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

We report on the detection with the HIFI instrument on board the Herschel satellite of the two hydrogen chloride isotopologues, (HCl)-Cl-35 and (HCl)-Cl-37, 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 similar to 625.9 and similar to 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 (HCl)-Cl-35 column density (a few times similar to 10(14) cm(-2)), and fractional abundance relative to H nuclei (similar to 7.5 x 10(-10)), supports an upper limit to the gas phase chlorine depletion of approximate to 200. Our best-fit model estimate of the (HCl)-Cl-35/(HCl)-Cl-37 abundance ratio is approximate to 2.1 +/- 0.5, slightly lower, but still compatible with the solar isotopic abundance ratio (approximate to 3.1). Since both species were observed simultaneously, this is the first accurate estimation of the [Cl-35]/[Cl-37] isotopic ratio in molecular clouds. Our models indicate that even for large line opacities and possible hyperfine intensity anomalies, the (HCl)-Cl-35 and (HCl)-Cl-37 J = 1-0 integrated line-intensity ratio provides a good estimate of the Cl-35/Cl-37 isotopic abundance ratio.