Depletion of chlorine into HCl ice in a protostellar core

The CHESS spectral survey of OMC-2 FIR 4^⋆

¹ Leiden Observatory, PO Box 9513 2300 RA, Leiden, The Netherlands
e-mail: mkama@strw.leidenuniv.nl
² Université de Toulouse, UPS-OMP, IRAP, Toulouse, France
³ CNRS, IRAP, 9 Av. colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
⁴ Université de Grenoble Alpes, IPAG, 38000 Grenoble, France
⁵ CNRS, IPAG, 38000 Grenoble, France
⁶ Univ. Bordeaux, LAB, UMR 5804, 33270 Floirac, France
⁷ CNRS, LAB, UMR 5804, 33270 Floirac, France
⁸ Astronomical Institute Anton Pannekoek, Science Park 904, 1098 XH Amsterdam, The Netherlands
⁹ Department of Astrophysics/IMAPP, Radboud University Nijmegen, Nijmegen, The Netherlands
¹⁰ LOMC – UMR 6294, CNRS-Université du Havre, 25 rue Philippe Lebon, BP 1123, 76063 Le Havre Cedex, France
¹¹ LERMA, Observatoire de Paris, PSL Research University, CNRS, UMR 8112, 75014, Paris, France
¹² Sorbonne Universités, Université Pierre et Marie Curie, Paris 6, CNRS, Observatoire de Paris, UMR 8112, LERMA, Paris, France
¹³ California Institute of Technology, Cahill Center for Astronomy and Astrophysics 301-17, CA 91125, Pasadena, USA
¹⁴ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Received: 2 August 2014
Accepted: 20 November 2014

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.