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A&A
Volume 690, October 2024
Article Number A24
Number of page(s) 7
Section Interstellar and circumstellar matter
DOI https://doi.org/10.1051/0004-6361/202451368
Published online 26 September 2024

© The Authors 2024

Licence Creative CommonsOpen Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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1 Introduction

Among the over 300 molecules detected in the interstellar medium to date1, those that contain sulfur constitute a particularly conspicuous chemical family. They are observed in the gas phase throughout a wide evolutionary time span, from clouds (e.g., Drdla et al. 1989; Navarro-Almaida et al. 2020; Spezzano et al. 2022; Esplugues et al. 2022) to protostars (e.g., Blake et al. 1987, 1994; van der Tak et al. 2003; Li et al. 2015; Drozdovskaya et al. 2018; Codella et al. 2021; Artur de la Villarmois et al. 2023; Fontani et al. 2023; Kushwahaa et al. 2023), protoplanetary disks (Fuente et al. 2010; Phuong et al. 2018; Semenov et al. 2018; Le Gal et al. 2019, 2021; Rivière-Marichalar et al. 2021; Booth et al. 2024), and Solar System bodies (Smith et al. 1980; Bockelée-Morvan et al. 2000; Hibbitts et al. 2000; Jessup et al. 2007; Moullet et al. 2008, 2013; Cartwright et al. 2020; Biver et al. 2021a,b; Calmonte et al. 2016; Altwegg et al. 2022). The species detected so far range in size, from simple diatomic molecules such as CS and SO to the thiols CH3SH and CH3CH2SH (Linke et al. 1979; Gibb et al. 2000; Cernicharo et al. 2012; Kolesniková et al. 2014; Zapata et al. 2015; Müller et al. 2016; Majumdar et al. 2016; Rodríguez-Almeida et al. 2021).

As opposed to gas-phase observations, however, only two sulfurated molecules have been detected in interstellar ices to date: OCS and SO2 (Palumbo et al. 1995; Boogert et al. 1997, 2022; Palumbo et al. 1997; Öberg et al. 2008; McClure et al. 2023; Rocha et al. 2024). This is probably in large part due to their low abundances, combined with the intrinsic limitations of solid-state observations, such as the broadness of the features and their high degeneracy. Nonetheless, these confirmed ice species already bring vast chemical ramifications, as they can act as reactants to form more complex organosulfur compounds, some of which have key biochemical roles (see, e.g., McAnally et al. 2024). However, despite their astrochemical significance, many open questions remain regarding the formation pathways of these small sulfur-bearing molecules in the solid state.

One particularly important case is that of carbonyl sulfide (OCS), a major gaseous sulfur carrier commonly detected toward young stellar objects (e.g., van der Tak et al. 2003; Herpin et al. 2009; Oya et al. 2016; Drozdovskaya et al. 2018; Kushwahaa et al. 2023; López-Gallifa et al. 2024; Santos et al. 2024). OCS is also frequently detected in interstellar ices (see, e.g., Palumbo et al. 1997; Boogert et al. 2022), in part facilitated by the characteristically large absorption band strength of its 4.9 μm feature in comparison to other ice species (e.g., ~1.2×10−16 cm molecule−1 as measured by Yarnall & Hudson 2022 for pure OCS ice at 10 K; See also Hudgins et al. 1993). Observed OCS abundances in both protostellar ices and hot-core gas point to a solid-state formation mechanism occurring predominately during the high-density pre-stellar core stage (AV > 9, nH ≳ 105 cm−3) after the onset of the CO catastrophic freeze-out (Palumbo et al. 1997; Boogert et al. 2022; Santos et al. 2024). This proposed icy origin of OCS is corroborated by gas-phase astrochemical models, which fail to reproduce observed OCS abundances (Loison et al. 2012). In the inner regions of the protostellar envelope, thermal heating by the protostar causes the volatile ice content to fully sublimate, enabling the bulk of the gaseous OCS observations.

So far, the two most commonly proposed routes to OCS involve either the oxidation of CS or the sulfurization of CO in the solid state (Palumbo et al. 1997; Ferrante et al. 2008; Adriaens et al. 2010; Chen et al. 2015; Laas & Caselli 2019; Shingledecker et al. 2020; Boogert et al. 2022): CS+O→OCS,$\[\mathrm{CS}+\mathrm{O} \rightarrow \mathrm{OCS},\]$(1) CO+S→OCS.$\[\mathrm{CO}+\mathrm{S} \rightarrow \mathrm{OCS}.\]$(2)

However, CS abundances are considerably smaller (≲2.5%) than those of OCS, casting doubts on the extent of the contribution from Reaction 1 to the interstellar OCS content (Boogert et al. 2022). Reaction 2 is more often invoked as the dominant route to OCS, especially due to the large availability of solid-state CO as a reactant in the post freeze-out stage.

Hydrogen in its atomic form is also abundant in pre-stellar cores (H/CO~1–10, Lacy et al. 1994; Goldsmith & Li 2005) and can trigger solid-state reactions of relevance to the sulfur network. Following adsorption onto dust grains, sulfur atoms are subject to successive hydrogenation reactions to form SH and H2S via the steps: S+H→SH,$\[\mathrm{S}+\mathrm{H} \rightarrow \mathrm{SH},\]$(3) SH+H→H2 S, $\[\mathrm{SH}+\mathrm{H} \rightarrow \mathrm{H}_2 \mathrm{~S} \text {, }\]$(4)

both of which are predicted by astrochemical models to proceed very efficiently (see, e.g., Garrod et al. 2007; Druard & Wakelam 2012; Esplugues et al. 2014; Vidal et al. 2017). Once H2S ice is formed, it can further react with another hydrogen atom via an abstraction route (Lamberts & Kästner 2017; Oba et al. 2018, 2019; Santos et al. 2023), or be dissociated through energetic processing (e.g., Jiménez-Escobar et al. 2014; Chen et al. 2015; Cazaux et al. 2022): H2 S+H→SH+H2,$\[\mathrm{H}_2 \mathrm{~S}+\mathrm{H} \rightarrow \mathrm{SH}+\mathrm{H}_2,\]$(5a) H2 S+hv→SH+H,$\[\mathrm{H}_2 \mathrm{~S}+h v \rightarrow \mathrm{SH}+\mathrm{H},\]$(5b)

thus partially replenishing the SH ice content. As a result, SH radicals will likely be present in the ice throughout a wide evolutionary time span, and could potentially serve as an alternative source of sulfur in the formation of OCS.

A particularly promising pathway is the reaction between SH and CO followed by a hydrogen abstraction step (Adriaens et al. 2010; Chen et al. 2015): SH+CO→HSCO,$\[\mathrm{SH}+\mathrm{CO} \rightarrow \mathrm{HSCO},\]$(6) HSCO+H→OCS+H2,$\[\mathrm{HSCO}+\mathrm{H} \rightarrow \mathrm{OCS}+\mathrm{H}_2,\]$(7)

which involves the formation of the intermediate complex HSCO. This pathway is analogous to the case of CO2 ice formation from CO and OH through the HOCO complex (e.g., Oba et al. 2010; Ioppolo et al. 2011; Noble et al. 2011; Qasim et al. 2019; Molpeceres et al. 2023). In the case of CO2, Molpeceres et al. (2023) find that the spontaneous dissociation of HOCO into H and CO2 is not energetically viable, and thus the formation of the latter is only possible through the interaction of the HOCO intermediate with a hydrogen atom. Similarly, HSCO is also prevented from spontaneously dissociating into OCS and H (Adriaens et al. 2010). In the laboratory, OCS is readily formed in energetically processed CO:H2S ice mixtures (Ferrante et al. 2008; Garozzo et al. 2010; Jiménez-Escobar et al. 2014; Chen et al. 2015), for which both reactions involving CO + S and CO + SH have been suggested as possible formation routes. Moreover, Nguyen et al. (2021) allude to the possibility of forming OCS through reactions (6) and (7) in a CO:H2S ice mixture exposed to hydrogen atoms, but without exploring this pathway further. Dedicated laboratory studies assessing this particular reaction pathway to OCS formation are therefore still highly warranted.

In the present experimental work, we investigate the viability of OCS formation via Reactions (6) and (7) under dark cloud conditions by simultaneously depositing H2S, CO, and H atoms at 10 K. The SH radicals are produced via Reaction 5a, and are subsequently subject to reaction with neighboring species. The experimental methods used in this work are described in Sec. 2. In Sec. 3 we present and discuss our main results. Their astrophysical implications are elaborated in Sec. 4, followed by a summary of our main findings in Sec. 5.

2 Experimental methods

This work uses the ultrahigh vacuum (UHV) setup SURFRESIDE3, for which detailed descriptions are provided elsewhere (Ioppolo et al. 2013; Qasim et al. 2020). Briefly, this setup contains a main chamber that operates at base pressures of ~5 × 10−10 mbar, and at the center of which a gold-plated copper substrate is mounted on the tip of a closed-cycle helium cryostat. Resistive heaters are used to vary the substrate temperature between 8 and 450 K, as monitored by two silicon diode sensors with a relative accuracy of 0.5 K. A hydrogen atom beam source (HABS; Tschersich 2000) generates H atoms that are subsequently cooled down by colliding with a nose-shaped quartz pipe before reaching the substrate. As H atoms are injected into the chamber, gases of H2S (Linde, 99.5% purity) and CO (Linde, 99.997% purity) are concomitantly admitted through two separate all-metal leak valves. The heavier isotopolog 13C18O (Sigma-Aldrich, 95% purity 18O, 99% purity 13C) is also used to assist in the product identification. Fourier-transform reflection-absorption infrared spectroscopy (FT-RAIRS) is used to monitor ice growth in the range of 700–4000 cm−1 with 1 cm−1 spectral resolution. When the deposition is complete, temperature-programmed desorption (TPD) experiments are performed by heating the sample at a constant rate of 5 K min−1. During TPD, the solid phase is monitored by RAIRS, while the sublimated species are immediately ionized by 70 eV electron impact and are recorded by a quadrupole mass spectrometer (QMS). The variation of the substrate temperature during the collection of each IR TPD spectrum is of ~10 K.

The absolute abundance of the ice species can be derived from their integrated infrared absorbance (∫ Abs(v)dv) using a modified Beer-Lambert law: NX=ln⁡10∫Abs(ν)dνA′(X),$\[N_X=\ln 10 \frac{\int A b s(\nu) d \nu}{A^{\prime}(X)},\]$(8)

where NX is the column density in molecules cm−2 and A′(X) is the apparent absorption band strength in cm molecule−1 of a given species. We use A′(CO)∼2142 cm−1∼(4.2±0.3)×10−17$\[A^{\prime}(\mathrm{CO})_{\sim 2142 \mathrm{~cm}^{-1}} \sim(4.2 \pm 0.3) \times10^{-17}\]$ cm molecule−1 and A′(H2 S)∼2553 cm−1∼(4.7±0.1)×10−17$\[A^{\prime}\left(\mathrm{H}_2 \mathrm{~S}\right)_{\sim 2553 \mathrm{~cm}-1} \sim(4.7 \pm 0.1) \times10^{-17}\]$ cm molecule−1, as measured for our reflection-mode IR settings using the laser-interference technique (see Santos et al. 2023 for the case of H2S). For the target product, OCS, we employ the band strength of A′(OCS)∼2043 cm−1∼(1.2±0.1)×10−16$\[A^{\prime}(\mathrm{OCS})_{\sim 2043 \mathrm{~cm}^{-1}} \sim(1.2 \pm 0.1) \times10^{-16}\]$ cm molecule−1 measured in transmission mode by Yarnall & Hudson (2022), and correct it by an averaged transmission-to-reflection conversion factor of 3.2 derived with our experimental setup (see Santos et al. 2023 for the case of H2S, which was later combined with CO ice depositions to derive the averaged setup-specific conversion factor).

Table 1 summarizes the experiments performed in this work. All depositions are carried out for 180 minutes with a constant substrate temperature of 10 K. The relative errors of the H-atom flux, as well as both molecule fluxes, are estimated to be ≲5%. The latter are estimated from evaluating the ice growth rate in multiple pure-ice deposition experiments. To estimate the former, multiple experiments to calibrate the relative H-atom flux are conducted. These consist of trapping H atoms inside O2 ice matrices and monitoring the formation of HO2, which is proportional to the H-atom flux provided that O2 is overabundant relative to H (Ioppolo et al. 2013; Fedoseev et al. 2015). The instrumental uncertainties in the integrated QMS and IR signals are derived from the corresponding integrated spectral noise for the same band width.

Table 1

Experiments performed in this work.

thumbnail Fig. 1

Infrared data confirming the formation of OCS from CO, H2S, and H atoms. Panel a: spectrum collected after deposition of CO + H2S + H (experiment 1, red) together with the control experiment without H atoms (experiment 2, gray) and the isotope-labeled experiment with 13C18O (experiment 3, blue). Small features assigned to 13CO and 12CO are due to minor contaminations from the gas bottle (respectively, ~4% and ~2% with respect to 13C18O). Panel b: infrared spectra acquired during TPD following the deposition of experiment 1. The spectrum in dark red (upper) is taken at ~70 K, and the one in light red (lower) is taken at ~100 K. Panel c: same as panel b, but for the isotope-labeled experiment. In all panels, only the relevant frequency range is shown and the spectra are offset for clarity.

3 Results and discussion

3.1 OCS formation

Panel a of Fig. 1 shows the infrared spectra obtained after codeposition of CO, H2S, and H atoms (1:1:30, experiment 1) at 10 K, as well as after a control experiment of only CO and H2S (1:1, experiment 2). When CO and H2S are exposed to hydrogen atoms, a new feature appears at ~2043 cm−1 (~4.89 μm) with a full-width at half maximum (FWHM) of ~15 cm−1 (~0.03 μm). Based on its peak position and the ice elemental composition (i.e., C, O, S, and H), this feature is assigned as the CO-stretching mode of OCS (v1, Yarnall & Hudson 2022)2. The band’s FWHM and peak position are also consistent with previously reported values for OCS in CO-rich ices measured in reflection mode (Ferrante et al. 2008). To confirm this assignment, an analogous experiment is performed with a heavier isotopolog of carbon monoxide, 13C18O (experiment 3: blue spectrum in Fig. 1a). In this case, the 18O13CS v1 feature appears at ~1954 cm−1 (~5.12 μm) in accordance with the expected redshift of the heavier isotopolog. During TPD, both features at ~2043 cm−1 and ~1954 cm−1 disappear from the ice in the same temperature interval of 70–100 K (Panels b and c in Fig. 1), thus signaling that the two bands correspond to the same molecule.

Complementarily to the infrared spectra, data obtained by the QMS during TPD can also be used to strengthen the OCS identification. Panel a of Fig. 2 displays the signal for m/z = 60 recorded during the warm-up of the ice sample in the CO + H2S + H experiment. This mass-to-charge ratio corresponds to the molecular ion of OCS, its strongest signal resulting from 70 eV electron impact as listed in NIST3. A desorption feature appears peaking at ~89 K, in agreement with previously reported desorption temperatures of OCS (Ferrante et al. 2008; Nguyen et al. 2021). In the isotope-labeled experiment, a similar desorption peak is observed for m/z = 63, consistent with the mass shift corresponding to the molecular ion of 18O13CS. Moreover, no peak signal is detected for m/z = 60, indicating that this same feature appearing in experiment 1 corresponds uniquely to carbonyl sulfide. The desorption temperature of OCS as measured by the QMS correlates with the disappearance of the ~2043 cm−1 feature in the infrared spectra taken during TPD, as indicated by the area of this infrared band as a function of substrate temperature (Fig. 2, panel b). Both the infrared and QMS data therefore provide unequivocal evidence for the formation of OCS as a result of interactions between H2S, CO, and H atoms in the solid state.

Aside from OCS, another S-bearing species that could presumably be formed under our experimental conditions is thioformic acid, HCOSH. Indeed, formation routes via either the hydrogenation of the HSCO intermediate or the interaction between HCO + SH have been proposed by Rodríguez-Almeida et al. (2021) to explain HCOSH observations toward the giant molecular cloud G+0.693–0.027, with the former subsequently verified theoretically by Molpeceres et al. (2021). Although HCOSH has been shown to form upon keV electron irradiation of H2S:CO ices (Wang et al. 2022), Nguyen et al. (2021) only tentatively identify this product in similar hydrogenation experiments on H2S:CO ices. No evidence for this species is observed in our experiments above the instrumental detection limit. Likewise, we do not detect any signal of volatile sulfur allotropes such as S2 or S3, nor of hydrogenated sulfur chains such as H2S2 or H2S3 – which could presumably be formed from the association of sulfur atoms produced via hydrogen abstraction reactions from SH radicals. This leads to the conclusion that the potential production of atomic S does not proceed efficiently under our experimental conditions. Moreover, as no H2S2 is detected, the recombination of two SH radicals is also likely not efficient in the present experiments – signaling that HS radicals are efficiently consumed, both by reacting with CO leading to OCS and by reacting with H to reform H2S.

Reactions (6) and (7) are therefore the predominant contributors to the formation of OCS in our experiments. As shown by Nguyen et al. (2021), however, the backward reactions are also viable. Once formed, OCS can be hydrogenated into the complex intermediate HSCO, which in turn can further react with another H atom to yield CO and H2S, as well as dissociate back into the reactants SH and CO (Adriaens et al. 2010; Nguyen et al. 2021; Molpeceres et al. 2021). The effective amount of OCS detected will therefore be subject to these two competing directions.

thumbnail Fig. 2

Complementary QMS and infrared data acquired during TPD confirming the OCS detection. Panel a: TPD-QMS signal recorded for m/z = 60 after the deposition of CO + H2S + H (experiment 1, gray), as well as those for m/z = 63 (dark red) and m/z = 60 (light red) after the deposition of 13C18O + H2S + H (experiment 3). The blue shadowed region denotes the range of desorption, and the dashed line highlights the peak desorption temperature. Signals from different experiments are offset for clarity. Panel b: area of the OCS v1 band as a function of temperature during TPD following the deposition of experiment 1. The uncertainties in the horizontal axis are due to the substrate temperature variation of ~10 K during the collection of each IR TPD spectrum. The blue shadowed area and dashed line are reproductions of the range and peak position shown in panel a.

3.2 Effects of larger CO fractions

Once the possibility of forming OCS from CO and SH is ascertained, the next question to arise is that regarding the viability of this route in more astronomically representative ice mixing ratios. To date, there are no convincing detections of H2S in interstellar ices, with upper limits estimated to be ≤0.7% with respect to water (Jiménez-Escobar & Muñoz Caro 2011). This would translate to ice abundance upper limits of roughly a few percent with respect to CO (e.g., N(H2S)/N(CO) ≲ 0.035 if assuming N(CO)/N(H2O) ~0.21 according to the median CO ice abundance value with respect to H2O derived for low-mass young stellar objects in Boogert et al. 2015). Accordingly, we performed similar depositions with larger fractions of CO with respect to H2S in order to assess how the dilution of the latter affects the yields of OCS. The deposition fluxes of H2S and atomic H are kept the same in all experiments, with only variations in the CO flux (see Table 1). Figure 3 compares the relative intensities of the OCS signals obtained from both the IR and QMS data for the different mixing ratios explored here (i.e., CO:H2S = 1:1, 5:1, 10:1, and 20:1). The areas of the OCS ν1 vibrational modes for each mixing ratio after deposition are normalized to that of experiment 1, which yielded the largest absolute amount of products. Similarly, the areas of the m/z = 60 desorption band at ~89 K are also normalized to that of experiment 1.

Overall, the infrared and QMS data provide consistent OCS yields within their respective error bars, and signal that dilution of H2S in CO reduces but does not impede OCS formation under our experimental conditions. Indeed, in the highest dilution case (5% H2S with respect to CO), OCS production diminishes by ~50% (the mean value between the infrared and QMS results) in comparison to the nondiluted reference experiment (100% H2S with respect to CO). This decrease in OCS yield is likely governed by the reduced number of available H atoms in experiments with larger CO fractions, as opposed to a diminishing effectiveness of the OCS formation. As well established by previous works, the interaction of CO with H atoms in the solid state initiates a chain of reactions leading to H2CO and CH3OH (Tielens & Hagen 1982; Charnley et al. 1992; Hiraoka et al. 1994; Watanabe & Kouchi 2002; Fuchs et al. 2009; Santos et al. 2022), which efficiently consumes the available atomic hydrogen in the system. Increasing the CO deposition flux while maintaining a fixed H-atom flux therefore results in less hydrogen being able to react with other species. This decrease in available H atoms will have a greater impact on the formation of SH radicals (Reaction (5a)) than on the final abstraction step (Reaction (7)), as the latter is predicted to be barrierless (Adriaens et al. 2010), whereas the former requires quantum tunneling through a barrier of ~1500 K (Lamberts & Kästner 2017). The slowly decreasing trend in Fig. 3 suggests that OCS can still be formed at 10 K in ice mixtures with CO:H2S of higher than 20:1 and H:CO of as low as 1.5:1. The formation of OCS from CO + SH could therefore make a non-negligible contribution to the observed OCS in interstellar ices, as discussed below.

thumbnail Fig. 3

Integrated intensities of the infrared and QMS features of OCS for different flux conditions: CO:H2S = 1:1, 5:1, 10:1, and 20:1. The infrared value is derived from the area of the ν1 mode of OCS, while the QMS counterpart is calculated from the integrated desorption band for m/z = 60 peaking at ~89 K. The resulting yields are normalized to that of experiment 1 (CO:H2S = 1:1). The upper x-axis shows the corresponding ratios of H/CO, signaling the decrease in H-atom availability with increasing CO fractions.

4 Astrophysical implications

The reaction network probed in this work is shown in Fig. 4. Aside from OCS, other closed-shell species also formed within this network are H2S2 (Santos et al. 2023), H2CO, and CH3OH (Tielens & Hagen 1982; Charnley et al. 1992; Hiraoka et al. 1994; Watanabe & Kouchi 2002; Fuchs et al. 2009; Santos et al. 2022). In our experiments, H2S is used as a source of SH radicals via a hydrogen abstraction (Reaction (5a)) to avoid reactions with S atoms to interfere with the analysis. In the ISM, however, SH will not only be produced from H2S but also from the hydrogenation of sulfur atoms that adsorb onto dust grains (Reaction (3)). Observations of singly and doubly deuterated H2S in Class 0 sources suggest that the bulk of its formation takes place early in the evolution of a cloud, before the catastrophic CO freeze-out stage begins (Ceccarelli et al. 2014). Accordingly, SH radicals will be available in the ice as early as during the low-density prestellar core stage. As the cloud evolves and the density of the environment increases, most of the available S atoms will be converted into H2S. Nonetheless, SH radicals can still be formed through Reaction (5a) or through dissociation reactions induced by energetic processing. As a result, SH will remain a viable reactant throughout a large fraction of a cloud’s lifetime.

Observations of OCS toward massive young stellar objects reveal that its 4.9 μm feature is best fitted by reference spectra of OCS in a CH3OH-rich environment (Palumbo et al. 1997; Boogert et al. 2022). Their column density ratios (N(OCS)/N(CH3OH)) in both ice and gas have similarly narrow distributions and comparable median values (within a factor of 3, Santos et al. 2024), strengthening the hypothesis that both coexist in similar ice environments. Moreover, observed ice abundances of OCS and CH3OH are correlated (Boogert et al. 2022), suggesting a strong chemical link between the two molecules. As methanol is known to be formed via the hydrogenation of CO in the solid phase, such observations are in line with CO being a common precursor between the two. This would imply that the bulk of OCS ice likely originates deep into dense clouds, in a dense environment post catastrophic CO freeze-out. Conversely, a large portion of the free sulfur atoms will likely be readily converted into H2S at such densities. The reaction route proposed here could partially circumvent this issue, as SH radicals can be efficiently produced through a top-down mechanism (Reaction (5a)). Moreover, estimated H2S upper limits in ices are larger than the detected OCS abundances (Palumbo et al. 1997; Jiménez-Escobar & Muñoz Caro 2011; Boogert et al. 2022), and thus this route remains viable in light of the observational evidence available so far. Overall, both CO + S and CO + SH routes likely contribute to the observed OCS abundances in interstellar ices, and exploration of the extent of this contribution warrants dedicated chemical modeling.

thumbnail Fig. 4

Reaction scheme proposed in this work. Blue boxes denote the deposited molecules and the orange box highlights the studied product, OCS. Hydrogen abstraction reactions from CH3OH, H2CO, and associated radicals are omitted for the sake of simplicity.

5 Conclusions

In this work, we conduct a systematic experimental investigation of the viability of forming OCS under dark cloud conditions from ice mixtures of CO, H2S, and H atoms. Our main findings are summarized below:

  • OCS can be efficiently formed at 10 K from solid-state reactions involving CO, H2S, and H. The proposed underlying mechanism is via CO + SH → HSCO followed by HSCO + H → OCS + H2, analogously to CO2 ice;

  • The OCS product yield decreases slowly with increasing initial CO/H2S ratios and decreasing H-atom availability, resulting in ~50% less OCS formation for a 20 times higher CO abundance relative to H2S and fixed H-atom fluxes. This trend suggests that OCS can be efficiently formed via the proposed route in more representative interstellar ices, where H2S is likely highly diluted in CO;

  • SH can be produced both through bottom-up (S + H → SH) and top-down (H2S + H → SH + H2) routes, thus making it a favorable reactant to form OCS during the high-density, post-CO-freeze-out stage of prestellar cores. This is in line with both ice and gas-phase observations of OCS in protostars.

These findings suggest that the CO + SH reaction is potentially responsible for a non-negligible share of the interstellar OCS ice and therefore could be a valuable new addition to gas-grain chemical models focusing on sulfur species. In turn, such models could help to disentangle the contributions of the CO + SH and CO + S routes to forming OCS.

Acknowledgements

This work has been supported by the Danish National Research Foundation through the Center of Excellence “InterCat” (Grant agreement no.: DNRF150); the Netherlands Research School for Astronomy (NOVA); and the Dutch Astrochemistry Network II (DANII). KJC is grateful for support from NWO via a VENI fellowship (VI.Veni.212.296).

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2

The numbers assigned to the CS and CO stretches of OCS are interchangeable. In this work, we adopt the notation from Yarnall & Hudson (2022).

All Tables

Table 1

Experiments performed in this work.

In the text

All Figures

thumbnail Fig. 1

Infrared data confirming the formation of OCS from CO, H2S, and H atoms. Panel a: spectrum collected after deposition of CO + H2S + H (experiment 1, red) together with the control experiment without H atoms (experiment 2, gray) and the isotope-labeled experiment with 13C18O (experiment 3, blue). Small features assigned to 13CO and 12CO are due to minor contaminations from the gas bottle (respectively, ~4% and ~2% with respect to 13C18O). Panel b: infrared spectra acquired during TPD following the deposition of experiment 1. The spectrum in dark red (upper) is taken at ~70 K, and the one in light red (lower) is taken at ~100 K. Panel c: same as panel b, but for the isotope-labeled experiment. In all panels, only the relevant frequency range is shown and the spectra are offset for clarity.
In the text
thumbnail Fig. 2

Complementary QMS and infrared data acquired during TPD confirming the OCS detection. Panel a: TPD-QMS signal recorded for m/z = 60 after the deposition of CO + H2S + H (experiment 1, gray), as well as those for m/z = 63 (dark red) and m/z = 60 (light red) after the deposition of 13C18O + H2S + H (experiment 3). The blue shadowed region denotes the range of desorption, and the dashed line highlights the peak desorption temperature. Signals from different experiments are offset for clarity. Panel b: area of the OCS v1 band as a function of temperature during TPD following the deposition of experiment 1. The uncertainties in the horizontal axis are due to the substrate temperature variation of ~10 K during the collection of each IR TPD spectrum. The blue shadowed area and dashed line are reproductions of the range and peak position shown in panel a.
In the text
thumbnail Fig. 3

Integrated intensities of the infrared and QMS features of OCS for different flux conditions: CO:H2S = 1:1, 5:1, 10:1, and 20:1. The infrared value is derived from the area of the ν1 mode of OCS, while the QMS counterpart is calculated from the integrated desorption band for m/z = 60 peaking at ~89 K. The resulting yields are normalized to that of experiment 1 (CO:H2S = 1:1). The upper x-axis shows the corresponding ratios of H/CO, signaling the decrease in H-atom availability with increasing CO fractions.
In the text
thumbnail Fig. 4

Reaction scheme proposed in this work. Blue boxes denote the deposited molecules and the orange box highlights the studied product, OCS. Hydrogen abstraction reactions from CH3OH, H2CO, and associated radicals are omitted for the sake of simplicity.
In the text

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