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

TL;DR

This study reports the first accurate measurement of the 35Cl/37Cl isotopic ratio in dense molecular clouds using Herschel/HIFI observations of hydrogen chloride, revealing insights into chlorine isotopic composition in star-forming regions.

Contribution

It provides the first precise estimation of the 35Cl/37Cl ratio in molecular clouds through detailed radiative transfer modeling of HIFI data.

Findings

H35Cl/H37Cl ratio estimated at ~2.1±0.5

Supports an upper limit of ~200 for chlorine depletion

Line intensity ratio effectively estimates isotopic ratio

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 W3A. 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 1e14 cm^-2), and fractional abundance relative to H nuclei (~7.5e^-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.