Spectroscopic Determination of Radius Changes of Cepheid Variable Stars

We introduce a simple method for determining pulsation velocities of Cepheid variables; namely, the matching of observed spectral line profiles in both shape and absolute position with model profiles. This approach avoids the ambiguous meaning of radial velocity for asymmetric lines, and it avoids the awkward projection factor (the “p” factor) used to expand radial velocities to pulsation velocities. In this investigation, we employ a simple pure-geometry model that reproduces all the major features in the observations. We illustrate the method using spectroscopic observations of z Gem, h Aql, and d Cep taken at the Elginfield Observatory, and derive the absolute radius changes.