Knowledge Bank

We have detected strong methyl cyanide $(CH3CN)$ emission lines from the hot core regions W51 e1 and W51 e2 using the BIMA Array. This is the first survey of $CH3CN$ toward W51 to utilize both $3mm(J=5-4\&6-5)$ and $1mm(J=12-11,13-12\&14-13)$ transitions as probes of the physical and chemical conditions present in these regions. To determine the true kinetic temperatures, densities and column densities of the emitting regions W51 e1 and e2, statistical equilibrium models were used to calculate the relative populations of each energy level. The best fit to the observed spectra toward W51 e1 is given by a temperature of $123(11)$ K, a hydrogen density of $5(1)\times 105cm-3$ and a total methyl cyanide column density of $1.4(1)\times 1016cm-2$. The best fit to the observed spectra toward W51 e2 is given by a temperature of $153(21)$ K, a hydrogen density of $5(2)\times 105cm-3$ and a total methyl cyanide column density of $3.8(7)\times 1016cm-2$. 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 W51 e1 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 $HCOOCH3$.