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 H₂ 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 H₂ 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 n_H = 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.

Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555.