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)
LINK
PDF
PS

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)
LINK
PDF
PS

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)
LINK
PDF
PS

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)
LINK
PDF
PS