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

TL;DR

This study presents a comprehensive survey of interstellar HCl isotopologues toward 28 high-mass star-forming regions using the APEX SEPIA660 receiver, targeting the ground-state transitions near $625$ GHz to constrain the $^{35}$Cl/$^{37}$Cl ratio across the Galaxy. By applying LTE radiative transfer modeling with XCLASS, including core and outflow components, the authors derive column densities and isotopic ratios, finding $N_{ m tot}$ in the range of a few $ imes10^{13} m \,cm^{-2}$ and $^{35}$Cl/$^{37}$Cl ratios between $1.6$ and $3.5$ for emission-only sources, with an average around $2.6\pm0.8$. The results show no strong gradient of the isotopic ratio with Galactocentric radius, in line with Galactic chemical evolution models, though local variations may reflect recent nucleosynthesis or metallicity histories, especially near the Galactic center where higher ratios are observed. In addition to tracing chlorine chemistry in dense cores, the study detects HCl emission from outflows, including explosive events, suggesting HCl can probe a broader range of environments; the authors emphasize that higher-resolution interferometry is required to localize the HCl emission and better understand its origin in outflows and explosive phenomena.

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.

Figures (9)

• Figure 1: Chemical network displaying the most relevant gas-phase formation and destruction pathways involved in interstellar chlorine chemistry adapted from Fig. 3 of neufeld2021chemistry.
• Figure 2: Distribution of the targets observed in this work across the Milky Way in white "$+$" symbols along with their identifiers, as listed in Table \ref{['tab:source-params']}. 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 reid2019trigo_distances. The sources are projected onto the Galactic plane with parallax-based distances from reid2019trigonometric. The underlying Milky Way spiral arm pattern follows the log-periodic model from the same study, with colours adapted from jacob2022hygal. 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.
• Figure 3: Atmospheric transmission at the APEX $12m$ sub-mm telescope site in the frequency range from $\SIrange{600}{630}{\giga\hertz}$ for typical values of pwv columns in our observations: $0.2mm$ (blue) and $0.4mm$ (green). The dashed and dash-dotted black lines correspond to the $J = 1$$-$$0$ transition of H$^{35}$Cl and H$^{37}$Cl, respectively, with the grey shaded regions marking the sideband coverage of our observations.
• Figure 4: Upper panel: Observed spectral line profiles of the H$^{35}$Cl and H$^{37}$Cl $J=1$--0 transitions towards G291.272$-$00.714, G009.621+00.194, G029.954$-$00.016 and HGAL 285.264$-$00.049, along with their XCLASS fitted models (red curves). The grey dashed lines mark the systemic velocities, while the grey dotted lines mark the fitted continuum temperatures around the presented lines for each source. Bottom panel: The hyperfine structure splitting deconvolved column density profiles. The derived d$N$/d$\upsilon$ distributions with the corresponding 1$\sigma$ errors are displayed by the blue solid curves and blue shaded regions, respectively.
• Figure 5: Upper panel: Observed line profiles of the H$^{35}$Cl and H$^{37}$Cl $J=1$--0 line towards W43-MM1, HGAL 000.546$-$00.852, IRAS 16352$-$4721 and IRAS 16547$-$4247, along with their XCLASS fitted models (red curves). The grey dashed lines mark the systemic velocities, while the grey dotted lines mark the fitted continuum temperatures around the presented lines for each source. Bottom panel: The hyperfine structure splitting deconvolved column density profiles. The derived d$N$/d$\upsilon$ distributions with the corresponding 1$\sigma$ errors are displayed by the blue solid curve and blue shaded region, respectively. The absorption components are analogously illustrated in red.