  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.
