Tracing the sulfur depletion in starless and pre-stellar cores

Abstract

Sulfur is one of the most abundant elements in the Universe, yet the sulfur budget inferred from the observed sulfur-bearing molecules in dense cores is significantly lower than expected. Starless and pre-stellar cores represent the earliest stages of star formation and provide a laboratory for studying the physical and chemical processes that cause sulfur depletion. We aim to constrain sulfur chemistry in dense cores by measuring abundances of sulfur-bearing molecules and how they reflect core evolution and environmental effects. We observed nine cores in the Taurus Molecular Cloud, targeting 13 sulfur-bearing molecules, including CS, CCS, C$_3$S, OCS, SO, SO$_2$, H$_2$CS, and isotopologs. Molecular abundances and six abundance ratios were compared to three evolutionary tracers: H$_2$ column density, N$_2$D$^+$/N$_2$H$^+$, and the CO depletion factor. We also compared observations with 0D chemical models with different initial sulfur abundances. We find variations in abundances across cores. L1517B exhibits low abundances and a high depletion factor, whereas L1495B shows enhanced levels in oxygen-bearing species within the L1495 filament. Ratios tracing carbon- and oxygen-bearing species (CCS/$^{34}$SO and C$^{34}$S/$^{34}$SO) decrease with increasing H$_2$ column density and N$_2$D$^+$/N$_2$H$^+$ ratio. Other species and ratios show weak or no correlation with tracers. Models reproduce OCS, H$_2$CS, and HDCS reasonably well, but not all species simultaneously, especially between carbon- and oxygen-bearing molecules. The variations and lack of consistent correlations suggest that a single evolutionary parameter cannot describe sulfur chemistry and that the local environmental conditions strongly influence the observed abundances. Reproducing the full sample of sulfur-bearing molecules would require improved chemical networks and models that account for the core's physical structure.