Letter to the Editor

The ³⁵Cl/³⁷Cl isotopic ratio in dense molecular clouds: HIFI observations of hydrogen chloride towards W3 A^*

¹ Centro de Astrobiología. CSIC-INTA. Carretera de Ajalvir, Km 4, Torrejón de Ardoz. 28850, Madrid, Spain e-mail: jcernicharo@cab.inta-csic.es
² LUTH, Observatoire de Paris-Meudon, 5 Place Jules Janssen, 92190 Meudon, France
³ Centre d'Étude Spatiale des Rayonnements, Université de Toulouse [UPS], 31062 Toulouse Cedex 9, France
⁴ Atacama Large Millimeter/Submillimeter Array, ALMA Office, Santiago, Chile
⁵ SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD Groningen, The Netherlands
⁶ Jet Propulsion Laboratory, 4800 Oak Grove Drive, MC 302-231, Pasadena, CA 91109, USA
⁷ MPI für Sonnensystemforschung, 37191 Katlenburg-Lindau, Germany
⁸ KOSMA, I. Physik. Institut, Universität zu Köln, Germany
⁹ With Raytheon Co., Fort Wayne, Indiana, USA, since March of 2009

Received: 31 March 2010
Accepted: 12 May 2010

Abstract

We report on the detection with the HIFI instrument on board the Herschel satellite of the two hydrogen chloride isotopologues, H³⁵Cl and H³⁷Cl, towards the massive star-forming region W3 A. The J = 1–0 line of both species was observed with receiver 1b of the HIFI instrument at ~625.9 and ~624.9 GHz. The different hyperfine components were resolved. The observations were modeled with a non-local, non-LTE radiative transfer model that includes hyperfine line overlap and radiative pumping by dust. Both effects are found to play an important role in the emerging intensity from the different hyperfine components. The inferred H³⁵Cl column density (a few times ~10¹⁴ cm^-2), and fractional abundance relative to H nuclei (~7.5 × 10^-10), supports an upper limit to the gas phase chlorine depletion of ≈200. Our best-fit model estimate of the H³⁵Cl/H³⁷Cl abundance ratio is ≈ 2.1 ± 0.5, slightly lower, but still compatible with the solar isotopic abundance ratio (≈3.1). Since both species were observed simultaneously, this is the first accurate estimation of the [³⁵Cl] /[³⁷Cl] isotopic ratio in molecular clouds. Our models indicate that even for large line opacities and possible hyperfine intensity anomalies, the H³⁵Cl and H³⁷Cl J = 1–0 integrated line-intensity ratio provides a good estimate of the ³⁵Cl/³⁷Cl isotopic abundance ratio.