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 o f the Opacity Project, are used to obtain data required for the calcul ation of radiative accelerations. These data are being made generally available through the Centre de Donnees de Strasbourg (CDS). The new a tomic data are used in an exploratory study of the abundances of Manga nese in HgMn stars. In most such stars observations show enhanced atmo spheric Mn abundances, which result from processes of diffusion. We de fine chi to be the factor by which abundances are enhanced (or deplete d). Due to saturation effects, the radiative accelerations g(rad)(chi) decrease as chi increases and may eventually leach values smaller tha t the gravitational acceleration g(grav);chi(stat) is defined to be th e value of chi for which the diffusion velocity is zero. Using results from ultra-violet observations one obtains abundances chi(obs) always a good deal smaller than chi(stat). It follows that there must be out flow at the stellar surfaces. Solutions are obtained of the equations for time-dependent diffusion. The initial conditions are taken to be c hi = 1 (unmodified abundances) at time t = 0. With such abundances the flux F = nv for Mn is found to be vary large at depths with temperatu res in the vicinity of log (T) similar or equal to 5.3, the region giv ing the ''Z-bump'' in Rosseland mean opacities. As time advances, thos e large fluxes move to the outer layers and give atmospheric abundance s in agreement with the maximum values which are observed. At later ti mes the atmospheric abundances decrease.