Tracing the sulfur depletion in starless and pre-stellar cores

Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany

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Received: 23 March 2026
Accepted: 6 May 2026

Abstract

Context. 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, thus providing an ideal laboratory to study the physical and chemical processes that cause sulfur depletion.

Aims. We aim to constrain the sulfur chemistry in dense cores by measuring the abundances of different sulfur-bearing molecules and examining how these abundances reflect core evolution and environmental effects.

Methods. We observed nine cores in the Taurus Molecular Cloud, targeting 13 sulfur-bearing molecules, including CS, CCS, C₃S, OCS, SO, SO₂, H₂CS, and various isotopologs. The molecular abundances and six abundance ratios were then compared to three evolutionary tracers: H₂ column density, N₂D⁺/N₂H⁺, and the CO depletion factor. We also compared the observed abundances with 0D chemical models with different initial sulfur abundances.

Results. We observe substantial variations in the abundances of individual sulfur-bearing molecules across the cores. L1517B exhibits consistently low abundances and a high depletion factor, whereas L1495B shows high abundances in oxygen-bearing species compared to the other cores within the L1495 filament. Ratios that probe the balance between carbon- and oxygen-bearing sulfur species (CCS/³⁴SO and C³⁴S/³⁴SO) decrease with increasing H₂ column density and N₂D⁺/N₂H⁺ ratio. In contrast, individual molecules and other ratios show weak or no correlation with the evolutionary tracers. The 0D chemical models reproduce the abundances of some molecules, such as OCS, H₂CS, and HDCS, reasonably well but cannot simultaneously account for all observed species. This difference is most evident between the carbon- and oxygen-bearing molecules.

Conclusions. The observed variations in sulfur abundances between the different cores and the lack of clear correlations with three standard evolutionary tracers indicate that a single evolutionary parameter cannot describe the sulfur chemistry. Instead, it is strongly influenced by local environmental factors. Reproducing the full sample of sulfur-bearing molecules would require improved chemical networks and models that account for the core’s physical structure.