The winds of hydrogen-rich central stars of planetary nebulae

We have modelled far-UV, UV and optical spectra of a sample of 10 hydrogen-rich central stars of planetary nebulae (CSPN) using stellar atmosphere codes to derive their photospheric and wind parameters. The resulting stellar temperatures range from 40 to 120 kK, well spanning the CSPN evolutionary phase and allowing certain trends to be discerned. In particular, an inhomogeneous wind structure and X-ray emission in the wind are required to match spectral diagnostic lines in many cases. For the majority of the sample, a wind clumping factor of 0.1 <= f <= 0.04 is derived (mainly from the PV lambda lambda 1118, 28 and OV lambda 1371 lines). Such factors correspond to clump densities of similar to 10-25 times that of the smooth wind density, with resulting mass-loss rates one-third to one-fifth the smooth wind values, which is of significant consequence to nebular dynamics, stellar and galactic evolution. Furthermore, we find clumping to begin at small radii (similar to 1.2 R-*), as has been found when modelling the winds of (massive) O stars. The inclusion of X-ray fluxes, presumably from shocks, in the model atmosphere calculations is found to improve the fit of the OVI lambda lambda 1032, 38 line (and other features) for stars with 55 <= T-eff <= 80 kK, and to be absolutely necessary to match this feature for the coolest stars in our sample (T-eff less than or similar to 45 kK). These findings suggest that shocks originating from line-driven wind instabilities leading to the formation of clumped winds and X-rays may be a common characteristic of CSPN, as has been found for the winds of massive O-type stars. We also find interesting results for some individual stars. NGC 1360 (T-eff similar or equal to 105 kK) displays the signature of a (previously undetected) weak stellar wind in its O VI 1032, 38 profile, and probably has the lowest mass-loss rate ((M) over dot less than or similar to 1 x 10(-10) M-circle dot yr(-1)) of any known CSPN. In contrast, we find the wind terminal velocity of NGC 2392 (T-eff similar or equal to 45 kK) to be nu(infinity) similar or equal to 300 km s(-1), one of the slowest CSPN wind known, probably related to its subsolar metallicity. We have included in the model calculations many elements and high-ionization species previously neglected in analyses of this type, providing additional wind diagnostics such as Ne VII lambda 973 and Ar VII lambda 1064. The effects of including these as well as other line-blanketing elements are discussed.