Sulphur-bearing molecule survey towards prestellar and starless cores in the Perseus molecular cloud

Abstract

Sulphur is drastically depleted from the gas phase in dense starless and prestellar cores, creating the long-standing "missing sulphur problem". Observations of sulphur-bearing molecules are crucial to understanding sulphur chemistry, core-environment relationships, and the physical evolution of star-forming regions. We present a survey of four sulphur-bearing molecules, HCS+, OCS, H2CS, and SO, towards 15 starless and prestellar cores in the Perseus molecular cloud using the Yebes 40m telescope. The HCS+ 1-0, OCS 3-2 and 4-3, H2CS 1_{0,1}-0_{0,0}, and SO 3_2-2_2 transitions were observed, fitted with Gaussian profiles, and column densities were calculated assuming local thermodynamic equilibrium (LTE). We also compare these with observations of CS, CCS, and CCCS from a previous study. HCS+, OCS, H2CS, and SO are detected in 100%, 93%, 100%, and 73% of the sample, respectively. Derived column densities are (2.2-17.0)x10^11 cm^-2 for HCS+, (1.9-25.0)x10^12 cm^-2 for OCS, (1.0-15.0)x10^12 cm^-2 for H2CS, and (0.3-17.0)x10^13 cm^-2 for SO, spanning over an order of magnitude. HCS+ and SO abundances strongly correlate (inversely and directly, respectively) with core volume density, reinforcing their early- and late-type status. Correlations indicate a differentiation between carbon and oxygen chemistry for sulphur-bearing molecules, with CS showing the strongest correlations as expected. Cores in protostellar outflow paths do not show consistently higher abundances. The chemical age (SO/CS) correlates with physical age (volume density), except for Per752, an outlier likely linked to an enhanced cosmic ray ionization rate. These results provide a robust observational baseline constraining chemical models of sulphur chemistry in early star formation.