A Cyanopolyyne-rich but Complex-organic-molecule-poor Massive Protostar: The First Hot Carbon-chain Chemistry Source G28.28-0.36

We present molecular emission line data from the massive young stellar object (MYSO) G28.28-0.36 (G28.28) obtained with the Atacama Large Millimeter/submillimeter Array Band 3. Cyanopolyynes (HC 3 N and HC 5 N) and three complex organic molecules (COMs; CH 3 OH, CH 3 CN, and CH 3 CHO) are detected from the MYSO G28.28. In addition, strong emission regions of cyanopolyynes are identified between G28.28 and a nearby ultracompact H ii region. The HC 5 N emission is coincident with the dust continuum peak, where an excitation temperature of 100 K is derived from CH 3 CN. These results suggest that the hot carbon-chain chemistry (HCCC) mechanism produces cyanopolyynes in the hot region around G28.28. We find that G28.28 exhibits a unique chemical feature: cyanopolyynes are abundant, but COMs are deficient, unlike the other MYSOs studied previously. These results imply that G28.28 is a counterpart of the warm carbon-chain chemistry (WCCC) low-mass source L1527. G28.28 is the first HCCC source identified so far.