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We study the astrochemical diagnostics of the isolated massive protostarG28.20-0.05. We analyze data from ALMA 1.3~mm observations with resolution of0.2 arcsec ($\sim$1,000 au). We detect emission from a wealth of species,including oxygen-bearing (e.g., $\rm{H_2CO}$, $\rm{CH_3OH}$, $\rm{CH_3OCH_3}$),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 possiblechemical origins. In the central region near the protostar, we identify threehot molecular cores (HMCs). HMC1 is part of a mm continuum ring-like structure,is closest in projection to the protostar, has the highest temperature of$\sim300\:$K, and shows the most line-rich spectra. HMC2 is on the other sideof the ring, has a temperature of $\sim250\:$K, and is of intermediate chemicalcomplexity. HMC3 is further away, $\sim3,000\:$au in projection, cooler($\sim70\:$K) and is the least line-rich. The three HMCs have similar masssurface densities ($\sim10\:{\rm{g\:cm}}^{-2}$), number densities ($n_{\rmH}\sim10^9\:{\rm{cm}}^{-3}$) and masses of a few $M_\odot$. The total gas massin the cores and in the region out to $3,000\:$au is $\sim 25\:M_\odot$, whichis comparable to that of the central protostar. Based on spatial distributionsof peak line intensities as a function of excitation energy, we infer that theHMCs are externally heated by the protostar. We estimate column densities andabundances of the detected species and discuss the implications for hot coreastrochemistry.

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