1998A&A...338...75U

Previous diffusion calculations failed to explain quantitatively the presence of helium in hot hydrogen rich white dwarfs and subdwarfs. We present the results of improved diffusion calculations in the range 40000K≤T_eff≤100000K, 6.0≤logg≤8.0 with a more sophisticated treatment of the momentum transfer from photons to matter and the inclusion of ionization- and flux-blocking effects. However, the effect of all these improvements turns out to be small in comparison to the possible effect of mass loss on the surface composition, which may, compared to the results of the diffusion calculations, lead to large over- or underabundances of helium. For T_eff=80000K, logg=7.0 and zero mass loss the surface helium abundance decreases from the solar one to about He/H=10^–3 within a few thousands of years, whereas in the presence of mass loss with {dot}(M)≃10^–13M_*/y helium sinks in time scales similar to cooling ages. In contrast, mass loss rates of {dot}(M)<10^–14M_*/y lead to a strong depletion of helium. Therefore mass loss is a possible explanation for the presence of helium in DAO white dwarfs as well as for the absence of helium in DA's. Depending on the number fraction of hydrogen hidden in hot helium rich white dwarfs, a weak wind may prevent or considerably retard the floating up of hydrogen.