Ultraviolet Molecular Hydrogen Fluorescence in IC 63: FUSE, HUT, and Rocket Observations

We present far-ultraviolet observations of IC 63, an emission/reflection nebula illuminated by the B0IV star γ Cassiopeia located 1.3 pc from the nebula. IC 63 was the first reflection nebula in which molecular hydrogen fluorescence was detected and has previously been studied in both the mid-UV (IUE) and far-UV (ORFEUS). Here we present Far Ultraviolet Spectroscopic Explorer (FUSE) observations towards three locations in the nebula, complemented by Hopkins Ultraviolet Telescope (HUT) data on the central nebular position and sounding rocket data on the stellar spectrum of γ Cas. Molecular hydrogen fluorescence is detected in all three FUSE pointing, but the intensities of this emission as well as the contributions from other species are seen to vary with position. The broad spectral coverage of the sounding rocket data (900-1590 Å) 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 mid and far ultraviolet observations and achieves a satisfactory agreement with the H2 rotational structure observed with FUSE.