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Session 102 - Pulsating Stars.
Display session, Thursday, January 18
North Banquet Hall, Convention Center

[102.07] The Effect of Velocity Gradients on Cepheid Spectra: Models

R. A. Bell (U. Maryland), R. P. Butler (UC Berkeley, San Francisco State U.), R. B. Hindsley (USNO)

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 20decrease the gamma velocity by 2 km/sec relative to the standard assumption of a co-moving 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 17for a co-moving atmosphere, while the mean Mv changes from -4.47 to -4.08.

Program listing for Thursday