The abundance, excitation and deuteration of interstellar H3+
Professor JH Black
Onsala Space Observatory; Chalmers University of Technology, Sweden
Observations of interstellar H3+ test a central part of theories of ion-molecule chemistry in space. It is possible to measure the abundances of cold H2, of H3+, and of CO in the same parcel of gas by means of infrared absorption spectroscopy toward obscured stars. The observed populations of the lowest rotational levels of H2 and H3+ confirm that reactive collisions tend to thermalize the ortho and para nuclear-spin species in interstellar clouds. The rotational population distributions in CO provide diagnostic information on physical conditions (density and kinetic temperature). Taken together, observations of these three molecules make it possible to infer the rate of ionization of neutral, molecular clouds by penetrating cosmic rays or X-rays.
Recent observations and their interpretation are summarized. The theoretical basis for describing the rotational and vibrational excitation of H3+ in interstellar gas is discussed. The abundances and excitation of H3+ and H2D+ are coupled and must be solved simultaneously in order to describe fully the mechanisms of deuteration of some important interstellar molecules. The prospects for detecting mm-wave masers in H3+ and H2D+ are considered. It is suggested that emission lines of H3+ and other reactive molecular ions might be useful probes of molecular gas near photoionized nebulae and active galactic nuclei.