Publication Abstract

High-Resolution Observations of Methyl Cyanide (CH$_3$CN) toward the Hot Core Regions W51e1/e2

Remijan, A., Sutton, E. C., Snyder, L. E., Friedel, D. N., Liu, S.-Y., and Pei, C.-C.

We have detected strong methyl cyanide (CH3CN) emission lines from the hot core regions W51e1 and W51e2, using the BIMA Array. This is the first survey of CH3CN toward W51 to utilize both 3 mm (J=5-4 and 6-5) and 1 mm (J=12-11, 13-12, and 14-13) transitions as probes of the physical and chemical conditions present in these regions. The emission reveals molecular clumps centered on the ultracompact H II regions found by Zhang and colleagues. The CH3CN lines show large optical depths in the lower K transitions toward both regions in W51. To determine the true kinetic temperatures, densities, and column densities of the emitting regions W51e1 and e2, statistical equilibrium models were used to calculate the relative populations of each energy level. The best fit to the observed spectra toward W51e1 is given by a temperature of 123(11) K, a hydrogen density of 5(1)×105 cm-3, and a total methyl cyanide column density of 1.4(1)×1016 cm-2. The uncertainties describe a nominal 90% confidence interval for the last digit given. The best fit to the observed spectra toward W51e2 is given by a temperature of 153(21) K, a hydrogen density of 5(2)×105 cm-3, and a total methyl cyanide column density of 3.8(7)×1016 cm-2. Our observations indicate that CH3CN can be used as a good probe of the physical conditions present in hot molecular cores and as a tracer of hard-to-detect large molecular species. Despite the differences in molecular structure and chemical formation mechanisms, methyl cyanide (CH3CN), ethyl cyanide (CH3CH2CN), and acetic acid (CH3COOH) are found to have similar abundances toward the W51e1 and e2 regions. In contrast, for a column density of CH3CN more than 15 times smaller than the column density of HCOOCH3 the integrated line flux is more than 7 times larger. Thus, because CH3CN lines are easy to detect, it appears to be a much better tracer of CH3CH2CN and CH3COOH than HCOOCH3.

Published in the Astrophysical Journal vol. 606, pp. 917-928

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© 2015, D. N. Friedel