Survey of Ices toward Massive Young Stellar Objects: I. OCS, CO, OCN$^-$, and CH$_3$OH

TL;DR

This survey of 23 massive YSOs reveals widespread OCS ice detection, its correlation with CH3OH and OCN-, and insights into the formation and composition of ices in star-forming regions, with implications for cometary ices.

Contribution

First comprehensive survey of OCS ice in massive YSOs, establishing its correlation with other ices and suggesting formation pathways deep inside dense clouds or protostellar envelopes.

Findings

OCS detected in 20 out of 23 YSOs, five times more than previously known.

OCS correlates with CH3OH and OCN-, but not with H2O or apolar CO.

Median ice composition includes CO, CH3OH, OCN-, and OCS in specific ratios.

Abstract

An important tracer of the origin and evolution of cometary ices is the comparison with ices found in dense clouds and towards Young Stellar Objects (YSOs). We present a survey of ices in the 2-5 micron spectra of 23 massive YSOs, taken with the NASA InfraRed Telescope Facility SpeX spectrometer. The 4.90 micron absorption band of OCS ice is detected in 20 sight-lines, more than five times the previously known detections. The absorption profile shows little variation and is consistent with OCS being embedded in CH3OH-rich ices, or proton-irradiated H$_2$S or SO$_2$-containing ices. The OCS column densities correlate well with those of CH$_3$OH and OCN$^-$, but not with H$_2$O and apolar CO ice. This association of OCS with CH$_3$OH and OCN$^-$ firmly establishes their formation location deep inside dense clouds or protostellar envelopes. The median composition of this ice phase towards massive YSOs, as a percentage of H$_2$O, is CO:CH$_3$OH:OCN$^-$:OCS=24:20:1.53:0.15. CS, due to its low abundance, is likely not the main precursor to OCS. Sulfurization of CO is likely needed, although the source of this sulfur is not well constrained. Compared to massive YSOs, low mass YSOs and dense clouds have similar CO and CH$_3$OH ice abundances, but less OCN$^-$ and more apolar CO, while OCS awaits detection. Comets tend to be under-abundant in carbon-bearing species, but this does not appear to be the case for OCS, perhaps signalling OCS production in protoplanetary disks.