Diffusion velocity of metals in Ap star atmospheres: the effect of ambipolar diffusion of H

Ambipolar diffusion of hydrogen is effective in the atmosphere of stars when hydrogen is partially ionized. This is precisely the case for Ap stars whose atmospheres are stable enough to allow atomic diffusion in building metals stratification. Now, ambipolar diffusion of H induces an additional term to the diffusion velocity usually considered for metals. For some chemical elements, this may significantly affect the diffusion flux, and so, their abundance distribution. This effect decreases for high effective temperatures since protons density becomes very dominant compared to neutral H (that is partial ionization of H may be neglected), and so, this process has been neglected in recent models of abundance distributions in Bp stars. In this study, we evaluate for many metals the role of ambipolar diffusion in view of future detailed modelling of atomic diffusion for chemically peculiar stars with $T_{\rm eff}\lesssim 12\, 000$ K. We discuss a possible effect that we name the ambipolar flooding.