Spectroscopic studies of very old hot stars. I. NGC 246 and its exciting star

A spectroscopic study has been made of the planetary nebula NGC 246 and its exciting star. Observations of the central star obtained by L. H. Aller at the Lick Observatory indicate that the star has one of the most highly excited spectra known to optical astronomers. The only lines definitely present in the stellar spectrum are lines of C IV and O VI, although lines of He II are probably present. Nitrogen lines, in any ionization state, are absent. The stellar spectrum, therefore, suggests that the triple-α process has operated in this star. Nebular observations obtained at the Laboratory for Optical Astronomy 36-inch (91 cm) telescope were used to estimate the atmospheric properties of the exciting star via the Zanstra method, with results that are in accord with the spectral appearance of the star. This procedure yielded the following probable values for the stellar parameters: T/sub eff/approx.200,000degreeK, log gapprox.6.8, and R/R/sub sun/+approx.0.05. Comparison of the properties of the star and surrounding nebula with Paczynski's evolutionary tracks for planetary nuclei suggests that the star is very near the point of exhaustion of its nuclear fuels