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OBSERVATIONAL PROJECTS - CHICAGO
Discovery of H3+ in the Diffuse Interstellar
Medium
While the abundance of H3+ in dense clouds is well
understood in terms of a simple chemical model
(more details),
our detection
of H3+ towards Cygnus OB2 12 was a big surprise.
The sightline towards Cygnus OB2 12 is heavily extinguished, but appears to
contain primarily diffuse material [based on the low CO column density].
In diffuse clouds, H3+ was expected to have a very
low steady-state abundance because of the efficiency of dissociative
recombination with electrons, which are abundantly produced by
photo-ionization of atomic carbon. However, we observed a column density
of H3+ towards Cygnus OB2 12 that is comparable to
that in dense cloud sources! This implies that either the absorption
path length consists of nearly the entire distance to the background star
(which is unrealistic), or that there is a serious problem with the
simple chemical model we use.
Some people suggested that Cygnus OB2 12 might be a special case, because
of the presence of an association of young hot stars. However, we
subsequently detected H3+ in a variety of diffuse
cloud sightlines, including HD 183143, which is the classic sightline
used by Herbig in his studies of the diffuse interstellar bands. These
observations implied that there is, in fact, a general and serious problem
with the chemical model for H3+, which simply consists
of a balance between formation by cosmic-ray ionization of H2
versus destruction by dissociative recombination.
References:
B. J. McCall, K. H. Hinkle, T. R. Geballe, G. H. Moriarty-Schieven,
N. J. Evans II, K. Kawaguchi, S. Takano, V. V. Smith, and T. Oka
Observations of H3+ in the Diffuse
Interstellar Medium
Astrophysical Journal, 567, 391 (2002)
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T. R. Geballe, B. J. McCall, K. H. Hinkle, and T. Oka
Detection of H3+ in the Diffuse Interstellar
Medium: The Galactic Center and Cygnus OB2 No. 12
Astrophysical Journal, 510, 251 (1999)
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B. J. McCall, K. H. Hinkle, T. R. Geballe, and T. Oka
H3+ in Dense and Diffuse Clouds
Faraday Discussions, vol. 109, 267-280 (1998)
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B. J. McCall, T. R. Geballe, K. H. Hinkle, and T. Oka
Detection of H3+ in the Diffuse Interstellar
Medium Toward
Cygnus OB2 No. 12
Science, 279, 1910 (1998)
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