ON SPECTRAL-LINE FORMATION AND MEASUREMENT IN CEPHEIDS - IMPLICATIONSTO DISTANCE DETERMINATION

The formation and measurement of Cepheid photospheric spectral lines a re explored to understand better the systematic effects involved in ra dial and pulsation velocity determination, and hence to help secure Ce pheid distance scale calibrations that depend on Cepheid velocities (e .g., the Baade-Wesselink method). Using high-resolution optical and in frared spectra and synthetic line profiles, we examine techniques for measuring the position of line center and the amount of asymmetry of C epheid absorption line profiles. The line asymmetry is observed to be a specific function of pulsation phase and to correlate with line cent er measurements, thus (1) the conversion factor between radial and pul sation velocity (p-factor) may not be assumed constant with phase and (2) there is a systematic offset of about 1 km s(-1) in the center-of- mass velocity. Our Cepheid models, which employ non-LTE radiative hydr odynamics, reproduce and describe (as an opacity effect) the observed unequal line asymmetry magnitudes during contraction and expansion sta ges. The models also allow us to study the phase dependence of the p-f actor and its influence on Baade-Wesselink radius calculations. We con clude that in order to reach an accuracy of better than 7% (0.15 mag) in the zero point of the Cepheid distance scale, these systematic effe cts should be taken into account.