Sulfur depletion in dense clouds and circumstellar regions I. H2S ice abundance and UV-photochemical reactions in the H2O-matrix

TL;DR

This study investigates sulfur depletion in dense interstellar environments by simulating UV photoprocessing of H2S and H2O ices, revealing formation of refractory sulfur polymers that may explain missing sulfur in space.

Contribution

It provides new laboratory data on H2S ice photochemistry and desorption, offering insights into sulfur chemistry and depletion in astrophysical environments.

Findings

H2S ice desorbs at specific temperatures depending on composition.

UV irradiation produces sulfur polymers up to S8 in ices.

Polymeric sulfur may account for missing sulfur in space.

Abstract

This work aims to study the unexplained sulfur depletion observed toward dense clouds and protostars. We made simulation experiments of the UV-photoprocessing and sublimation of H2S and H2S:H2O ice in dense clouds and circumstellar regions, using the Interstellar Astrochemistry Chamber (ISAC), a state-of-the-art ultra-high-vacuum setup. The ice was monitored in situ by mid-infrared spectroscopy in transmittance. Temperature-programmed desorption (TPD) of the ice was performed using a quadrupole mass spectrometer (QMS) to detect the volatiles desorbing from the ice. Comparing our laboratory data to infrared observations of protostars we obtained a more accurate upper limit of the abundance of H2S ice toward these objects. We determined the desorption temperature of H2S ice, which depends on the initial H2S:H2O ratio. UV-photoprocessing of H2S:H2O ice led to the formation of several species. Among them, H2S2 was found to photodissociate forming S2 and, by elongation, other species up to S8, which are refractory at room temperature. A large fraction of the missing sulfur in dense clouds and circumstellar regions could thus be polymeric sulfur residing in dust grains.