Contact Binaries

TL;DR

This paper reviews the structural understanding of W UMa contact binaries, highlighting the challenges in modeling their energy exchange, thermal equilibrium, and the differences between early and late-type systems.

Contribution

It provides a comprehensive overview of the current state of modeling contact binaries and identifies gaps in understanding their energy and mass exchange mechanisms.

Findings

Complete equilibrium models explain only a few W UMa binaries.

Departures from thermal equilibrium can match later-type binaries but cause relaxation oscillations.

The nature of energy exchange in massive, early-type contact binaries remains largely unexplored.

Abstract

Despite being the most abundant (by space density) of interacting binary stars, W Ursae Majoris (W UMa) stars are not understood structurally. Their stellar components are in physical contact, and share a common convective envelope. Observations demand large-scale energy exchange between components, as do considerations of hydrostatic and thermal equilibrium. Yet solutions in complete equilibrium can account for only a few of the bluest W UMa binaries. Models which permit departures from thermal equilibrium can reproduce the observable properties of later-type W UMa binaries, but they develop relaxation oscillations, during a substantial portion of which the binary components break thermal contact and develop very different effective temperatures, contrary to observational statistics. Massive, early-type contact binaries are also known to exist, but the nature of energy and mass exchange in common radiative envelopes remains virtually unexplored.