APEX survey of interstellar HCl

³⁵Cl/³⁷Cl isotopic ratios in dense cores and outflows

¹ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching bei München, Germany
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany

^★ Corresponding author.

Received: 8 August 2025
Accepted: 25 November 2025

Abstract

Context. Despite being only the nineteenth most abundant element in the interstellar medium, chlorine’s reactivity and volatility give rise to a unique interstellar chemistry, favouring the formation of several chlorine-bearing hydrides. Further, the ³⁵Cl/³⁷Cl ratio – shaped in supernovae and evolved stars – probes nucleosynthesis across the Galaxy. Yet, studies of Cl-bearing molecules have remained limited to a few sight lines due to observational challenges.

Aims. We systematically investigated the Galactic distribution of HCl and the [H³⁵Cl]/[H³⁷Cl] ratio in high-mass star-forming regions. As a probe of a region’s nucleosynthesis history, this ratio may constrain predictions of Galactic chemical evolution models.

Methods. We surveyed the ground-state J = 1–0 transitions of H³⁵Cl and H³⁷Cl near 625 GHz towards 28 sources with the SEPIA660 receiver on the APEX 12 m sub-millimetre telescope. This survey more than doubles the number of sources with HCl detections and reveals HCl emission arising from both the background core and associated outflows. The spectra were modelled with XCLASS to derive column densities, isotopic ratios, and the kinematics of both the core and the outflow components.

Results. H³⁵Cl was detected in all sources, H³⁷Cl in all but two, with spectral line profiles ranging from those with only emission to complex emission–absorption mixtures. Column densities span from 2.3–22.8 × 10¹³ cm⁻² for H³⁵Cl and 0.6–12.5 × 10¹³ cm⁻² for H³⁷Cl, resulting in isotopic ratios between 1.6 and 3.5 in emission-only sources.

Conclusions. The derived [H³⁵Cl]/[H³⁷Cl] aligns with Galactic chemical evolution models and shows no trend with Galactocentric radius. However, local variations may reflect recent nucleosynthesis. Overall, the results suggest that most Galactic chlorine was synthesised during epochs of lower average metallicity in the Galaxy. Notably, we detect H³⁵Cl emission arising from outflows – particularly explosive ones – hinting at its presence in a broader range of environments. The present single-dish observations cannot reveal the origin of HCl in outflows, necessitating interferometric follow-up observations.