The presence of fe VII and of low-ionization features in the UV spectra of central stars of planetary nebulae.

Abstract

The correlation between the presence of N I and Fe VII absorption lines in the spectra of central stars of planetary nebulae reported by Feibelman & Bruhweiler (1990) and Cheng et al. (1991) is refuted. Many spectra that they discuss are severely underexposed; moreover, 12 of them reveal no continuum whatsoever. These therefore cannot be used in the discussion of the presence or absence of absorption lines. Contrary to their claim that the Fe VII lines bear no relation to central-star properties, these lines correlate strongly with photospheric temperature. N I appears in all the spectra which have at least moderately well exposed continua, and a correlation between the equivalent width of the 1200-A triplet and the surface gravity of the central star - which can be interpreted as an age index of the star - is shown to be a selection effect. Consequently, there is no evidence that the lines in the IUE spectra have anything but a wholly interstellar origin.