Radiative Ionization of the Filaments in the Crab Nebula

Abstract

We have attempted to explain the observed excitation conditions in the filamentary system of the Crab Nebula in terms of ionization and heating by high frequency radiation. It was found that it is possible to reasonably fit most of the observed lines by assuming either (1) an ultraviolet continuum which smoothly joins the optical and X-ray data and a gas composition in which the number densities of hydrogen and helium are equal, (2) a spectrum which drops off about as steeply as v-2 and therefore does not smoothly fit the X-ray spectrum, or (3) an ultraviolet continuum which smoothly joins the optical and X-ray data plus strong emission-line features near 20 eV. A UV continuum which is much larger than in these models results in too much ionization in the filaments. The calculations suggest that most of the filaments consist of outer ionized regions with cores of neutral gas, so that the mass of the filamentary shell may be considerably larger than the 1.45 M⊙ required to explain the emission line intensities.