Herschel observations of interstellar chloronium

Abstract

Using the Herschel Space Observatory's Heterodyne Instrument for the Far-Infrared (HIFI), we have observed para-chloronium (H2Cl+) toward six sources in the Galaxy. We detected interstellar chloronium absorption in foreground molecular clouds along the sight-lines to the bright submillimeter continuum sources Sgr A (+50 km/s cloud) and W31C. Both the para-H2-35Cl+ and para-H2-37Cl+ isotopologues were detected, through observations of their 1(11)-0(00) transitions at rest frequencies of 485.42 and 484.23 GHz, respectively. For an assumed ortho-to-para ratio of 3, the observed optical depths imply that chloronium accounts for ~ 4 - 12% of chlorine nuclei in the gas phase. We detected interstellar chloronium emission from two sources in the Orion Molecular Cloud 1: the Orion Bar photodissociation region and the Orion South condensation. For an assumed ortho-to-para ratio of 3 for chloronium, the observed emission line fluxes imply total beam-averaged column densities of ~ 2.0E+13 cm-2 and ~ 1.2E+13 cm-2, respectively, for chloronium in these two sources. We obtained upper limits on the para-H2-35Cl+ line strengths toward H2 Peak 1 in the Orion Molecular cloud and toward the massive young star AFGL 2591. The chloronium abundances inferred in this study are typically at least a factor ~10 larger than the predictions of steady-state theoretical models for the chemistry of interstellar molecules containing chlorine. Several explanations for this discrepancy were investigated, but none has proven satisfactory, and thus the large observed abundances of chloronium remain puzzling.

Accepted for publication in the Astrophysical Journal

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• Formation of different isotopomers of chloronium in the interstellar medium
• The Role of Terahertz and Far-IR Spectroscopy in Understanding the Formation and Evolution of Interstellar Prebiotic Molecules
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• The HIFI spectral survey of AFGL 2591 (CHESS). II. Summary of the survey
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• The ortho-to-para ratio of H${}_2$Cl${}^{+}$: Quasi-classical trajectory calculations and new simulations in light of new observations
• The first results from the Herschel-HIFI mission
• The Galactic Chemical Evolution of Chlorine
• The chemistry of chlorine-bearing species in the diffuse interstellar medium, and new SOFIA/GREAT observations of HCl${}^{+}$
• The high resolution infrared spectrum of HCl${}^{+}$
• Cl+ and HCl+ in Reaction with H2 and Isotopologues: A Glance into H Abstraction and Indirect Exchange at Astrophysical Conditions
• Rotational excitation cross sections for chloronium based on a new 5D interaction potential with molecular hydrogen