Gas phase Elemental abundances in Molecular cloudS (GEMS). VII. Sulfur elemental abundance

Abstract

Context. Gas phase Elemental abundances in molecular CloudS (GEMS) is an IRAM 30-m Large Program aimed at determining the elemental abundances of carbon (C), oxygen (O), nitrogen (N), and sulfur (S) in a selected set of prototypical star-forming filaments. In particular, the elemental abundance of S remains uncertain by several orders of magnitude, and its determination is one of the most challenging goals of this program.
Aims: This paper aims to constrain the sulfur elemental abundance in Taurus, Perseus, and Orion A based on the GEMS molecular database. The selected regions are prototypes of low-mass, intermediate-mass, and high-mass star-forming regions, respectively, providing useful templates for the study of interstellar chemistry.
Methods: We have carried out an extensive chemical modeling of the fractional abundances of CO, HCO⁺, HCN, HNC, CS, SO, H₂S, OCS, and HCS⁺ to determine the sulfur depletion toward the 244 positions in the GEMS database. These positions sample visual extinctions from A_V ~ 3 mag to >50 mag, molecular hydrogen densities ranging from a few × 10³ cm⁻³ to 3 × 10⁶ cm⁻³, and T_k ~ 10-35 K. We investigate the possible relationship between sulfur depletion and the grain charge distribution in different environments.
Results: Most of the positions in Taurus and Perseus are best fitted assuming early-time chemistry, t = 0.1 Myr, ζ_H2 ~ (0.5−1) × 10⁻¹⁶ s⁻¹, and [S/H] ~ 1.5 × 10⁻⁶. On the contrary, most of the positions in Orion are fitted with t = 1 Myr and ζ_H2 ~ 10⁻¹⁷ s⁻¹. Moreover, ~40% of the positions in Orion are best fitted assuming the undepleted sulfur abundance, [S/H] ~ 1.5 × 10⁻⁵. We find a tentative trend of sulfur depletion increasing with density.
Conclusions: Our results suggest that sulfur depletion depends on the environment. While the abundances of sulfur-bearing species are consistent with undepleted sulfur in Orion, a depletion factor of ~20 is required to explain those observed in Taurus and Perseus. We propose that differences in the grain charge distribution might explain these variations. Grains become negatively charged at a visual extinction of A_V ~ 3.5 mag in Taurus and Perseus. At this low visual extinction, the S⁺ abundance is high, X(S⁺) > 10⁻⁶, and the electrostatic attraction between S⁺ and negatively charged grains could contribute to enhance sulfur depletion. In Orion, the net charge of grains remains approximately zero until higher visual extinctions (A_V ~ 5.5 mag), where the abundance of S⁺ is already low because of the higher densities, thus reducing sulfur accretion. The shocks associated with past and ongoing star formation could also contribute to enhance [S/H].