Evolutions of CH$_3$CN abundance in molecular clumps

Abstract

To investigate the effects of massive star evolution on surrounding molecules, we select 9 massive clumps previously observed with the Atacama Pathfinder Experiment (APEX) telescope and the Submillimeter Array (SMA) telescope. Based on the observations of APEX, we obtain luminosity to mass ratio L$_{\rm clump}$/M$_{\rm clump}$ that range from 10 to 154 L$_{\sun}$/M$_{\sun}$, where some of them embedded Ultra Compact (UC) H\,{\footnotesize II} region. Using the SMA, CH$_3$CN (12$_{\rm K}$--11$_{\rm K}$) transitions were observed toward 9 massive star-forming regions. We derive the CH$_3$CN rotational temperature and column density using XCLASS program, and calculate its fractional abundance. We find that CH$_3$CN temperature seems to increase with the increase of L$_{\rm clump}$/M$_{\rm clump}$ when the ratio is between 10 to 40 L$_{\sun}$/M$_{\sun}$, then decrease when L$_{\rm clump}$/M$_{\rm clump}$ $\ge$ 40 L$_{\sun}$/M$_{\sun}$. Assuming the CH$_3$CN gas is heated by radiation from the central star, the effective distance of CH$_3$CN relative to the central star is estimated. The distance range from $\sim$ 0.003 to $\sim$ 0.083 pc, which accounts for $\sim$ 1/100 to $\sim$ 1/1000 of clump size. The effective distance increases slightly as L$_{\rm clump}$/M$_{\rm clump}$ increases (R$_{\rm eff}$ $\sim$ (L$_{\rm clump}$/M$_{\rm clump}$)$^{0.5\pm0.2}$). Overall, the CH$_3$CN abundance is found to decrease as the clumps evolve, e.g., X$_{\rm CH_3CN}$ $\sim$ (L$_{\rm clump}$/M$_{\rm clump}$)$^{-1.0\pm0.7}$. The steady decline of CH$_3$CN abundance as the clumps evolution can be interpreted as a result of photodissociation.