A Comparison of Variable and Non-Variable Stars in the Cepheid Strip

Abstract

Spectra of three Cepheids (RX Aur, X Cyg and T Mon) and 11 non -variable supergiants are analyzed. Additional evidence is presented for the presence of non -variables within the Cepheid strip. Temperatures determined from Ha line profiles and broad band photometry were used together with model atmospheres and line strengths from the spectra to determine the gravities, turbulent velocities and element abundances. The gravities of the Cepheids appear to be lower than for the non -variables at the same luminosity while the turbulent velocities are higher. No differences of significance are found in the element abundances. The gravity differences can be explained either in terms of mass loss in the Cepheids or very low helium abundance in the non -variables. The former interpretation fails to explain why some Cepheid strip stars do not vary while the latter raises problems in connection with the origin of the elements. If we assume that pulsation causes some mass loss a more consistent picture emerges in which only moderate helium deficiencies are required in the non-variables.