Gas phase Elemental abundances in Molecular cloudS (GEMS)

Abstract

Context. Sulphur is one of the most abundant elements in the Universe. Surprisingly, sulphuretted molecules are not as abundant as expected in the interstellar medium and the identity of the main sulphur reservoir is still an open question. Aims. Our goal is to investigate the H2S chemistry in dark clouds, as this stable molecule is a potential sulphur reservoir. Methods. Using millimeter observations of CS, SO, H2S, and their isotopologues, we determine the physical conditions and H2S abundances along the cores TMC 1-C, TMC 1-CP, and Barnard 1b. The gas-grain model NAUTILUS is used to model the sulphur chemistry and explore the impact of photo-desorption and chemical desorption on the H2S abundance. Results. Our modeling shows that chemical desorption is the main source of gas-phase H2S in dark cores. The measured H2S abundance can only be fitted if we assume that the chemical desorption rate decreases by more than a factor of 10 when nH > 2 × 104. This change in the desorption rate is consistent with the formation of thick H2O and CO ice mantles on grain surfaces. The observed SO and H2S abundances are in good agreement with our predictions adopting an undepleted value of the sulphur abundance. However, the CS abundance is overestimated by a factor of 5−10. Along the three cores, atomic S is predicted to be the main sulphur reservoir. Conclusions. The gaseous H2S abundance is well reproduced, assuming undepleted sulphur abundance and chemical desorption as the main source of H2S. The behavior of the observed H2S abundance suggests a changing desorption efficiency, which would probe the snowline in these cold cores. Our model, however, highly overestimates the observed gas-phase CS abundance. Given the uncertainty in the sulphur chemistry, we can only conclude that our data are consistent with a cosmic elemental S abundance with an uncertainty of a factor of 10.

High Spatial Resolution Observations of Molecular Lines toward the Protoplanetary Disk around TW Hya with ALMA

“…For the CS line, we only compare the TW Hya dust model (D2) with the models (A2) and (A3) since we need to modify the sulfur elemental abundance to the values (A2) 2.0×10 −9 and (A3) 4.0 × 10 −9 , respectively, in order to better fit the observation. Elemental abundance of sulfur in gas-phase is an unknown factor even in molecular clouds, and freeze-out in ice and/or transformation into refractory material have been suggested (e.g., Navarro-Almaida et al 2020). Further observations of other sulfur-bearing species are needed in order to better constrain the sulfur chemistry in the disk (e.g., Semenov et al 2018;Le Gal et al 2019).…”

Probing the kinematics and chemistry of the hot core Mon R 2 IRS 3 using ALMA observations

“…During the collapse, the bulk of the sulphur reservoir is locked onto grain mantles (see e.g. Vidal et al 2017;Navarro-Almaida et al 2020). These species are released to the gas phase again when dust temperatures increase to >100 K and the ice mantles are evaporated in the hot core/corino stage.…”

“…Although a large theoretical and observational effort has been undertaken in the last five years to understand sulphur chemistry, there is still a big debate about the main sulphur reservoirs in gas phase and volatiles, and eventually about the sulphur elemental abundance in molecular clouds (Fuente et al 2016;Vidal et al 2017;Fuente et al 2019;Le Gal et al 2019;Laas & Caselli 2019;Navarro-Almaida et al 2020;Shingledecker et al 2020;Bulut et al 2021)). According to chemical models, SO and SO 2 are the main gas-phase sulphur reservoirs in gas phase under the conditions of high temperature and density prevailing in hot cores (Esplugues et al 2014;Vidal et al 2017;Gieser et al 2019).…”

Quantifying the chemical desorption of H$_2$S and PH$_3$ from amorphous water ice surfaces

“…Gaseous H 2 S has been detected in various evolutionary stages of star and planet formation, cold dense clouds (e.g., Minh et al 1989;Ohishi et al 1992;Navarro-Almaida et al 2020), envelope around protostars (e.g., Blake et al 1994;Wakelam et al 2004), and protoplanetary disks (Phuong et al 2018;Rivière-Marichalar et al 2021). The formation of H 2 S in the gas phase is inefficient because of the presence of endothermic reactions in the sequence of reaction pathways to convert S + or atomic S into H 2 S (e.g., Yamamoto 2017); the reaction of S + with H 2 to form SH + is endothermic, as is the reaction of SH + (which can be formed by the reaction between atomic S and H 3 + ) with H 2 to form SH + 2 .…”