The effect of velocity gradients on Cepheid spectra: Models

High-resolution observations of eta Aql and other Cepheids have revealed differences in the radial velocity curves from the spectral lines of elements having differing ionization states and excitation energies. The effect of atmospheric velocity gradients on these differential radial 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 these models. It is shown that a significant velocity gradient is needed near the phase of maximum infall velocity to account for the line profile asymmetries and velocity differences observed in the spectrum of eta Aql. The effect of this velocity gradient is to reduce the amplitude 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 standard assumption of a comoving atmosphere. The Barnes-Evans method has been used to analyze eta Aql, taking this velocity gradient into account. The resulting radius and distance are reduced by about 17%, compared to the result found earlier for a comoving atmosphere, while the mean M(v) changes from -4.47 to -4.08.