ALMA ACA study of the H$_2$S/OCS ratio in low-mass protostars

Abstract

The identification of the main sulfur reservoir on its way from the diffuse interstellar medium to the cold dense star-forming cores and eventually to protostars is a long-standing problem. Despite sulfur's astrochemical relevance, the abundance of S-bearing molecules in dense cores and regions around protostars is still insufficiently constrained. The goal of this investigation is to derive the gas-phase H$_2$S/OCS ratio for several low-mass protostars, which could provide crucial information about the physical and chemical conditions in the birth cloud of Sun-like stars. Using ALMA ACA Band 6 observations, H$_2$S, OCS, and their isotopologs are searched for in 10 Class 0/I protostars with different source properties such as age, mass, and environmental conditions. An LTE model is used to fit synthetic spectra to the detected lines and to derive the column densities based solely on optically thin lines. The H$_2$S and OCS column densities span four orders of magnitude across the sample. The H$_2$S/OCS ratio is found to be in the range from 0.2 to above 9.7. IRAS 16293-2422 A and Ser-SMM3 have the lowest ratio, while BHR71-IRS1 has the highest. Only the H$_2$S/OCS ratio of BHR71-IRS1 agress within uncertainties with the ratio in comet 67P/C$-$G. The determined gas-phase H$_2$S/OCS ratios can be below the upper limits on the solid-state ratios by as much as an order of magnitude. The H$_2$S/OCS ratio depends significantly on the environment of the birth cloud, such as UV-irradiation and heating received prior to the formation of a protostar. The highly isolated birth environment of BHR71-IRS1 is hypothesized to be the reason for its high gaseous H$_2$S/OCS ratio due to lower rates of photoreactions and more efficient hydrogenation reactions under such dark, cold conditions. The gaseous inventory of S-bearing molecules in BHR71-IRS1 appears to be most similar to that of interstellar ices.