Models of interstellar clouds. I - The Zeta Ophiuchi cloud

Abstract

Techniques are described for constructing complex models of diffuse interstellar clouds designed to take advantage of Copernicus UV line data and to reproduce observations of the amounts of atomic and molecular hydrogen, the abundances of various simple molecules, the fractionation of HD, the degree of ionization of minor atomic species, and the populations of the rotational levels of the ground state of H2. The construction of such models is shown to involve simultaneous treatment of the transfer of photons that dissociate molecules, ionize atoms, and contribute to heating; the chemical and ionization equilibria of important constituents, and the statistical equilibrium describing level populations in H2. Application of the techniques is illustrated by developing a successful model of the diffuse interstellar cloud toward Zeta Ophiuchi. The results presented indicate that all the available observational data for this cloud can be adequately accounted for by a two-component model consisting of an outer region with a temperature of 110 K and a total density of 500 per cu cm and an inner core having a temperature of 22 K and a total density of 2500 per cu cm.