Depletion of chlorine into HCl ice in a protostellar core
The CHESS spectral survey of OMC-2 FIR 4⋆
PO Box 9513
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
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.
Key words: stars: formation / astrochemistry / ISM: abundances
Appendices are available in electronic form at http://www.aanda.org
© ESO, 2015