Radiative forces on lithium: effects of electron recoil and background opacities

Recent results on momentum transfer during photoionization processes determine how the ionizing photon momentum is split between the ejected electron and the recoiling ion. This leads to improved radiative force calculations. The consequences of this recoil for radiative forces are demonstrated by comparing for lithium the new accurate results with calculations based on simpler approximations commonly used in the past. We show that at some densities and temperatures typical of deep stellar envelopes, the total radiative force on lithium becomes negative (outgoing stellar radiation pushing the ions inwards); however this negative force is then negligible compared to gravity. In stellar envelope calculations, the exact results for hydrogenoid states are well approximated by the simpler formula that Sommerfeld obtained for the hydrogen 1s state.We also show that the force on Li depends significantly on the abundance of other elements, especially helium and oxygen, that block photon flux at critical wavelengths.