THE EFFECT OF VELOCITY-GRADIENTS ON CEPHEID SPECTRA - MODELS

High-resolution observations of eta Aql and other Cepheids have reveal ed differences in the radial velocity curves from the spectral lines o f elements having differing ionization states and excitation energies. The effect of atmospheric velocity gradients on these differential ra dial velocity curves is investigated with synthetic spectra calculated from model hydrostatic stellar atmospheres appropriate for Cepheids. A number of velocity gradients have been kinematically introduced in t hese models. It is shown that a significant velocity gradient is neede d near the phase of maximum infall velocity to account for the line pr ofile asymmetries and velocity differences observed in the spectrum of eta Aql. The effect of this velocity gradient is to reduce the amplit ude of the pulsational velocity curve at optical depth tau = 2/3 by 20 % and to decrease the gamma velocity by 2 km s(-1) relative to the sta ndard assumption of a comoving atmosphere. The Barnes-Evans method has been used to analyze eta Aql, taking this velocity gradient into acco unt. The resulting radius and distance are reduced by about 17%, compa red to the result found earlier for a comoving atmosphere, while the m ean M(v) changes from -4.47 to -4.08.