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 AV ~ 3 mag to >50 mag, molecular hydrogen densities ranging from a few × 103 cm−3 to 3 × 106 cm−3, and Tk ~ 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, ζH2 ~ (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 ζH2 ~ 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 AV ~ 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 (AV ~ 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].
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- February 2023
- DOI:
- arXiv:
- arXiv:2212.03742
- Bibcode:
- 2023A&A...670A.114F
- Keywords:
-
- astrochemistry;
- ISM: abundances;
- ISM: kinematics and dynamics;
- ISM: molecules;
- stars: formation;
- stars: low-mass;
- Astrophysics - Astrophysics of Galaxies;
- Astrophysics - Solar and Stellar Astrophysics
- E-Print:
- 22 pages, 15 figures, Astronomy and Astrophysics, in press