Astronomy and Astrophysics – Astronomy
Scientific paper
May 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008aas...212.4604m&link_type=abstract
American Astronomical Society, AAS Meeting #212, #46.04; Bulletin of the American Astronomical Society, Vol. 40, p.244
Astronomy and Astrophysics
Astronomy
Scientific paper
H3+, the simplest polyatomic molecule, was long expected to be abundant in dense molecular clouds, initiating a network of ion-molecule reactions that produces most of the observed interstellar molecules. However, it was quite a surprise when abundant H3+ was discovered in diffuse molecular clouds, and its abundance has now revealed that the cosmic ray ionization rate in such clouds is roughly an order of magnitude larger than previously assumed.
The detection of interstellar H3+ itself would not have been possible without a supporting laboratory spectrum. Observers often take for granted the existence of such spectra, but in this case detecting the laboratory spectrum required a multi-year laboratory spectroscopic effort by Takeshi Oka at the NRC in Canada, which succeeded in 1980.
Even after H3+ was detected in the diffuse interstellar medium in 1998, a final interpretation of the result had to await the resolution of a long-standing controversy in the field of dissociative recombination. Various laboratory experiments (and theoretical calculations) had suggested wildly different values of the rate coefficient for recombination between H3+ and an electron, and this rate coefficient is a key ingredient in interpreting the H3+ abundance. It was not until 2003 that a new experiment combining accelerator techniques and a supersonic expansion seemed to offer the definitive value for this rate coefficient under interstellar conditions. Now, in 2008, the controversy appears to be finally resolved, and H3+ observations have reached their full potential in allowing direct measurements of the cosmic ray ionization rate.
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