Chemistry of the High-mass Protostellar Molecular Clump IRAS 16562-3959

Abstract

We present molecular line observations of the high-mass molecular clump IRAS 16562−3959 taken at 3 mm using the Atacama Large Millimeter/submillimeter Array (ALMA) at 1. 007 angular resolution (0.014 pc spatial resolution). This clump hosts the actively accreting high-mass young stellar object (HMYSO) G345.4938+01.4677, associated with a hypercompact Hii region. We identify and analyze emission lines from 22 molecular species (encompassing 34 isomers) and classify them into two groups, depending on their spatial distribution within the clump. One of these groups gathers shock tracers (e.g., SiO, SO, HNCO) and species formed in dust grains like methanol (CH3OH), ethenone or ketene (H2CCO), and acetaldehyde (CH3CHO). The second group collects species resembling more the dust continuum emission morphology and are formed mainly in the gas-phase, like hydrocarbons (CCH, c-C3H2, CH3CCH), cyanopolyynes (HC3N and HC5N) and cyanides (HCN and CH3C3N). Emission from complex organic molecules (COMs) like CH3OH, propanenitrile (CH3CH2CN), and methoxymethane (CH3OCH3) arise from gas in the vicinity of a hot molecular core (T & 100 K) associated with the HMYSO. Other COMs such as propyne (CH3CCH), acrylonitrile (CH2CHCN), and acetaldehyde seem to better trace warm (T . 80 K) dense gas. In addition, deuterated ammonia (NH2D) is detected mostly in the outskirts of IRAS 16562−3959 and associated with near-infrared dark globules, probably gaseous remnants of the clump’s prestellar phase. The spatial distribution of molecules in IRAS 16562−3959 supports the view that in protostellar clumps, chemical tracers associated with different evolutionary stages — starless to hot cores/Hii regions — exist coevally.