Depletion of chlorine into HCl ice in a protostellar core

Abstract : 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. Our aim here is to study the depletion of chlorine in the protostellar core, OMC-2 FIR 4. We observed transitions of HCl and H2Cl+ 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. 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 9e-11, a factor of only 0.001 that of volatile elemental chlorine. The H2Cl+ lines are detected in absorption and trace a tenuous foreground cloud, where we find no depletion of volatile chlorine. 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 <1e-10 in most of the protostellar core. We find the 35Cl/37Cl ratio in OMC-2 FIR 4 to be 3.2\pm0.1, consistent with the solar system value.
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Astronomy and Astrophysics - A&A, EDP Sciences, 2015, 574, pp.id.A107. 〈10.1051/0004-6361/201424737〉
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https://hal.archives-ouvertes.fr/hal-01090994
Contributeur : Marie-Paule Pomies <>
Soumis le : jeudi 4 décembre 2014 - 14:37:12
Dernière modification le : mercredi 25 octobre 2017 - 09:42:02

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M., Kama, E., Caux, A., Lopez-Sepulcre, Valentine Wakelam, C., Dominik, et al.. Depletion of chlorine into HCl ice in a protostellar core. Astronomy and Astrophysics - A&A, EDP Sciences, 2015, 574, pp.id.A107. 〈10.1051/0004-6361/201424737〉. 〈hal-01090994〉

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