Fluorescent Molecular Hydrogen Emission in IC 63: FUSE , Hopkins Ultraviolet Telescope, and Rocket Observations

Abstract

We present far-ultraviolet observations of IC 63, an emission/reflection nebula illuminated by the B0.5 IV star gamma Cas, located 1.3 pc from the nebula. Molecular hydrogen fluorescence was detected first in IC 63 by IUE and later at shorter wavelengths by ORFEUS. Here we present Far Ultraviolet Spectroscopic Explorer (FUSE) observations toward three locations in the nebula, complemented by Hopkins Ultraviolet Telescope (HUT) data on the central nebular position. In addition, we present a sounding rocket calibration of a FUSE spectrum of gamma 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 FUSE.