1995A&A...301..545N

The results of a spectroscopic investigation of central stars of old planetary nebulae (PN) are reported. Classifications are provided for a total of 38 central stars, most of them previously unclassified. The sample is dominated by hydrogen-rich (pre-)white dwarfs, and the ratio of hydrogen-rich to -poor stars (≃4:1) is consistent with results of previous investigations of central stars and hot white dwarfs. The class of hydrogen-poor central stars of planetary nebulae (CPN) is represented by helium- and carbon-rich PG1159 stars: seven new PG1159 central stars were detected, increasing the number of known objects of this rare class by more than 100%. A previously unknown class has been established by "hybrid" central stars: their spectra resemble closely those of PG1159 stars, but with strong Balmer lines indicating that they are hydrogen-rich in contrast to the PG1159 stars. We derived V magnitudes from our spectrophotometric observations and used the interstellar NaD lines for distance estimates of those PNe, which are close to the galactic plane. Since this method is independent of "ad hoc" assumptions about nebular properties it is superior to the widely used "statistical" methods. Kinematical ages (t_kin) are calculated from published expansion velocities and angular diameters and are used to construct a M_V-t_kin diagram. A comparison with theoretical evolutionary tracks yielded a mean mass of 0.62±0.09M_☉ in good agreement with generally accepted values for white dwarfs. It is shown that the widely adopted "statistical" distances of old PNe are obviously too small by more than a factor of two.