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Herschel/HIFI Discoveryof a Far-Infrared DIB Analog†‡

Published online by Cambridge University Press:  21 February 2014

H. S. P. Müller ,
P. Schilke ,
M. Gerin ,
D. C. Lis ,
E. A. Bergin ,
J. C. Pearson ,
M. De Luca  and
J. H. Black
H. S. P. Müller
Affiliation:
I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany email: [email protected]
P. Schilke
Affiliation:
I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany email: [email protected]
M. Gerin
Affiliation:
LERMA, Observatoire de Paris, ENS and CNRS, 75014 Paris, France
D. C. Lis
Affiliation:
California Institute of Technology, Pasadena, CA 91125, USA
E. A. Bergin
Affiliation:
Department of Astronomy, University of Michigan, Ann Arbor, MI 48109, USA
J. C. Pearson
Affiliation:
Jet Propulsion Laboratory, Caltech, Pasadena, CA 91109, USA
M. De Luca
Affiliation:
LERMA, Observatoire de Paris, ENS and CNRS, 75014 Paris, France
J. H. Black
Affiliation:
Earth & Space Sciences, Chalmers University of Technology, Onsala, Sweden
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Abstract

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The Herschel Space Observatory carried out observations at far-infrared wavelengths, which significantly increased our knowledge of the interstellar medium and the star-formation process in the Milky Way and external galaxies, as well as our understanding of astrochemistry.

Absorption features, known, e.g., from observations at millimeter wavelengths, are more commonly observed in the far-infrared, in particular toward strong dust continuum sources. The lowest energy transitions are not only observed at LSR-velocities related to the source, but often also at velocities associated with diffuse molecular clouds along the line of sight toward the background source.

Unbiased spectral line surveys of the massive and very luminous Galactic Center sources Sagittarius B2(M) and (N) were carried out across the entire frequency range of the high-resolution Heterodyne Instrument for Far-Infrared Astronomy (HIFI). An absorption feature was detected toward both sources at about 617.531 GHz, corresponding to 20.599 cm−1, 485.47 μm, or 2.5539 meV. This feature is unique in its appearance at all velocity components associated with diffuse foreground molecular clouds, together with its conspicuous absence at velocities related to the sources themselves. The carriers of at least a substantial part of the DIBs are thought to reside in the diffuse interstellar medium. Therefore, we consider this absorption feature to be a far-infrared DIB analog.

Subsequent dedicated observations confirmed that the line is present only in the foreground clouds on the line of sight toward other massive star-forming regions in the Galactic disk. There is indication that the feature has substructure, possibly of fine or hyperfine nature. Attempts to assign the feature to atomic or molecular species have been unsuccessful so far.

Keywords

Submillimeter: ISMISM: lines and bandsISM: cloudsLine: identification
Type
Contributed Papers
Information
Proceedings of the International Astronomical Union , Volume 9 , Symposium S297: The Diffuse Interstellar Bands , May 2013 , pp. 197 - 202
Copyright
Copyright © International Astronomical Union 2014 

Footnotes

†

Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.

‡

Note added in proof: M. Barlow et al. recently detected the same line toward a supernova remnant with the identification as the ground state rotational transition of 36ArH+ (“The Universe Explored by Herschel”, Oct. 15–18, Noordwijk, the Netherlands). While the line is no longer unidentified, all other aspects of the discussion presented here remain valid. The correct assignment was confused by the fact that terrestrial Ar consists predominantly of 40Ar, which, however, originates predominantly from the radioactive decay of 40K. In addition, the initial CDMS entry of 36ArH+ was incorrect.

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