International ultraviolet explorer observations of the central stars of the planetary nebulae NGC 6853 and NGC 7293.

Abstract

Absolute flux measurements have been obtained by the IUE in the wavelength range from 1200 to 3100 A for the central stars of the planetary nebulae NGC 6853 and NGC 7293. For NGC 6853, a value of E(B-V)=0.06 can be derived from visual photometry. For NGC 7293, an E(B-V)=0.012 is deduced from the interstellar Lyman alpha absorption line and the known gas-to-dust ratio. The effective temperatures of 144,000 K for NGC 6853 and 123,000 K for NGC 7293 are determined by the Zanstra method and model atmosphere calculations with standard helium abundances. The observed flux curves fit theoretical model atmospheres at these temperatures within the approximately 10% uncertainty of the measurements. Additional stellar parameters derived for NGC 6853 and NGC 7293, respectively, are surface gravities of log g = 7.7 and 8.0 for a 1 solar mass star, stellar radii of 0.023 and 0.017 solar radii, and luminosities of 200 and 60 solar luminosities.