On Spectral Line Formation and Measurement in Cepheids: Implications to Distance Determination

Abstract

The formation and measurement of Cepheid photospheric spectral lines are explored to understand better the systematic effects involved in radial and pulsation velocity determination, and hence to help secure Cepheid distance scale calibrations that depend on Cepheid velocities (e.g., the Baade-Wesselink method). Using high-resolution optical and infrared spectra and synthetic line profiles, we examine techniques for measuring the position of line center and the amount of asymmetry of Cepheid absorption line profile&amp; The line asymmetry is observed to be a specific function of pulsation phase and to correlate with line center measurements, thus (1) the conversion factor between radial and pulsation velocity (p4actor) may not be assumed constant with phase and (2) there is a systematic offset of about 1 km s<SUP>-1</SUP> in the center-of-mass velocity. Our Cepheid models, which employ non-LTE radiative hydrodynamics, reproduce and describe (as an opacity effect) the observed unequal line asymmetry magnitudes during contraction and expansion stages. The models also allow us to study the phase dependence of the p4actor and its influence on Baade-Wesselink radius calculations. We conclude 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 effects should be taken into account.