Spectroscopic studies of very old hot stars. II - Spectral classification, absolute magnitudes, and distances of O-type planetary nuclei

This paper describes a study of high-dispersion spectrograms of central stars previously classified from low-dispersion plates as 0-type stars. The main results of this study are, first, that the spectra of this group of stars show a wide range in excitation (roughly from O7 to 03), but no central star appeared to have a higher excitation than that of the young star HD 93129. Second, the spectra of this group of stars show a wider range in gravity-dependent spectral features than do young 0 stars. Third, some central stars have spectra identical to those of young 0 stars, and in these cases, the method of spectroscopic parallax (along with an assumed stellar mass) yields distances in accord with Cudworth's recent distance scale to planetary nebulae. Fourth, the stellar rotational velocities appear to be in a narrow range, with a typical full width at half-maximum (FWHM) corresponding to about 100 km s'. Fifth, a few central stars show evidence of mass loss. Subject headings: nebulae: planetary stars: early-type - stars: luminosities - stars: spectral classification