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B. J. McCall (UC Berkeley), T. R. Geballe (Gemini), T. Oka (U. Chicago)
H3+, the simplest polyatomic molecule, plays a key role in dense molecular clouds as the initiator of ion-molecule chemistry. The detection of H3+ in diffuse clouds came as a surprise, however, and revealed a serious (two orders of magnitude!) problem in the chemical model of this simple ion. The three possible sources of the discrepancy are the cosmic ray ionization rate (\zeta), the H3+ dissociative recombination rate constant (ke), and the electron fraction (e/H2).
Using CGS4 at UKIRT, we have recently detected H3+ in absorption towards \zeta Persei, a well-studied classical diffuse cloud sightline. The fact that C+ and H2 have both been observed in this sightline in the ultraviolet means that the electron fraction can be directly estimated, and the resulting estimate is consistent with the complete ionization of atomic carbon, as expected. This leaves only \zeta and ke as the possible sources of the discrepancy between theory and observation.
A recent laboratory experiment, using a supersonic expansion discharge source and the ion storage ring CRYRING, has measured the value of ke at interstellar temperatures, and has demonstrated that it is only a factor of 2 lower than the value previously adopted in the chemical models (the previous value was derived from experiments with rotationally hot H3+ ions). Given a reasonable estimate of the absorption pathlength (from other measurements), we derive a value of \zeta ~1.2 x 10-15 s-1, some 40 times higher than the "canonical" value of ~3 x 10-17 s-1. Because observations of H3+ in dense clouds are consistent with the canonical value, we suggest the possible existence of a high flux of low energy cosmic rays that can penetrate diffuse but not dense clouds.
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Bulletin of the American Astronomical Society, 35#5
© 2003. The American Astronomical Soceity.