SO₂ and OCS toward high-mass protostars

A comparative study of ice and gas

¹ Laboratory for Astrophysics, Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
² Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
³ European Southern Observatory, Garching, Germany
⁴ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany

Received: 16 May 2024
Accepted: 17 July 2024

Abstract

Context. OCS and SO₂ are both major carriers of gaseous sulfur and are the only sulfurated molecules detected in interstellar ices to date. They are thus the ideal candidates for exploring the evolution of the volatile sulfur content throughout the different stages of star formation.

Aims. We aim to investigate the chemical history of interstellar OCS and SO₂ by deriving a statistically significant sample of gas-phase column densities toward massive protostars and comparing them to observations of gas and ices toward other sources, from dark clouds to comets.

Methods. We analyzed a subset of 26 line-rich massive protostars observed by ALMA in Band 6 as part of the High Mass Protocluster Formation in the Galaxy (ALMAGAL) survey. Column densities were derived for OCS and SO₂ from their rare isotopologs O¹³CS and ³⁴SO₂ toward the compact gas around the hot cores. We compared the abundance ratios of gaseous OCS, SO₂, and CH₃OH with ice detections toward both high- and low-mass sources as well as dark clouds and comets.

Results. We find that gas-phase column density ratios of OCS and SO₂ with respect to methanol remain fairly constant as a function of luminosity between low- and high-mass sources, despite their very different physical conditions. In our dataset, OCS and SO₂ are weakly correlated. The derived gaseous OCS and SO₂ abundances relative to CH₃OH are overall similar to protostellar ice values, with a significantly larger scatter for SO₂ than for OCS. Cometary and dark-cloud ice values agree well with protostellar gas-phase ratios for OCS, whereas higher abundances of SO₂ are generally seen in comets compared to the other sources. Gaseous SO₂/OCS ratios are consistent with ices toward dark clouds, protostars, and comets, albeit with some scatter.

Conclusions. The constant gas-phase column density ratios throughout low- and high-mass sources indicate an early-stage formation before intense environmental differentiation begins. Icy protostellar values are similar to the gas-phase medians and are compatible with an icy origin for these species followed by thermal sublimation. The larger spread in SO₂ compared to OCS ratios with respect to CH₃OH is likely due to a more water-rich chemical environment associated with the former, as opposed to a CO-rich origin for the latter. Post-sublimation gas-phase processing of SO₂ can also contribute to the large spread. Comparisons to ices in dark clouds and comets point to a significant inheritance of OCS from earlier to later evolutionary stages.