He II Lyman α transfer and Bowen fluorescence in a Crab Nebula filament.

Spectroscopy of several locations in the Crab Nebula was obtained in November 1981 using the intensified image dissector scanner on the 2.1-m KPNO telescope, and the Henry and MacAlpine (1982) Crab Nebula filament Model 8.1 is selected to theoretically study He II Ly-alpha transfer and Bowen related 0(+2) Bowen resonance fluorescence. For a range of conditions which could be expected in the Crab Nebula filament, the fraction of diffuse 304-A photons directly absorbed by H(O), He(O) and other elements is found to be about 0.55-0.65, and the resultant gas energy input for some collisonally-excited lines is shown to yield intensities twice as weak as those predicted by 100 percent He II Ly-alpha absorption model calculations. Calculations for a number of situations are consistent with a 0.15 measured Bowen efficiency upper limit at filament position FK10, and it is suggested that low Bowen efficiencies may be expected due to high helium abundances.