The Chemistry of H3+ in the Diffuse Interstellar Medium
Ben McCall
Department of Chemistry, Department of Astronomy, University of Illinois
at Urbana-Champaign
Abstract
H3+, the simplest polyatomic molecule, plays a key role in dense
interstellar clouds as the initiator of ion-molecule chemistry. The
detection of H3+ in diffuse interstellar clouds came as a surprise,
however, and suggested a serious (factor of ~100) problem in the simple
model of diffuse cloud chemistry. In particular, this observation
raised questions as to the applicability of laboratory measurements of
the H3+ dissociative recombination rate to interstellar conditions. We
have recently measured the dissociative recombination rate of
rotationally cold H3+ ions in an ion storage ring, and have also
detected H3+ in the classical diffuse cloud towards _ Persei, where the
electron fraction is known from previous observations. This combination
of new laboratory measurements and astronomical observations has
eliminated two of the primary uncertainties in the chemical model, and
implies a previously unrecognized and significant enhancement in the
cosmic-ray ionization rate in the diffuse interstellar medium.