Interstellar H2 toward HD 147888

Abstract

The ultraviolet and far--ultraviolet spectra of HD 147888 allows to access the v=0 as well as higher vibrational levels of the ground H2 electronic level. We have determined column densities of the H2 molecule on vibrational levels v=0-5 and rotational levels J=0-6. The ortho to para H2 ratio for the excited vibrational states equals to 1.2. For the lowest vibrational state v=0 and rotational level J=1 the ortho to para H2 ratio is only 0.16. The temperature of ortho-para thermodynamical equilibrium is T_OP=43+-3 K. The large number of H2 absorption lines in the HST spectra allows to determine column densities even from a noisy spectra. The measurements of H2 column densities on excited vibrational levels (from the HST spectra) leads to constrains of radiation field in photon-dominated regions (PDR) models of interstellar cloud towards HD 147888.

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• Formation of ${\text{H}}_2$ on polycyclic aromatic hydrocarbons under conditions of the ISM: an ab initio molecular dynamics study