Enigma of H3 + in Diffuse Interstellar Clouds

1. INTRODUCTION H 3 + is of fundamental importance to interstellar chemistry both because it is a hydrogenic species (and >90% of all nuclei in the universe are hydrogen nuclei; see Figure 1) and because it serves as the universal protonator, initiating a chain of ion-neutral reactions that is responsible for the formation of most interstellar molecules. The chemistry of H 3 + in dense (molecular) clouds is now well understood, 1 and observations of H 3 + in these environments now permit measurements of the path lengths, number densities, and kinetic temperatures of dense clouds. However, as discussed in more detail in a separate paper 2 in this volume, the situation in diffuse clouds is much less clear. H 3 + has now been observed 3,4 in several diffuse cloud sources with unexpectedly large absorption strength. This enigma requires at least one of the model parameters (cosmic ray ionization rate, electron fraction, and the rate constant for dissociative recombination of H 3 +) to be changed by at least one to two orders of magnitude! In this paper we describe in more detail the observations of H 3 + in diffuse interstellar clouds, and the simple chemical model which is so obviously in error. We appeal to the specialists of dissociative recombination to resolve