The sulfur depletion problem: upper limits on the H₂S₂, HS^·₂, and S₂gas-phase abundances toward the low-mass warm core IRAS 16293-2422

Abstract

Context. A fraction of the missing sulfur in dense clouds and circumstellar regions could be in the form of three species not yet detected in the interstellar medium: H 2 S 2 , HS . 2 , and S 2 according to experimental simulations performed under astrophysically relevant conditions. These S-S bonded molecules can be formed by the energetic processing of H 2 S-bearing ice mantles on dust grains, and subsequently desorb to the gas phase. Aims. The detection of these species could partially solve the sulfur depletion problem, and would help to improve our knowledge of the poorly known chemistry of sulfur in the interstellar medium. To this purpose we calculated the frequencies and expected intensities of the rotational transitions not previously reported, and performed dedicated ground-based observations toward the low-mass warm core IRAS 16293-2422, a region with one of the highest measured gas-phase H 2 S abundances. Methods. Observations in the submillimeter regime were obtained with the APEX 12 m telescope during 15 h of observation. A total of ∼16 GHz were covered in a range of about 100 GHz, targeting a wide selection of the predicted rotational transitions of the three molecules.Results. The 1σ noise rms values were extracted in the spectral regions where the targeted species should have been detected. These values were a factor of 2−7 lower than those reached by previous observations toward the same source, and allowed us to estimate a 1σ upper limit to their molecular abundances of ≤8.1 × 10 −9 , ≤1.1 × 10 −8 , and ≤2.9 × 10 −7 relative to H 2 , for H 2 S 2 , HS . 2 , and S 2 , respectively. Conclusions. The upper limit abundances of the three molecules containing the S 2 unit are up to two orders of magnitude lower than the H 2 S abundance in the source, and one order of magnitude lower than the expected abundances from the experimental simulations using ice analogs. Subsequent gas-phase chemistry after desorption could lower the abundances of the three species to undetectable levels in our observations.

Thermal and energetic processing of astrophysical ice analogues rich in SO2

“…These findings stimulated recent efforts to observe H 2 S 2 , HS 2 , and S 2 in the gas phase toward the low-mass warm core IRAS 16293-2422(Martín-Doménech et al 2016. Estimated upper limit abundances of these molecules are up to two orders of magnitude lower than the H 2 S abundance in the source.…”

“…It is believed that the desorption of grain mantle species into the gas phase, for example after warming by a protostar or sputtering by energetic cosmic ions, gives an important contribution to the gas phase composition (Modica & Palumbo 2010;Palumbo et al 2008). As outlined by Martín-Doménech et al (2016) it seems plausible that an important fraction of the observed S-bearing gas phase species is released from grains because molecules such as H 2 S, SO 2 , OCS, SO, H 2 CS, HCS + , and NS have abundances that cannot A&A 604, A68 (2017) be explained with gas-phase-only chemical models (Doty et al 2004;Viti et al 2004;Wakelam et al 2011;Woods et al 2015).…”

On the Detectability of the HSS, HSO, and HOS Radicals in the Interstellar Medium

“…1-89;Papousek & Aliev 1982), can produce comparison to experiment to better than 5.0 cm −1 for vibrational frequencies and 30 MHz for rotational constants Fortenberry et al 2012bFortenberry et al , 2014aHuang et al 2013a;Zhao et al 2014;Kitchens & Fortenberry 2016). Such a CcCRE PES computed for HSS has produced useful spectroscopic data (Peterson et al 2008) to aid in the interstellar search for this simple, yet hidden disulfide radical (Martín-Doménech et al 2016). However, the spectral data set is still somewhat incomplete, and additional corroboration for the known theoretical or even experimental data would give astronomers more trust in utilizing theoretical data for comparison to interstellar spectra.…”

“…For instance, a recent survey of the low-mass warm core IRAS 16293-2422 returned a non-detection for this radical and inconclusive results about its possible interstellar abundance (Martín-Doménech et al 2016). This is perplexing since HSS is believed to exist in relatively high abundances in icy grains and may be even be the most common form of sulfur in such environnments, even more than H 2 S based on observations of the Hale-Bopp comet (Bockelée-Morvan et al 1995).…”

“…The aforementioned work searching for HSS in IRAS 16293-2422(Martín-Doménech et al 2016 relied upon low-resolution data obtained several instrumental generations ago (Yamamoto & Saito 1994). The accuracy of these experimental measurements has not been fully verified, and errors could still be on the order of tens of MHz.…”