Astrochemical Diagnostics of the Isolated Massive Protostar G28.20-0.05

Abstract

We study the astrochemical diagnostics of the isolated massive protostar G28.20-0.05. We analyze data from Atacama Large Millimeter/submillimeter Array 1.3 mm observations with a resolution of 0.″2 (∼1000 au). We detect emission from a wealth of species, including oxygen-bearing (e.g., H _2 CO, CH _3 OH, CH _3 OCH _3 ), sulfur-bearing (SO _2 , H _2 S), and nitrogen-bearing (e.g., HNCO, NH _2 CHO, C _2 H _3 CN, C _2 H _5 CN) molecules. We discuss their spatial distributions, physical conditions, correlation between different species, and possible chemical origins. In the central region near the protostar, we identify three hot molecular cores (HMCs). HMC1 is part of a millimeter continuum ring-like structure, is closest in projection to the protostar, has the highest temperature of ∼300 K, and shows the most line-rich spectra. HMC2 is on the other side of the ring, has a temperature of ∼250 K, and is of intermediate chemical complexity. HMC3 is further away, ∼3000 au in projection, cooler (∼70 K), and is the least line-rich. The three HMCs have similar mass surface densities (∼10 g cm ^−2 ), number densities ( n _H ∼ 10 ^9 cm ^−3 ), and masses of a few solar masses. The total gas mass in the cores and in the region out to 3000 au is ∼25 M _⊙ , which is comparable to that of the central protostar. Based on spatial distributions of peak line intensities as a function of excitation energy, we infer that the HMCs are externally heated by the protostar. We estimate column densities and abundances of the detected species and discuss the implications for hot core astrochemistry.

Keywords

• Star formation
• Astrochemistry
• Complex organic molecules
• Spectral line identification
• Massive stars
• Interstellar medium