2004ApJ...606..917R

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)x10⁵/cm3, and a total methyl cyanide column density of 1.4(1)x10¹⁶/cm2. 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)x10⁵/cm3, and a total methyl cyanide column density of 3.8(7)x10¹⁶/cm2. 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 HCOOCH₃the 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.