Depletion of chlorine into HCl ice in a protostellar core

Abstract

Context. The freezeout of gas-phase species onto cold dust grains can drastically alter the chemistry and the heating-cooling balance of protostellar material. In contrast to well-known species such as carbon monoxide (CO), the freezeout of various carriers of elements with abundances < 10 −5 has not yet been well studied. Aims. Our aim here is to study the depletion of chlorine in the protostellar core, OMC-2 FIR 4. Methods. We observed transitions of HCl and H 2 Cl + towards OMC-2 FIR 4 using the Herschel Space Observatory and Caltech Submillimeter Observatory facilities. Our analysis makes use of state of the art chlorine gas-grain chemical models and newly calculated HCl-H 2 hyperfine collisional excitation rate coefficients. Results. A narrow emission component in the HCl lines traces the extended envelope, and a broad one traces a more compact central region. The gas-phase HCl abundance in FIR 4 is 9 × 10 −11 , a factor of only 10 −3 that of volatile elemental chlorine. The H 2 Cl + lines are detected in absorption and trace a tenuous foreground cloud, where we find no depletion of volatile chlorine. Conclusions. Gas-phase HCl is the tip of the chlorine iceberg in protostellar cores. Using a gas-grain chemical model, we show that the hydrogenation of atomic chlorine on grain surfaces in the dark cloud stage sequesters at least 90% of the volatile chlorine into HCl ice, where it remains in the protostellar stage. About 10% of chlorine is in gaseous atomic form. Gas-phase HCl is a minor, but diagnostically key reservoir, with an abundance of 10 −10 in most of the protostellar core. We find the [ 35 Cl]/[ 37 Cl] ratio in OMC-2 FIR 4 to be 3.2 ± 0.1, consistent with the solar system value.

Halogens as tracers of protosolar nebula material in comet 67P/Churyumov–Gerasimenko

“…This result is compatible with a protosolar origin in view of our understanding of the chemical history of halogens in molecular clouds, where observations and models point to a halogen deficit in the gas phase whereas halogens end up as hydrogen halides on dust grains (Kama et al 2015).…”

“…To Lodders (2010). c protostellar core value in present-day molecular clouds in the local galactic environment from Kama et al (2015). d range for volatiles believed to be from Vesta, values from Barrett et al (2016).…”

“…Astronomical detection of HCl in such clouds indicates that it is strongly depleted in the gas phase (Blake, Keene & Phillips 1985), which suggests that HCl is frozen out on grains in these cold environments (Peng et al 2010;Emprechtinger et al 2012). Kama et al (2015) presented the first gas-grain chemical model for chlorine. Using data of various chlorine species from the Herschel Space Observatory and from the Caltech Submillimeter Observatory, they estimate that in protostellar cores at least 90 per cent of the full chlorine inventory is locked in HCl ice.…”

Water deuteration and ortho-to-para nuclear spin ratio of H2in molecular clouds formed via the accumulation of H I gas

“…The success of the LM abundances implies that the depletion of heavy metals from the gas phase occurs during the evolution from diffuse clouds to denser clouds, which is supported by some observational evidence (Joseph et al 1986). Plausible mechanisms of the depletion are thought to be entrapment in ices (Andersson et al 2013;Kama et al 2015) and incorporation into refractory dust grains (Keller et al 2002). The former process is naturally included in our simulations, while the latter is not.…”

Evidence for distributed gas sources of hydrogen halides in the coma of comet 67P/Churyumov–Gerasimenko

“…Dhooghe et al (2017) report the detection of the hydrogen halides HF, HCl, and HBr in the coma. They explain their presence by unprocessed incorporation of interstellar grains in the comet nucleus at the time of formation, since the observed depletion of the hydrogen halides in the gas phase of dense molecular clouds strongly suggests that these halides reside on the surface of the grains in such clouds (see also Kama et al 2015;Peng et al 2010;Emprechtinger et al 2012). The present paper uses the same data set to study the abundances of the hydrogen halides observed by DFMS in the atmosphere of 67P in more detail.…”