High‐Resolution Observations of Methyl Cyanide (CH 3 CN) toward the Hot Core Regions W51e1/e2

Abstract

We have detected strong methyl cyanide (CH3CN) emission lines fromthe hot core regions W51e1 and W51e2, using the BIMA Array. Thisis the first survey of CH3CN toward W51 to utilize both 3 mm (J=5-4and 6-5) and 1 mm (J=12-11, 13-12, and 14-13) transitions as probesof the physical and chemical conditions present in these regions.The emission reveals molecular clumps centered on the ultracompactH II regions found by Zhang and colleagues. The CH3CN lines showlarge optical depths in the lower K transitions toward both regionsin W51. To determine the true kinetic temperatures, densities, andcolumn densities of the emitting regions W51e1 and e2, statisticalequilibrium models were used to calculate the relative populationsof each energy level. The best fit to the observed spectra towardW51e1 is given by a temperature of 123(11) K, a hydrogen densityof 5(1)�105 cm-3, and a total methyl cyanide column density of 1.4(1)�1016cm-2. The uncertainties describe a nominal 90% confidence intervalfor the last digit given. The best fit to the observed spectra towardW51e2 is given by a temperature of 153(21) K, a hydrogen densityof 5(2)�105 cm-3, and a total methyl cyanide column density of 3.8(7)�1016cm-2. Our observations indicate that CH3CN can be used as a goodprobe of the physical conditions present in hot molecular cores andas a tracer of hard-to-detect large molecular species. Despite thedifferences 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 ande2 regions. In contrast, for a column density of CH3CN more than15 times smaller than the column density of HCOOCH3 the integratedline flux is more than 7 times larger. Thus, because CH3CN linesare easy to detect, it appears to be a much better tracer of CH3CH2CNand CH3COOH than HCOOCH3.