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${}_{\mathrm{clump}}$/M${}_{\mathrm{clump}}$ 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${}_{\mathrm{K}}$--11${}_{\mathrm{K}}$) 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${}_{\mathrm{clump}}$/M${}_{\mathrm{clump}}$ when the ratio is between 10 to 40 L$_{\sun}$/M$_{\sun}$, then decrease when L${}_{\mathrm{clump}}$/M${}_{\mathrm{clump}}$ $\ge$ 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${}_{\mathrm{clump}}$/M${}_{\mathrm{clump}}$ increases (R${}_{\mathrm{eff}}$ $\sim$ (L${}_{\mathrm{clump}}$/M${}_{\mathrm{clump}}$)${}^{0.5\pm 0.2}$). Overall, the CH${}_3$CN abundance is found to decrease as the clumps evolve, e.g., X${}_{\mathrm{C}{\mathrm{H}}_3\mathrm{CN}}$ $\sim$ (L${}_{\mathrm{clump}}$/M${}_{\mathrm{clump}}$)${}^{-1.0\pm 0.7}$. The steady decline of CH${}_3$CN abundance as the clumps evolution can be interpreted as a result of photodissociation.