Abstract
Abundance anomalies are obtained in a large fraction of both evolved and main sequence stars. They involve most chemical elements and abundances can vary by many orders of magnitude. On the main sequence they are seen in stars as cool as 6400K and as hot as 27000K. They are apparently caused by chemical separation. The competition between gravity and selective radiative acceleration leads to the appearance of either overabundances or underabundances. Calculations of radiative accelerations require large amounts of atomic data.
In this paper I concentrate on abundance anomalies observed on nonmagnetic stars with 6000 < Teff < 10000K. Some 20% of the stars in that Teff range (Teff stands for effective temperature) are involved. In them the separation goes on below the visible region. It is shown that, to obtain accurate enough radiative accelerations, the f values are needed to an accuracy of 30% for the important lines of all states of ionization, up to the 18th for Fe, for instance. An analysis of the needed atomic data is done in detail.
To construct models that successfully reproduce the observations will require much more atomic data than available. They are needed to properly analyze the observations produced by modern electronic detectors and by space observations. These developments lead to a better understanding of hydrodynamical processes in stellar envelopes. In particular it is shown here how mass loss modifies the effect of chemical separation and it is shown that a mass loss rate of 10−15 M0 yr−1 is implied by the anomalies observed in many objects.