2001ApJ...553L..59M

Based on recent high-resolution Hubble Space Telescope Space Telescope Imaging Spectrograph observations over the 1160-1360 Å wavelength region, we have discovered that HD 37903 (the illuminating star of the bright reflection nebula NGC 2023 in Orion) exhibits a rich spectrum of vibrationally excited interstellar H₂ absorption. We have identified a total of over 500 H₂ lines arising from various rotational levels (as high as J=13) in all 14 excited vibrational states of the ground electronic state. Prior to this study, the only published UV absorption measurements of vibrationally excited interstellar H₂ consisted of two weak lines toward the star ζ Ophiuchi. In terms of abundance, the v=3 J=1 H₂ column density toward HD 37903 is over 200 times greater than that toward ζ Oph. Overall, the populations of the excited H₂ rovibrational states toward HD 37903 exhibit a nonthermal distribution that is consistent with the UV fluorescent excitation of dense (n_H~10⁴ cm^–3) molecular gas located about 0.5 pc from the B1.5 V star. We contrast these results with those provided by near-IR H₂ emission-line studies of NGC 2023 and discuss opportunities for future UV observations of vibrationally excited H₂ toward HD 37903 and other sight lines.