Herschel observations of interstellar chloronium
Journal article, 2012

Using the Herschel Space Observatory's Heterodyne Instrument for the Far-Infrared, 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–1 cloud) and W31C. Both the para-H35 2Cl+ and para-H37 2Cl+ isotopologues were detected, through observations of their 111-000 transitions at rest frequencies of 485.42 and 484.23 GHz, respectively. For an assumed ortho-to-para ratio (OPR) 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 OPR of 3 for chloronium, the observed emission line fluxes imply total beam-averaged column densities of ~2 × 1013 cm–2 and ~1.2 × 1013 cm–2, respectively, for chloronium in these two sources. We obtained upper limits on the para-H35 2Cl+ 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.

submillimeter: ISM

molecular processes

ISM: molecules


D. A. Neufeld

Johns Hopkins University

Evelyne Roueff

Observatoire de Paris-Meudon

Ronald Snell

University of Massachusetts

D. C. Lis

California Institute of Technology (Caltech)

A. O. Benz

Swiss Federal Institute of Technology in Zürich (ETH)

S. Bruderer

Max Planck Society

John H Black

Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics

Massimo De Luca

LERMA - Laboratoire d'Etudes du Rayonnement et de la Matiere en Astrophysique et Atmospheres

M. Gerin

LERMA - Laboratoire d'Etudes du Rayonnement et de la Matiere en Astrophysique et Atmospheres

Paul F. Goldsmith

Jet Propulsion Laboratory, California Institute of Technology

H. Gupta

Jet Propulsion Laboratory, California Institute of Technology

N. Indriolo

Johns Hopkins University

Jacques Le Bourlot

Observatoire de Paris-Meudon

Franck Le Petit

Observatoire de Paris-Meudon

Bengt Larsson

Chalmers, Earth and Space Sciences

G. J. Melnick

Harvard-Smithsonian Center for Astrophysics

K. M. Menten

Max Planck Society

Raquel Monje

California Institute of Technology (Caltech)

Zsofia Nagy

University of Groningen

T.G. Phillips

California Institute of Technology (Caltech)

A. Sandqvist

Stockholm Observatory

P. Sonnentrucker

Space Telescope Science Institute (STScI)

F. F. S. van der Tak

Netherlands Institute for Space Research (SRON)

University of Groningen

M. Wolfire

University of Maryland

Astrophysical Journal

0004-637X (ISSN) 1538-4357 (eISSN)

Vol. 748 2012 March 20 1-11 37

Subject Categories

Astronomy, Astrophysics and Cosmology


Basic sciences


Onsala Space Observatory



More information

Latest update