Contact-binary evolution with energy transfer and saturated magnetic braking

TL;DR

This paper investigates the evolution of low-mass contact binaries, emphasizing the roles of saturated magnetic braking and energy transfer in shaping their period evolution and low mass ratios.

Contribution

It provides new evidence supporting saturated magnetic braking and highlights the importance of energy transfer in the evolution of contact binaries.

Findings

Magnetic braking likely saturates, affecting period evolution.

Energy transfer influences low mass ratios.

Period distribution supports saturated magnetic braking evidence.

Abstract

The evolution of low-mass contact binaries is influenced by angular-momentum loss, mass and energy transfer, and the nuclear evolution of the components. They have periods shorter than one day, and we expect their period evolution to be dominated by magnetic braking. Evidence for saturated magnetic braking was presented by studying the period distribution of detached eclipsing binaries. This means the strength of magnetic braking likely does not cause a steep period-shrinking relation derived from the widely used Skumanich law. We find further evidence for saturated magnetic braking by considering evolutionary models of low-mass contact binaries. We also show that energy transfer must play an important role over a wide parameter range in producing the observed low mass ratios of contact binaries.