Studies of hot B subdwarfs. VI - Detailed calculations of radiative forces on metals in the envelopes and atmospheres of hydrogen-rich subdwarfs

Detailed calculations of radiative accelerations on C, N, and Si are presented in model envelopes and atmospheres of hot, hydrogen-rich subdwarf (sdB, sdOB) stars. These calculations are carried out through several levels of approximation in order to examine the sensitivity of grad to the input physics used. Departures from LTE are also considered, with the help of an approximate treatment of the NLTE ionization balance. The radiative acceleration profiles are used to calculate a depth-dependent equilibrium abundance profile for C, N, and Si in various models. These can be, occasionally, far from homogeneous and provide a self-consistent input to line profile calculations. Radiative accelerations are found to be large enough to counterbalance the efficient gravitational settling of elements in these high-gravity atmospheres.