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

TL;DR

This study uses high-resolution ALMA observations to analyze molecular emissions in a high-mass protostellar clump, revealing diverse chemical environments and supporting the coexistence of different evolutionary stages within the same region.

Contribution

It provides detailed molecular line analysis of IRAS 16562-3959, identifying distinct chemical groups and their spatial distributions, advancing understanding of chemical complexity in high-mass star-forming regions.

Findings

Identification of shock tracers and grain-surface molecules in different regions.

Detection of complex organic molecules near the hot core.

Evidence for coeval presence of multiple evolutionary stages.

Abstract

We present molecular line observations of the high-mass molecular clump IRAS 16562$-$3959 taken at 3 mm using the Atacama Large Millimeter/submillimeter Array (ALMA) at 1.$\!\!^{\prime\prime}$7 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 (CH$_3$OH), ethenone or ketene (H$_2$CCO), and acetaldehyde (CH$_3$CHO). The second group collects species resembling the dust continuum emission morphology and which are formed mainly in the gas-phase, like hydrocarbons (CCH, c-C$_3$H$_2$, CH$_3$CCH), cyanopolyynes (HC$_3$N and HC$_5$N) and cyanides (HCN and CH$_3$C$_3$N). Emission from complex organic molecules (COMs) like CH$_3$OH, propanenitrile (CH$_3$CH$_2$CN), and methoxymethane (CH$_3$OCH$_3$) arise from gas in the vicinity of a hot molecular core ($T\gtrsim100$ K) associated with the HMYSO. Other COMs such as propyne (CH$_3$CCH), acrylonitrile (CH$_2$CHCN), and acetaldehyde seem to better trace warm ($T\lesssim80$ K) dense gas. In addition, deuterated ammonia (NH$_2$D) is detected mostly in the outskirts of IRAS 16562$-$3959, associated with near-infrared dark globules, probably gaseous remnants of the clump's prestellar phase. The spatial distribution of molecules in IRAS 16562$-$3959 supports the view that in protostellar clumps, chemical tracers associated with different evolutionary stages --- starless to hot cores/HII regions --- exist coevally.