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Formation of OCS in interstellar ices

Rafael, Martin Domenech

Publisher
Zenodo
DOI
10.5281/zenodo.17108807
Repository
zenodo.org
Language
en

Abstract

The high sensitivity and spectral resolution offered by the JWST have allowed us to expand the search for new species, both in the gas and the solid phases. The S-bearing species are of particular interest due to its relevance in the star-formation process (Fuente et al. 2023) and the origin of life (Leustek 2002, Francioso et al. 2020). Unfortunately, current observations cannot account for the expected sulfur abundance in star-forming regions (Laas & Caselli 2019). The missing sulfur is expected to be locked in the solid phase (Millar & Herbst 1990). However, JWST observations of ice mantles in dense molecular clouds have only been able to detect OCS (McClure et al. 2023) and, tentatively, SO2 (Rocha et al. 2024), with abundances that only account for <5% of the cosmic sulfur abundance. Understanding how this reservoir of sulfur in interstellar ices is built is of vital importance to constrain the evolution of the sulfur chemistry during star-formation. In the case of OCS, in situ formation on the grain surface through the CO+S and/or CS+O pathways needs to be invoked (Boogert et al. 2022). We present laboratory experiments comparing the relative contribution of these two pathways to the OCS formation in interstellar ices. We found that the CS+O pathway is 3-6 times more efficient, depending on the ice temperature. In addition, a significant fraction of the initial S in our experiments seemed to form sulfur chains, that could contain part of the missing sulfur in the ISM.

Full text

Formation of OCS in interstellar ices Rafael Martín-Doménech, Karin I. Öberg, Guillermo M. Muñoz Caro, Héctor Carrascosa, Asunción Fuente, Mahesh Rajappan 10/03/25 TOWARDS NEW FRONTIERS | 10-14 MARCH 2025 | ESO GARCHING 1.- Introduction S chemistry in the dense ISM Laas & Caselli 2019Calmonte et al. 2016McClure et al. 2023 Relative abundances of S-bearing molecules in ice mantles S/H Credits: A. Fuente S locked in ices? Ice observations with JWST •Melissa McClure •Zak Smith •Angèle Taillard (detectability of S-bearing) •Katerina Slavicinska (detection of NH4SH) 2/17 H2O:CS2 3:1 CO:CS2 7:1 CO2:CS2 7:1 2.- Laboratory Experiments Methodology 1 3 4 13 2 6 9 810 12 12 5 11 14 1 UHV chamber 2 Turbomolecular pump 3 Cryostat 4 Gas line 5 Gas doser 6 Electron gun 7a/7b Nd:YAG lasers 8 Dye laser 9 OPO 10 VUV cell 11 VUV guiding arm 12 FTIR spectrometer 13 QMS 14 Re-ToF MS (hindered) 7a 7b Maksyutenko et al. 2022 The SPACE TIGER setup at the CfA 3/17 H218O:CS2 3:1 13C18O:CS2 7:1 13C18O2:CS2 7:1 Laboratory experiments under astrophysically relevant conditions 2.- Laboratory Experiments Methodology UHV chamber (base pressure in the 10-10 torr range) ELG-2/EGPS-1022 low energy electron source. Irradiation equivalent to CR during ~5x106 years Substrate can be cooled down to 7 K 4/17 FTIR spectrometer 2.- Methodology Monitoring the ice Laboratory experiments under astrophysically relevant conditions 5/17 2.- Methodology Monitoring the gas Quadrupole Mass Spectrometer (QMS) •Thermal desorption → Temperature Programmed Desorption (TPD) Temperature m/z = 44 signal Laboratory experiments under astrophysically relevant conditions 6/17 3.- Results H218O : CS2 + eirradiation S chemistry Martín-Doménech et al. 2024 7/17 3.- Results H218O : CS2 + eirradiation S chemistry Martín-Doménech et al. 2024 8/17 3.- Results H218O : CS2 + eirradiation S chemistry Martín-Doménech et al. 2024 9/17 4.- Conclusions •Dissociation of CS2 molecules upon energetic processing leads to the formation of several detectable S-bearing products: OCS, SO2, C3S2, S2. •Sulfur chemistry in CS2-bearing ices could be dominated by the formation of undetectable long sulfur chains. •Formation of OCS through the CS+O → OCS pathway seems to be more efficient than CO+S → OCS under the same conditions. CS+O could play a role especially in warm regions with no CO ice. 16/17 LSAIP Guillermo M. Muñoz Caro Héctor Carrascosa de Lucas Bruno Escribano Carlos del Burgo Olivares SUL4LIFE Asunción Fuente Angèle Taillard David Navarro Almaida Öberg Astrochemistry Group Karin I. Öberg Mahesh Rajappan Elettra Piacentino Suchitra Narayanan Alexia Simon Marissa Maney LCF/BQ/PI22/11910030 Danke schön! 1.- Introduction OCS - experiments ➢In general, two OCS ice formation pathways have been proposed in the literature CS + O → OCS CO + S → OCS ➢Both pathways have been tested in the laboratory •CS2 + O at 15-70 K (Ward et al. 2012) •eirradiation of CS2:O2 at 12 K (Maity & Kaiser 2013) •H+ irradiation of CO:H2S, CO2:H2S, CO:SO2, CO2:SO2 at 10 K (Ferrante et al. 2008) •H+ irradiation of CO:H2S, CO:SO2 at 16-20 K (Garozzo et al. 2010) •UV irradiation of CO:H2S, CO2:H2S at 14 K (Chen et al. 2015) OCS formation pathways CO source (CO or CO2) + S source (H2S or SO2) + irradiation CS source (CS2) + O source (O or O2) + irradiation CO+S preferred, but contribution depends on the availability of reactants AND relative reaction rates 1.- Introduction Models ➢In general, two OCS ice formation pathways have been proposed in the literature CS + O → OCS CO + S → OCS OCS formation pathways THIS WORK: CO:CS2 and CO2:CS2 + eirradiation at 7-50 K 1. Compare the relative efficiencies of both pathways under the same conditions (CO, CS, O, S present) and study the effect of irradiation temperature 2. Explore the chemistry of CS2 molecules in realistic ice environments (also H2O ice matrix) ➢Detected in comets ➢Predicted in ice models (Laas & Caselli 2019) ➢Detected as one of the products upon energetic processing of CO:H2S and CO:SO2 ice mixtures ➢Proposed parent molecule of the detected CS in the gas phase Appendix CS2:O2 + eirradiation Maity & Kaiser 2013 CS2:O2 + eirradiation Appendix Reported experiments Martín-Doménech et al. 2024 Appendix Mass balance Column densities (x 1E15 molecules/cm2) Experiment 1 Experiment 2 Species Initial Final Initial Final 13C18O2 182 135 172 124 CS2 24 10 31 12 13C18O 057 058 C18O 012 013 C18O2 0 1.1 0 1.3 S18O2 0 8 0 9 S18O3 0 1.1 0 1.3 18O13CS 0 0.3 0 0.3 18OCS 0 1.2 0 1.2 C3S2 0 0.4 0 0.4 S2 0 0.7 0 0.8 Total Sbearing 24 21 Total Sulfur 48 33 Appendix 13C18O2 : CS2 + eirradiation Martín-Doménech et al. 2024 Appendix 13C18O2 : CS2 + eirradiation 18O13CS IR assignment Martín-Doménech et al. 2024 Appendix 13C18O2 : CS2 + eirradiation TPD curves of products other than OCS at 7 K Martín-Doménech et al. 2024 Appendix 13C18O2 : CS2 + VUV photon irradiation IR evolution during TPD of the ice sample irradiated at 10 K Additional contribution to the IR band (probably 13C18O3) that disappears at 80 K %18OCS (CS+O) at the end of the experiment = 78% Martín-Doménech et al. 2024 3.- Results 13C18O : CS2 + eirradiation CS2CS S 13C18O 18O 13C 18O13CS 18OCS + + This pathway is favored!!... But we can still study the S chemistry and the evolution of OCS formation with T OCS formation pathways Appendix 13C18O : CS2 + eirradiation Martín-Doménech et al. 2024 Appendix 13C18O : CS2 + eirradiation OCS formation pathways at 7-25 K Martín-Doménech et al. 2024 Appendix 13C18O : CS2 + eirradiation OCS formation pathways at 7-25 K 7 K 15 K 25 K Martín-Doménech et al. 2024 Appendix 13C18O : CS2 + eirradiation TPD curves of products other than OCS at 7 K Martín-Doménech et al. 2024 Appendix 13C18O : CS2 + eirradiation Martín-Doménech et al. 2024 Appendix 13C18O : CS2 + eirradiation Detection of C2S Appendix H2O:CS2 / H218O:CS2 + eirradiation 3.- Results H218O : CS2 + eirradiation CS2CS H218O 18OH. H 18OCS + + S