Interstellar H2 toward HD 37903

TL;DR

This paper analyzes interstellar molecular hydrogen toward HD 37903, extending previous models by including new vibrational data, and estimates physical conditions like temperature, density, and star-cloud distance in the nebula.

Contribution

It extends prior H2 analysis by incorporating FUSE data on vibrational levels, refining the physical parameters of the interstellar cloud around HD 37903.

Findings

Gas temperature Tkin=110-377 K

Hydrogen density nH=1874-544 cm^-3

Star-cloud distance of 0.45 pc

Abstract

We present an analysis of interstellar H2 toward HD 37903, which is a hot, B 1.5 V star located in the NGC 2023 reflection nebula. Meyer et al. (2001) have used a rich spectrum of vibrationally excited H2 observed by the HST to calculate a model of the interstellar cloud toward HD 37903. We extend Mayer's analysis by including the v"=0 vibrational level observed by the FUSE satellite. The T01 temperature should not be interpreted as a rotational temperature, but rather as a temperature of thermal equilibrium between the ortho and para H2. The ortho to para H2 ratio is lower for collisionally populated levels than for the levels populated by fluorescence. The PDR model of the cloud located in front of HD 37903 points to a gas temperature Tkin=110-377 K, hydrogen density nH=1874-544 cm^-3 and the star-cloud distance of 0.45 pc.