A new mechanism of long-term period variations for W UMa-type contact binaries

TL;DR

This paper proposes a new CCE-dominated mechanism explaining long-term orbital period variations in W UMa-type contact binaries, emphasizing the role of mass transfer between the common convective envelope and stellar components.

Contribution

It introduces a novel CCE-based mechanism supported by numerical results, offering a new perspective on period variations and the evolution of contact binaries.

Findings

Orbital periods vary significantly with fill-out factors at high mass ratios.

A simplified model predicts long-term oscillations in orbital periods.

The mechanism explains short-period, high-mass-ratio, deep contact binaries.

Abstract

W UMa-type contact binaries belong to close binary systems whose components exactly overflow their Roche lobes and share a common convective envelope (CCE). In the last twenty years, the long-term variations of their orbital periods have been thought to depend on several mechanisms. Now, we suggest a new mechanism: CCE-dominated mechanism. The CCE-dominated mechanism is found based on our numerical result, especially at high mass ratios, that the orbital periods (P) of contact binaries change very much with their fill-out factors (f). Because f is taken as a measurement of the thickness of CCE, the physical cause for the variation of P is a mass transfer between CCE and components. Further, an f-dominated simplification model for this mechanism is introduced. According to it, P may change in a long-term oscillation way with a similar time scale of the thermal modulation, meanwhile q is decreasing slowly till the two components merge. It could be also applied to explain the presence of extremely short period, high mass ratio and deep contact binaries. Moreover, the CCE-dominated mechanism should always work due to mass transfer and mass loss both occurring via CCE. Therefor, the effect of CCE on the variations of orbital periods may have been underestimated before.