Spectrum synthesis of sharp-lined A and B stars
  I have carried out spectrum synthesis of R = 120,000 spectra of several A and B stars having v sin i less than about 6 km/s. The following conclusions emerge: (1) As T_e descends from 12,000 to 8,000 K, microturbulent velocity xi deduced from abundance analysis rises steadily from 0 to about 5 km/s. (2) Stars with xi >= 1 km/s show direct evidence in their line profiles of the presence of macroscopic gas motions in the form of line asymmetry (bisector curvature) which grows with increasing xi. (3) Above T_e ~ 9000 K, both weak and strong spectral lines can be modelled with reasonable accuracy by conventional LTE spectrum synthesis with a single assumed model atmosphere, abundance table, v sin i, and an appropriate (constant) value of xi. (4) In contrast, at T_e ~ 8,000 K the weak spectral lines are much narrower than the strong lines. If the synthesis model is constrained in v sin i and xi by the weak lines, NO satisfactory model can be found for the strong spectral lines. Consequently, chemical abundances for such stars based only on strong lines may be significantly in error. 
J. D. Landstreet (Univ. of Western Ontario, Canada, and Obs.   Midi-Pyrenees, France)