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D.M. Meyer, J.T. Lauroesch (Northwestern U.), U.J. Sofia (Whitman College), B.T. Draine (Princeton U.), F. Bertoldi (MPIfR-Bonn)
Over the past 20 years, there have been an increasing number of near-IR emission-line studies of vibrationally-excited interstellar H2 due to advances in near-IR detectors and the utility of H2 as a physical probe of photodissociation regions (PDRs) and shock-heated gas. Although a UV absorption-line approach has a number of advantages in studying the nature of vibrationally-excited H2, the only UV detections published to date consist of two lines toward the star \zeta~Oph (Federman et al.\ 1995, ApJ, 445, 325). Based on recent high-resolution (\Deltav~\approx~3~km~s-1) HST STIS echelle 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 very 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 some cases) in all 14 excited vibrational states of the ground electronic state. In terms of abundance, the v=3~J=1~H2 column density toward HD~37903 is over 200 times greater than that toward \zeta~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 a dense H2 cloud within 1~pc of the star. We will discuss the implications of these results and opportunities for future HST observations of vibrationally-excited H2 in other sightlines.
The author(s) of this abstract have provided an email address for comments about the abstract: davemeyer@nwu.edu