2011A&A...532A.122G

We present an analysis of interstellar H₂ toward object HD 37903 , which is a hot B 1.5 V star located in the NGC 2023 reflection nebula. Meyer et al. (2001ApJ...553L..59M) 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 110K on the observer side of the cloud to 377K on the star side. The hydrogen density changes from n_H=1874cm^–3 on the observer side to 544cm^–3 on the star side. The star-cloud distance in our model is 0.45pc.