The Case for Surface Helium Enhancement in Cepheids

Abstract

Cepheid masses obtained from pulsation theory are almost always less than those derived from observed luminosities and the no-mass-loss stellar evolution theory mass-luminosity relation. This may indicate mass loss in the red giant region. However, evolution theory indicates that more than ten percent mass loss will prevent the blue loops that move most of the stars into the Cepheid pulsation instability strip. The mass anomaly problem is about thirty percent for the long period (> 15 days) Cepheids with observed Baade-Wesselink method radii and for those with bumps in their light and velocity curves (5.5 - 13 day periods). Chemically homogeneous models produce the correct bump phases only with lower than evolution theory masses. The mass anomaly is a factor of three or four for the double mode Cepheids and AC Andromedae, the only triple mode Cepheid. The long standing problem is to reconcile the differing masses obtained from evolution and pulsation theories.