Directional Radiation and Photodissociation Regions in Molecular Hydrogen Clouds

TL;DR

This paper investigates the broadening of molecular hydrogen absorption lines in astrophysical clouds, analyzing radiative transfer and photodissociation effects to better understand velocity distributions at excited rotational levels.

Contribution

It introduces a detailed analysis of line broadening mechanisms, including radiative pumping and photodissociation regions, in molecular hydrogen clouds under directional ultraviolet radiation.

Findings

Line broadening affects velocity distribution measurements.

Saturation of resonance lines influences excited rotational levels.

Photodissociation regions significantly impact line profiles.

Abstract

Some astrophysical observations of molecular hydrogen point to a broadening of the velocity distribution for molecules at excited rotational levels. This effect is observed in both Galactic and high redshift clouds. Analysis of H_2, HD, and CI absorption lines has revealed the broadening effect in the absorption system of QSO 1232+082 (z_{abs}=2.33771). We analyze line broadening mechanisms by considering in detail the transfer of ultraviolet radiation (in the resonance lines of the Lyman and Werner H_2 molecular bands) for various velocity distributions at excited rotational levels. The mechanism we suggest includes the saturation of the lines that populate excited rotational levels (radiative pumping) and manifests itself most clearly in the case of directional radiation in the medium. Based on the calculated structure of a molecular hydrogen cloud in rotational level populations, we have considered an additional mechanism that takes into account the presence of a photodissociation region. Note that disregarding the broadening effects we investigated can lead to a significant systematic error when the data are processed.