The Rich Ultraviolet Spectrum of Vibrationally Excited Interstellar H[TINF]2[/TINF] toward HD 37903

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Abstract
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 (nH ≈ 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.

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