Spectroscopic investigation of old planetaries IV. Model atmosphere analysis

The results of a NLTE model atmosphere analysis of 27 hydrogen-rich central stars of old planetary nebulae (PN) are reported. These stars were selected from a previous paper in this series, where we gave classifications for a total of 38 central stars. Most of the analyzed central stars fill a previously reported gap in the hydrogen-rich evolutionary sequence. Our observations imply the existence of two separated spectral evolutionary sequences for hydrogen-rich and -poor central stars/white dwarfs. This is in line with theoretical evolutionary calculations, which predict that most post-AGB stars reach the white dwarf domain with a thick hydrogen envelope of approx. 10^-4 Msun. We determine stellar masses from the comparison with evolutionary tracks and derive a mass distribution for the hydrogen-rich central stars of old PNe. The peak mass and the general shape of the distribution is in agreement with recent determinations of the white dwarf mass distribution. The properties of most analyzed stars are well explained by standard post-AGB evolution. However, for eight stars of the sample other scenarios have to be invoked. A wide spread of helium abundances is observed in the photospheres of central stars of old PNe. It is shown that a good correlation between helium abundances and luminosity is present. It is inferred that when the stars' luminosities fall below L = 300Lsun depletion starts and the helium abundance steadily decreases with decreasing luminosity. The existence of this correlation is in qualitative agreement with recent theoretical calculations of gravitational settling in the presence of a stellar wind.