Chemical Diversity in Three Massive Young Stellar Objects associated with 6.7 GHz CH${}_3$OH Masers

Abstract

We have carried out observations in the 42$-$46 and 82$-$103 GHz bands with the Nobeyama 45-m radio telescope, and in the 338.2$-$339.2 and 348.45$-$349.45 GHz bands with the ASTE 10-m telescope toward three high-mass star-forming regions containing massive young stellar objects (MYSOs), G12.89+0.49, G16.86$-$2.16, and G28.28$-$0.36. We have detected HC${}_3$N including its ${}^{13}$C and D isotopologues, CH${}_3$OH, CH${}_3$CCH, and several complex organic molecules (COMs). Combining our previous results of HC${}_5$N in these sources, we compare the $N$(HC${}_5$N)/$N$(CH${}_3$OH) ratios in the three observed sources. The ratio in G28.28$-$0.36 is derived to be $0.091_{-0.039}^{+0.109}$, which is higher than that in G12.89+0.49 by one order of magnitude, and that in G16.86$-$2.16 by a factor of $\sim 5$. We investigate the relationship between the $N$(HC${}_5$N)/$N$(CH${}_3$OH) ratio and the $N$(CH${}_3$CCH)/$N$(CH${}_3$OH) ratio. The relationships of the two column density ratios in G28.28$-$0.36 and G16.86$-$2.16 are similar to each other, while HC${}_5$N is less abundant when compared to CH${}_3$CCH in G12.89+0.49. These results imply a chemical diversity in the lukewarm ($T\sim 20-30$ K) envelope around MYSOs. Besides, several spectral lines from complex organic molecules, including very-high-excitation energy lines, have been detected toward G12.89+0.49, while the line density is significantly low in G28.28$-$0.36. These results suggest that organic-poor MYSOs are surrounded by a carbon-chain-rich lukewarm envelope (G28.28$-$0.36), while organic-rich MYSOs, namely hot cores, are surrounded by a CH${}_3$OH-rich lukewarm envelope (G12.89+0.49 and G16.86$-$2.16).

24 pages, 11 figures, 10 tables, Accepted for publication in The Astrophysical Journal