Interpretation of the level population distribution of highly rotationally excited H2 molecules in diffuse clouds

Abstract

The H2 level population distribution of the rotational levels J = 5, 6 and 7 is analysed using Copernicus observations. The measured distribution is shown to be compatible with an H2 rotational excitation mechanism based on either H2 formation or collisions with hydrogen atoms; it is incompatible with UV pumping. The H2 formation hypothesis is more plausible, because the collisional excitation mechanism requires physical conditions which are not consistent with other observations of diffuse clouds. If H2 formation on grains is the source of the observed H2 level population distribution, the newly created H2 molecules are rotationally hot. Assuming that H2 molecules have a narrow initial rotational energy distribution when they are formed, the evolution time of diffuse clouds from an initially atomic state is estimated for a number of observed diffuse clouds. These cloud ages are typically |$\widetilde\lt$| 106yr. Chemical models for diffuse clouds are also strongly influenced by the assumption that H2 formation rather than UV pumping is responsible for the H2 excitation, because the UV field irradiating the clouds can in this case be much weaker than for previous models adopted. A chemical model for the ζ Oph cloud using the H2formation mechanism for reproducing the observed H2 level population distribution matches the observations for all detected chemical species (except CH+) remarkably well. In particular, the calculated CO, CN and CS abundances match those observed, and a large fraction of cosmic carbon is assumed to be locked in dust.