Astronomy Colloquium / Fall 2008

November 12

Prof. Ben Mc Call

University of Illinois at Urbana-Champaign

H3+ and the Interstellar Cosmic-Ray Ionization Rate

H3+ has long been recognized as playing a key role in dense interstellar clouds as the initiator of ion-molecule chemistry, which leads to the production of well over one hundred interstellar molecules. The detection of H3+ in diffuse interstellar clouds came as a surprise, however, and suggested a serious 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 measured the dissociative recombination rate of rotationally cold H3+ ions in an ion storage ring, and observed H3+ in a large sample of diffuse clouds. 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. However, standard theoretical models of cosmic-ray production by supernovae and propagation through the interstellar medium cannot explain this enhanced ionization rate. What seems to be needed is a large and previously unrecognized flux of low energy (2-10 MeV) cosmic-rays. We have concocted an ad hoc theoretical cosmic-ray spectrum that can explain the H3+ observations in both dense and diffuse clouds, and does not violate other observational constraints, such as light element abundances. We will discuss this theoretical spectrum, its implications, and further observational tests that are on the horizon.