A Wideband Chemical Survey of Massive Star-forming Regions at Subarcsecond Resolution with the Submillimeter Array

TL;DR

This study uses high-resolution, wideband submillimeter observations to analyze the complex molecular chemistry in massive star-forming regions, revealing chemical evolution and spatial variations in molecular distributions.

Contribution

It provides the first wideband (32 GHz) subarcsecond-resolution survey of multiple massive star-forming regions, detecting hundreds of molecular lines and analyzing their spatial and chemical properties.

Findings

Detected hundreds of lines from over 60 molecules, including complex organic molecules.

Identified chemical evolution from hot cores to HII regions across the sample.

Found spatial offsets among molecular families indicating physical and chemical structure variations.

Abstract

Massive star-forming regions exhibit a rich chemistry with complex gas distributions, especially on small scales. While surveys have yielded constraints on typical gas conditions, they often have coarse spatial resolution and limited bandwidths. Thus, to establish an interpretative framework for these efforts, detailed observations that simultaneously provide high sensitivity, spatial resolution, and large bandwidths for a subset of diverse sources are needed. Here, we present wideband (32 GHz) Submillimeter Array observations of four high-mass star-forming regions (G28.20-0.05, G20.08-0.14 N, G35.58-0.03, W33 Main) at subarcsecond resolution, where we detect and spatially-resolve 100s of lines from over 60 molecules, including many complex organic molecules (COMs). The chemical richness of our sample is consistent with an evolutionary sequence from the line-rich hot cores and HC HII regions of G28.20-0.05 and G20.08-0.14 N to the more chemically-modest UC HII regions in G35.58-0.03, followed by the molecule-poor HII region W33 Main. We detect lines across a range of excitation conditions (Eu=20 to >800 K) and from numerous isotopologues, which enables robust estimates of gas properties. We derive nearly constant COM column density ratios that agree with literature values in other low- and high-mass protostellar cores, supporting the idea that COM abundances are set during the pre-stellar phase. In all regions, we identify spatial offsets among different molecular families, due to a combination of source physical structure and chemistry. In particular, we find potential evidence of carbon grain sublimation in G28.20-0.05 and identify an elemental oxygen gradient and rich sulfur-chemistry in G35.58-0.03. Overall, these results demonstrate that the SMA's wide bandwidth is a powerful tool to untangle the complex molecular gas structures associated with massive star formation.