Interstellar H2 toward HD 147888

TL;DR

This study measures the vibrational and rotational levels of interstellar H2 toward HD 147888 using ultraviolet spectra, providing insights into the molecular environment and radiation field in the region.

Contribution

It presents new measurements of H2 vibrational and rotational levels and their ratios, constraining the radiation field in the interstellar cloud towards HD 147888.

Findings

H2 column densities determined for vibrational levels v=0-5 and rotational levels J=0-6.

Ortho to para H2 ratio varies between excited and lowest vibrational states.

Constraints on the radiation field in photon-dominated regions derived from H2 absorption lines.

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.