Far-Ultraviolet Fluorescence of Molecular Hydrogen in IC 63

Abstract

We present observations of H2 fluorescence at wavelengths between 1000 and 1200 A from the bright reflection nebula IC 63. Observations were performed with the Berkeley spectrograph aboard the ORFEUS II mission. To the best of our knowledge, this is the first detection of astrophysical H2 fluorescent emission at these wavelengths (excluding planetary atmospheres). The shape of the spectrum is well described by the model of Sternberg. The absolute intensity, however, is fainter by a factor of 10 than an extrapolation from observations at longer ultraviolet wavelengths. Of the mechanisms that might help to reconcile these observations, optical depth effects in the fluorescing H2 itself are the most promising (or at least the most difficult to rule out).