A Spectroscopic Study of Nuclear Processing and Production of Anomalously Strong Lines in the Crab Nebula
We present and discuss correlations for optical and near-infrared (5500-10030 angstroms) line intensity measurements at many positions in the Crab Nebula. These correlations suggest the existence of gas produced by a range of nuclear processing, from material in which synthesis ended with the CNO-cycle, to some helium-burning and nitrogen depletion, to regions containing enriched products of oxygen-burning. The latter exhibit a gradual, linear rise of [Ni II] emission with increasing argon enrichment, whereas gas with less nuclear processing shows markedly different [Ni II] emission characteristics, including the highest derived abundances. This suggests two origins for stable, neutron-rich nickel in the nebula: a type of "alpha-rich freezeout" in the more highly processed material, and possibly removal of ions from the neutron star in other regions. In addition, the data indicate that anomalously strong observed [C I] emission comes from broad, low-ionization zones. Although the strongest He I emission could also be enhanced in similar low-ionization gas, correlations between relevant line ratios argue against that interpretation, strengthening the case for an exceptionally high helium mass fraction in some locations.