Interstellar H₂ toward HD 37903^⋆

Institute of Theoretical Physics and Astrophysics, University of Gdańsk, ul. Wita Stwosza 57, 80-952 Gdańsk, Poland
e-mail: pg@iftia9.univ.gda.pl

Received: 15 February 2010
Accepted: 22 April 2011

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.