Searches for HCl and HF in comets 103P/Hartley 2 and C/2009 P1 (Garradd) with the Herschel Space Observatory^⋆

¹ LESIA, Observatoire de Paris, CNRS, UPMC, Université Paris-Diderot, 5 place Jules Janssen, 92195 Meudon, France
e-mail: dominique.bockelee@obspm.fr
² California Institute of Technology, MC 301-17, Pasadena CA 91125, USA
³ Max-Planck-Institut für Sonnensystemforschung, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany
⁴ Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA
⁵ Space Research Centre, Polish Academy of Science, Bartycka 18a, 00-716 Warszawa, Poland
⁶ Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, Domaine Universitaire, 38406 Saint Martin d’ Hères, France
⁷ Department of Astrophysics, CAB, INTA-CSIC, Crta Torrejón-Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain
⁸ NASA Goddard Space Flight Center, Greenbelt MD 20770, USA
⁹ Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, 2455 Hayward Street, Ann Arbor MI 48109-2143, USA
¹⁰ ALMA Observatory, Alonso de Córdova 3107, 7630000 Vitacura, Santiago, Chile
¹¹ LERMA, Observatoire de Paris, CNRS, UPMC, 61 avenue de l’Observatoire, 75014 Paris, France
¹² European Space Astronomy, ESAC, PO Box 78, 28691 Villanueva de la Cañada, Spain
¹³ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany

Received: 29 October 2013
Accepted: 18 December 2013

Abstract

Context. Hydrogen chloride (HCl) and hydrogen fluoride (HF) are expected to be the main reservoirs of fluorine and chlorine over a wide range of conditions, wherever hydrogen is predominantly molecular. They are found to be strongly depleted in dense molecular clouds, suggesting freeze-out onto grains in such cold environments. We can then expect that HCl and HF were also the major carriers of Cl and F in the gas and icy phases of the outer solar nebula, and were incorporated into comets.

Aims. We aimed to measure the HCl and HF abundances in cometary ices as they can provide insights on the halogen chemistry in the early solar nebula.

Methods. We searched for the J(1−0) lines of HCl and HF at 626 and 1232 GHz, respectively, using the Heterodyne Instrument for the Far-Infrared (HIFI) onboard the Herschel Space Observatory. HCl was searched for in comets 103P/Hartley 2 and C/2009 P1 (Garradd), whereas observations of HF were conducted in comet C/2009 P1 (Garradd). In addition, observations of H₂O and H₂¹⁸O lines were performed in C/2009 P1 (Garradd) to measure the H₂O production rate at the time of the HCl and HF observations. Three lines of CH₃OH were serendipitously observed in the HCl receiver setting.

Results. HCl is not detected, whereas a marginal (3.6-σ) detection of HF is obtained. The upper limits for the HCl abundance relative to water are 0.011% and 0.022%, for comet 103P/Hartley 2 and C/2009 P1 (Garradd), respectively, showing that HCl is depleted with respect to the solar Cl/O abundance by a factor more than 6$\begin{array}{c}\mathrm{+}\mathrm{6}\\ -3\end{array}$ in 103P/Hartley 2, where the error is related to the uncertainty in the chlorine solar abundance. The marginal HF detection obtained in C/2009 P1 (Garradd) corresponds to an HF abundance relative to water of (1.8 ± 0.5) × 10^-4, which is approximately consistent with a solar photospheric F/O abundance. The inferred water production rate in comet C/2009 P1 (Garradd) is (1.1 ± 0.3) × 10²⁹ s^-1 and (0.75 ± 0.05) × 10²⁹ s^-1 on 17 and 23 February 2012, respectively. CH₃OH abundances relative to water are 2.7 ± 0.3% and 3.4  ±  0.6%, for comets 103P/Hartley 2 and C/2009 P1 (Garradd), respectively.

Conclusions. The observed depletion of HCl suggests that HCl was not the main reservoir of chlorine in the regions of the solar nebula where these comets formed. HF was possibly the main fluorine compound in the gas phase of the outer solar nebula. However, this needs to be confirmed by future measurements.