Ultraviolet spectra of the central stars of large planetary nebulae.

Abstract

The authors have examined the ultraviolet spectra of 32 planetary nebula nuclei with the IUE. Almost all the nebulae are quite large, with radii greater than 0.2 pc, and all are hot, with Zanstra temperatures ≥70,000K. In most cases, the fluxes at 1500 Å are closely consistent with the visual magnitudes and the assumption of a hot blackbody. The authors determine ultraviolet and UV-to-optical flux ratios from which they derive color temperatures. The distribution of these color temperatures ranges from rough agreement with the Zanstra temperatures upward to infinite values, far in excess of those derived from the Zanstra method. There is evidence that some stars have UV flux distributions at or near the Rayleigh-Jeans limit. The authors believe that the color temperatures based on the standard calibration represent upper limits to the effective temperatures. A detailed discussion of the Zanstra discrepancy is presented. The effects of the IUE calibration, of stellar winds, circumstellar material and nebular optical depth on determining effective temperatures are considered.