Depletion of chlorine into HCl ice in a protostellar core. The CHESS spectral survey of OMC-2 FIR 4

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 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-H₂ 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 H₂Cl⁺ 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 [³⁵Cl]/[³⁷Cl] ratio in OMC-2 FIR 4 to be 3.2 ± 0.1, consistent with the solar system value.

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