Depletion of chlorine into HCl ice in a protostellar core: The CHESS spectral survey of OMC-2 FIR 4
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_2Cl^+ 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_2Cl^+ 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.
© 2015 ESO. Received 2 August 2014. Accepted 20 November 2014. Published online 03 February 2015. We would like to thank the anonymous referee for constructive comments that helped to improve the manuscript. We also thank Catherine Walsh, Alexandre Faure, Yulia Kalugina, Laurent Wiesenfeld and Ewine van Dishoeck for useful discussions; Charlotte Vastel for help with molecular data; and Evelyne Roueff for support with the Meudon code. Astrochemistry in Leiden is supported by the Netherlands Research School for Astronomy (NOVA), by a Royal Netherlands Academy of Arts and Sciences (KNAW) professor prize, and by the European Union A-ERC grant 291141 CHEMPLAN. V.W. acknowledges funding by the ERC Starting Grant 3DICE (grant agreement 336474). F.L. and M.L. acknowledge support by the Agence Nationale de la Recherche (ANR-HYDRIDES), contract ANR-12-BS05-0011-01, by the CNRS national program "Physique et Chimie du Milieu Interstellaire" and by the CPER Haute-Normandie/CNRT/Énergie, Électronique, Matériaux. Support for this work was provided by NASA (Herschel OT funding) through an award issued by JPL/Caltech. We gratefully acknowledge Göran Pilbratt for granting Herschel Director's Discretionary Time that greatly improved the HIFI data sensitivity. HIFI has been designed and built by a consortium of institutes and university departments from across Europe, Canada and the United States under the leadership of SRON Netherlands Institute for Space Research, Groningen, The Netherlands and with major contributions from Germany, France and the US. Consortium members are: Canada: CSA, U. Waterloo; France: CESR, LAB, LERMA, IRAM; Germany: KOSMA, MPIfR, MPS; Ireland, NUI Maynooth; Italy: ASI, IFSI-INAF, Osservatorio Astrofisico di Arcetri-INAF; Netherlands: SRON, TUD; Poland: CAMK, CBK; Spain: Observatorio Astronómico Nacional (IGN), Centro de Astrobiología (CSIC-INTA). Sweden: Chalmers University of Technology – MC2, RSS & GARD; Onsala Space Observatory; Swedish National Space Board, Stockholm University – Stockholm Observatory; Switzerland: ETH Zurich, FHNW; USA: Caltech, JPL, NHSC. The Caltech Submillimeter Observatory is operated by the California Institute of Technology under cooperative agreement with the National Science Foundation (AST-0838261). Based on analysis carried out with the CASSIS software. CASSIS has been developed by IRAP-UPS/CNRS.
Submitted - 1411.6483v1.pdf
Published - aa24737-14.pdf