Astrochemistry

APEX Survey Of Interstellar HCl: 35Cl/37Cl Isotopic Ratios In Dense Cores And Outflows

By Keith Cowing

Status Report

astro-ph.GA

December 28, 2025

Filed under astro-ph.GA, astrochemistry, astronomy, Interstellar, interstellar medium, metallicity, Spectroscopy

APEX Survey Of Interstellar HCl: 35Cl/37Cl Isotopic Ratios In Dense Cores And Outflows — Distribution of the targets observed in this work across the Milky Way in white “+” symbols along with their identifiers, as listed in Table 1. The red asterisk and Sun symbols mark the Galactic centre at (0, 0) kpc and the position of the Solar System at (0, −8.15) kpc, respectively (Reid et al. 2019). The sources are projected onto the Galactic plane with parallax-based distances from Reid et al. (2019). The underlying Milky Way spiral arm pattern follows the log-periodic model from the same study, with colours adapted from Jacob et al. (2022). The background image, is an artist’s impression of the large-scale structure of the Milky Way, courtesy of ESA/Gaia/DPAC/Stefan Payne-Wardenaar. — astro-ph.GA

Despite being only the 19th 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 probes nucleosynthesis across the Galaxy. Yet, studies of Cl-bearing molecules have remained limited to a few sightlines due to observational challenges.

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.

We observed the ground-state J=1−0 lines of H³⁵Cl and H³⁷Cl toward 28 high-mass star-forming regions with SEPIA660 on APEX, more than doubling the number of known HCl detections and revealing with XCLASS models emission from both cores and outflows.

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. We find column densities of the order of 1013cm−2 for H³⁵Cl and isotopic ratios between 1.6 and 3.5 in emission-only sources.

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 synthesized 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.

Lennart M. Böhm, Arshia M. Jacob, Friedrich Wyrowski, Karl M. Menten, Katharina Immer, Ashley T. Barnes

Comments: Accepted for publication in A&A (15 pages, 9 figures, 4 tables)
Subjects: Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2511.21813 [astro-ph.GA] (or arXiv:2511.21813v1 [astro-ph.GA] for this version)
https://doi.org/10.48550/arXiv.2511.21813
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Submission history
From: Lennart Böhm
[v1] Wed, 26 Nov 2025 19:00:01 UTC (21,175 KB)
https://arxiv.org/abs/2511.21813
Astrobiology, Astrochemistry,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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