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[61.16] High-Resolution Observations of Methyl Cyanide (CH₃CN) toward the Hot Core Regions W51e1/e2

A. Remijan (NASA Goddard Space Flight Center), E. C. Sutton, L. E. Snyder, D. N. Friedel (University of Illinois), S.-Y. Liu (Institute of Astronomy and Astrophysics, Academia Sinica), C.-C. Pei (Purple Mountain Observatory, National Astronomical Observatories, Chinese Academy of Sciences)

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 & 6-5) and 1 mm (J=12-11, 13-12 & 14-13) transitions as probes of the physical and chemical conditions present in these regions. The emission reveals molecular clumps centered on the ultra-compact HII 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. 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 rather than HCOOCH₃.

The author(s) of this abstract have provided an email address for comments about the abstract: aremijan@pop900.gsfc.nasa.gov

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Bulletin of the American Astronomical Society, 36 #2
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