Fluorescent Molecular Hydrogen Emission in Ic 63 : Fuse , Hut , and Rocket Observations

We present far-ultraviolet observations of IC 63, an emission/reflection nebula illuminated by the B0.5IV star γ Cassiopeia, located 1.3 pc from the nebula. Molecular hydrogen fluorescence was detected first in IC 63 by IU E and later at shorter wavelengths by ORF EU S. Here we present F ar U ltraviolet Spectroscopic Explorer (F U SE) observations towards three locations in the neb-ula, complemented by Hopkins Ultraviolet Telescope (HUT) data on the central nebular position. In addition, we present a sounding rocket calibration of a F U SE spectrum of γ Cas. Molecular hydrogen fluorescence is detected in all three FUSE pointings. The intensity of this emission as well as the contributions from other species are seen to vary with position. The absolute flux calibration of the sounding rocket data allows us to reliably predict the radiation field incident on IC 63. We use these data to test models of the fluorescent process. Our modeling resolves the perceived discrepancy between the existing ultraviolet observations and achieves a satisfactory agreement with the H 2 rotational structure observed with F U SE. Ultraviolet fluorescence is the initial step in the process that gives rise to the near-infrared (IR) emission spectrum of photo-excited molecular hydrogen (H 2) observed in a wide range of astronomical environments. Hydrogen molecules make the transition to an excited electronic state (predominantly B 1 Σ + u and C 1 Π u) by absorbing ultraviolet (UV) photons. The transition back to the ground electronic state produces the characteristic ultraviolet spectrum and leaves the molecules in excited rovibrational levels. The near-IR lines are emitted as the molecules return to the ground vibrational level through quadrupole transitions. Infrared emission lines can also be populated collisionally, and are a common diagnostic used to probe the molecular gas phase of many classes of astronomical objects (e.g., star forming regions: Luhman Ultraviolet emission lines of H 2 cannot be produced thermally because the molecules would dissociate before the upper electronic states could be populated, so they must be pumped by ultraviolet photons or non-thermal electrons. The detection of these lines is a clear indication of non-thermal excitation occurring in at least some portion of the molecular gas. The far-ultraviolet emission from molecular hydrogen was first predicted to be detectable in diffuse objects by Duley & Williams (1980). Witt et al. (1989) detected this emission in IC 63 with the Short Wavelength Primary (SWP) camera …