Evolutions of CH3CN abundance in molecular clumps

Abstract

To investigate the effects of massive star evolution on surrounding molecules, we select nine 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 ratios L clump/M clump that range from 10 to 154 L ⊙/M ⊙, where some of them embedded ultra compact (UC) HII region. Using the SMA, CH3CN (12K–11K) transitions were observed toward nine massive star-forming regions. We derive the CH3CN rotational temperature and column density using the XCLASS program, and calculate its fractional abundance. We find that CH3CN temperature seems to increase with the increase of L clump/M clump when the ratio is between 10 to 40 L ⊙/M ⊙, then decrease when L clump/M clump ≥ 40 L ⊙/M ⊙. Assuming that the CH3CN gas is heated by radiation from the central star, the effective distance of CH3CN relative to the central star is estimated. The distance ranges from ∼ 0.003 to ∼ 0.083 pc, which accounts for ∼ 1/100 to ∼ 1/1000 of clump size. The effective distance increases slightly as L clump/M clump increases (R eff ∼ (L clump/M clump)0.5±0.2). Overall, the CH3CN abundance is found to decrease as the clumps evolve, e.g., X CH3CN ∼ (L clump/M clump)−1.0 ± 0.7. The steady decline of CH3CN abundance as the clumps evolution can be interpreted as a result of photodissociation.