APEX survey of interstellar HCl: ${}^{35}$Cl/${}^{37}$Cl isotopic ratios in dense cores and outflows

Abstract

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 ${}^{35}\text{Cl}{/}^{37}$Cl 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 [H${}^{35}$Cl]/[H${}^{37}$Cl] 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$$-$0 lines of H${}^{35}$Cl and H${}^{37}$Cl toward 28 high-mass star-forming regions with SEPIA660 on APEX, more than doubling the number of known HCl detections and revealing with XCLASS models emission from both cores and outflows. H${}^{35}$Cl was detected in all sources, H${}^{37}$Cl 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 $10^{13}{\mathrm{cm}}^{-2}$ for H${}^{35}$Cl and isotopic ratios between $1.6$ and $3.5$ in emission-only sources. The derived [H${}^{35}$Cl]/[H${}^{37}$Cl] 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 H${}^{35}$Cl 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.

arXiv commentAccepted for publication in A&A (15 pages, 9 figures, 4 tables)

References in corpus (27)

• SciPy 1.0--Fundamental Algorithms for Scientific Computing in Python
• The Cologne Database for Molecular Spectroscopy, CDMS, in the Virtual Atomic and Molecular Data Centre, VAMDC
• Trigonometric Parallaxes Of High-Mass Star Forming Regions: Our View Of The Milky Way
• Nucleosynthesis Yields of Core-Collapse Supernovae and Hypernovae, and Galactic Chemical Evolution
• ATLASGAL - The APEX Telescope Large Area Survey of the Galaxy at 870 microns
• Hi-GAL: the Herschel infrared Galactic Plane Survey
• High-Mass Star and Massive Cluster Formation in the Milky Way
• Complex organic molecules in the interstellar medium: IRAM 30 m line survey of Sagittarius B2(N) and (M)
• The Evolution of Isotope Ratios in the Milky Way Galaxy
• High-resolution wide-band Fast Fourier Transform spectrometers
• Star formation near the Sun is driven by expansion of the Local Bubble
• The chemistry of interstellar molecules containing the halogen elements
• Interstellar Hydrides
• eXtended CASA Line Analysis Software Suite (XCLASS)
• ATLASGAL-selected massive clumps in the inner Galaxy: I. CO depletion and isotopic ratios
• Explosive Outflows Powered by the Decay of Non-Hierarchical Multiple Systems of Massive Stars: Orion BN/KL
• The Hi-GAL compact source catalogue -- II. The 360° catalogue of clump physical properties
• Herschel/HIFI discovery of interstellar chloronium (H${}_2$Cl${}^{+}$)
• A high resolution line survey of IRC+10216 with Herschel. First results: Detection of warm silicon dicarbide SiC2
• SEPIA - a new single pixel receiver at the APEX Telescope
• Zooming in on Individual Star Formation: Low- and High-mass Stars
• A Comprehensive Survey of Hydrogen Chloride in the Galaxy
• The 35Cl/37Cl isotopic ratio in dense molecular clouds: HIFI observations of hydrogen chloride towards W3A
• HyGAL: Characterizing the Galactic ISM with observations of hydrides and other small molecules -- I. Survey description and a first look toward W3(OH), W3 IRS5 and NGC 7538 IRS1
• Chlorine Abundances in Cool Stars
• Chlorine Isotope Ratios in M Giants
• The chemistry of chlorine-bearing species in the diffuse interstellar medium, and new SOFIA/GREAT observations of HCl${}^{+}$