RADIATIVE FORCES ON LITHIUM - EFFECTS OF ELECTRON RECOIL AND BACKGROUND OPACITIES

Recent results on momentum transfer during photoionization processes d etermine how the ionizing photon momentum is split between the ejected electron and the recoiling ion. This leads to improved radiative forc e calculations. The consequences of this recoil for radiative forces a re demonstrated by comparing for lithium the new accurate results with calculations based on simpler approximations commonly used in the pas t. We show that at some densities and temperatures typical of deep ste llar envelopes? the total radiative force on lithium becomes negative (outgoing stellar radiation pushing the ions inwards); however this ne gative force is then negligible compared to gravity. In stellar envelo pe calculations, the exact results for hydrogenoid states are well app roximated by the simpler formula that Sommerfeld obtained for the hydr ogen Is state. We also show that the force on Li depends significantly on the abundance of other elements, especially helium and oxygen, tha t block photon flux at critical wavelengths.