DETECTION OF ATMOSPHERIC VELOCITY-FIELDS IN A-TYPE STARS

High signal-to-noise spectra with spectral resolution of more than 10( 5) have been obtained of one normal B9.5V, one normal A1V, two Am star s, and two HgMn B stars having v sin i less than 6 km s(-1). These spe ctra are modeled with LTE line profile synthesis to test the extent to which the spectrum of each star can be modeled correctly with a singl e set of parameters T-e, log g, chemical abundances, v sin i, and (dep th-independent) microturbulent velocity xi. The answer to this questio n is important for abundance analysis of A and B stars; if conventiona l line synthesis does not reproduce the line profiles observed in star s of small v sin i, results obtained from such analysis are not likely to be very precise. The comparison of models with observations is the n used to search for direct evidence of atmospheric motions, including line-strength dependent broadening, line core shape, and line asymmet ries, in order to study how the microturbulence derived from abundance analysis is related to more direct evidence of atmospheric velocity f ields. It is found for the three stars with 12,000 greater than or equ al to T-e greater than or equal to 10,200 K (the normal star 21 Peg an d the two HgMn stars 53 Tau and HD 193452) that xi is less than 1 km s (-1), and line profiles are reproduced accurately by the synthesis wit h a single set of parameters. The slightly cooler (T-e approximate to 9800 K) star HD 72660 has only a slightly stronger surface convective layer than the hotter stars, but for this star xi approximate to 2.2 k m s(-1). Strong spectral lines all show significant asymmetry, with th e blue line wing deeper than the red wing, and have line bisectors whi ch have curvature towards the blue with a span of about 0.5 to 1.0 km s(-1). A single model fits all lines satisfactorily. The two Am stars (HD 108642 and 32 Aqr), with T-e approximate to 8000 K, are found to h ave much larger values of xi (4 to 5 km s(-1)). The strong spectral li nes of these two stars are extremely asymmetric, with depressed blue w ings, and the bisectors have spans of order 3 km s(-1). No consistent fit to all lines can be found with a single model of the type used her e. It is concluded (a) that classical LTE line synthesis is able to re produce with considerable accuracy the line profiles of late B and ear ly A stars with T-e above about 9500 K, but that the LTE model with de pth-independent microturbulence provides a very poor approximation for cooler A stars, (b) that curve-of-growth microturbulent velocities in A stars are related to directly detectable atmospheric velocity field s, and (c) that the discrepancies between calculated and observed line profiles in stars with temperatures in the vicinity of 8000 K are so large that abundances derived mainly from saturated lines may well con tain significant errors. As a by-product, laboratory gf values for Fe II between 3800 and 5300 Angstrom have been combined to form a set of data optimized for internal consistency of the g f values.