The³⁵Cl/³⁷Cl isotopic ratio in dense molecular clouds: HIFI observations of hydrogen chloride towards W3 A

Abstract

We report on the detection with the HIFI instrument on board the <i>Herschel<i/> satellite of the two hydrogen chloride isotopologues, H<sup>35<sup/>Cl and H<sup>37<sup/>Cl, towards the massive star-forming region W3 A. The <i>J<i/> = 1–0 line of both species was observed with receiver <i>1b<i/> 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 H<sup>35<sup/>Cl column density (a few times ~10<sup>14<sup/> cm<sup>-2<sup/>), and fractional abundance relative to H nuclei (~7.5 × 10<sup>-10<sup/>), supports an upper limit to the gas phase chlorine depletion of <i>≈<i/>200. Our best-fit model estimate of the H<sup>35<sup/>Cl/H<sup>37<sup/>Cl abundance ratio is <i>≈<i/> 2.1 <i>±<i/> 0.5, slightly lower, but still compatible with the solar isotopic abundance ratio (<i>≈<i/>3.1). Since both species were observed simultaneously, this is the first accurate estimation of the [<sup>35<sup/>Cl] /[<sup>37<sup/>Cl] isotopic ratio in molecular clouds. Our models indicate that even for large line opacities and possible hyperfine intensity anomalies, the H<sup>35<sup/>Cl and H<sup>37<sup/>Cl <i>J<i/> = 1–0 integrated line-intensity ratio provides a good estimate of the <sup>35<sup/>Cl/<sup>37<sup/>Cl isotopic abundance ratio.