Interstellar H2 toward HD 37903

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, ApJ, 553, L59) used a rich spectrum of vibrationally excited H 2 observed by the HST to calculate a model of the interstellar cloud toward HD 37903. We extend their analysis by including the ν′′ = 0 vibrational level observed by the FUSE satellite. The temperature calculated from the two lowest rotational levels (J′′ = 0, 1) of the H 2 molecule 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 the lowest rotational levels than for the higher levels populated by fluorescence. The PDR model of the cloud located in front of HD 37903 points to a gas kinetic temperature ranging from 110 K on the observer side of the cloud to 377 K on the star side. The hydrogen density changes from nH = 1874 cm-3 on the observer side to 544 cm-3 on the star side. The star-cloud distance in our model is 0.45 pc. © 2011 ESO.