The effect of stellar evolution upon Population II Contact Binaries in the Period-Color Relation. I. Equal-Mass, Marginal Contact Systems

Field W UMa binaries observe a well known Period-Color Relation such that systems containing more massive stars are bluer and have longer orbital periods than those systems with lower mass components. However, it has been known for a decade that metal-poor W UMa's are too blue, have too short an orbital period, or both. Correcting the observed color for the reduced line blanketing in the atmosphere of a Pop II star only accounts for part of the observed discrepancy. As others have suggested, and Rucinski (2000) show, the smaller radii of Pop II stars and the correspondingly shorter orbital periods are responsible for the remainder. In this paper I investigate the effect of evolution upon the location in the period-color plane. This paper addresses the restricted case of equal mass components in critical contact with their inner Roche lobes, but should be applicable to the more general cases to the extent that the relative sizes of stellar components are preserved with metallicity changes. The calculated metallicity-age dependent Period-Color Relations substantially agree with Rucinski & Duerbeck's (1997) empirically derived corrections to the Period-Color Relation over most of the investigated range of periods. However, our predictions deviate to a greater degree as stellar age increases since their parameterization does not include the effect of evolution.