Atomic data for the calculation of radiative accelerations and diffusion of Mn in HgMn stars

Archived monochromatic opacities, obtained in the course of the work of the Opacity Project, are used to obtain data required for the calculation of radiative accelerations. These data are being made generally available through the Centre de Donnees de Strasbourg (CDS).

The new atomic data are used in an exploratory study of the abundances of Manganese in HgMn stars. In most such stars observations show enhanced atmospheric Mn abundances, which result from processes of diffusion. We define χ to be the factor by which abundances are enhanced (or depleted). Due to saturation effects, the radiative accelerations g_rad(χ) decrease as χ increases and may eventually reach values smaller that the gravitational acceleration g_grav: χ_stat is defined to be the value of χ for which the diffusion velocity is zero. Using results from ultra-violet observations one obtains abundances χ_obs always a good deal smaller than χ_stat. It follows that there must be outflow at the stellar surfaces.

Solutions are obtained of the equations for time-dependent diffusion. The initial conditions are taken to be χ = 1 (unmodified abundances) at time t = 0. With such abundances the flux F = nv for Mn is found to be very large at depths with temperatures in the vicinity of log (T) ≃ 5.3, the region giving the "Z-bump" in Rosseland mean opacities. As time advances, those large fluxes move to the outer layers and give atmospheric abundances in agreement with the maximum values which are observed. At later times the atmospheric abundances decrease.