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We report on the detection with the HIFI instrument on board the Herschelsatellite of the two hydrogen chloride isotopologues, H35Cl and H37Cl, towardsthe massive star-forming region W3A. The J=1-0 line of both species wasobserved with receiver 1b of the HIFI instrument at 625.9 and 624.9 GHz. Thedifferent hyperfine components were resolved. The observations were modeledwith a non-local, non-LTE radiative transfer model that includes hyperfine lineoverlap and radiative pumping by dust. Both effects are found to play animportant role in the emerging intensity from the different hyperfinecomponents. The inferred H35Cl column density (a few times 1e14 cm^-2), andfractional abundance relative to H nuclei (~7.5e^-10), supports an upper limitto the gas phase chlorine depletion of ~200. Our best-fit model estimate of theH35Cl/H37Cl abundance ratio is ~2.1+/-0.5, slightly lower, but still compatiblewith the solar isotopic abundance ratio (~3.1). Since both species wereobserved simultaneously, this is the first accurate estimation of the[35Cl]/[37Cl] isotopic ratio in molecular clouds. Our models indicate that evenfor large line opacities and possible hyperfine intensity anomalies, the H35Cland H37Cl J=1-0 integrated line-intensity ratio provides a good estimate of the35Cl/37Cl isotopic abundance ratio.

The 35Cl/37Cl isotopic ratio in dense molecular clouds: HIFI observations of hydrogen chloride towards W3A