with observations. We found that the inclusion of clumps into the chemical model can reproduce the observed abundances of C+ and C. However, due to the greater weight placed on the photon dominated region in smaller clumps, clumps have a detrimental effect on reproducing the abundances of other species. Models with a range of C/O ratios were also compared to the abundances measured in each cloud. Good agreement between model and observations was found for early times (t ~105 yr) and for C/O ~ 0.8. The implications of these results on the evolutionary state of GMC cores are discussed.

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