The energy transfer and its effects on the secondaries in W Ursae Majoris-type contact binaries

Abstract

The energy transfer of W UMa contact binaries and its effects on the secondary in W UMa contact binaries are investigated. Relations are given between the mass ratio (q) and the relative energy transfer rates, i.e. U1, the ratio of the transferred luminosity to the surface luminosity of the primary and U2, the ratio of the transferred luminosity to the nuclear luminosity of the secondary. The theoretical curves(U1 vs q and U2 vs q) are derived. Although these curves can reflect the distribution of U1 vs q and U2 vs q, some observational systems are significantly deviated from these curves. It is mainly resulted from the difference in the effective temperatures of the components in W UMa systems.The radius and the density of the secondary are related to the relative energy tran-sfer rate U2: the higher is U2, the greater is the expansion and the lower is the density of the secondaries in W UMa systems. The temperature difference of W UMa binary components is correlated with the relative energy transfer rate U1 and decreases with increasing U1. This might suggest that there is a thermal coupling between two components in W UMa contact binaries, and that the classif-ication of W UMa contact binaries into A- or W-types depends on the energy tran-sfer from the primary to the secondary. The temperature difference of W UMa bin-ary components is poorly correlated with the mass of the primary. This suggests that the properties of the common envelope of W UMa contact binaries might not have a significant effect on the energy transfer between two components.

7 pages,8 figures, accepted by MNRAS

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