Attributing the O'Connell effect in contact binaries to a cooling mass-transfer stream

TL;DR

This paper models contact binary systems to attribute the O'Connell effect to a cooling mass-transfer stream, challenging the common magnetic spot explanation and providing a new perspective on heat transfer mechanisms.

Contribution

The study introduces a parametric model of a mass-transfer stream with variable heat capacity to explain the O'Connell effect in contact binaries, offering an alternative to magnetic spot hypotheses.

Findings

A low-heat capacity stream can reproduce the O'Connell effect.

Surface flows significantly influence heat transfer in contact binaries.

Method applicable to large photometric survey samples.

Abstract

Contact binaries are very short-period systems that are continuously interacting by transferring mass and energy. Obtaining large, statistical samples of contact binaries from photometric surveys can put valuable constraints on the various processes involved in their evolution. Modeling those systems however present some challenges. In some contact-binary light curves, the O'Connell effect is visible, where the maxima at both quarter phases are unequal. In the literature, this effect is typically attributed to magnetic spots on the surface of the binary. In this work, we model contact-binary surfaces using PHOEBE, and include a parametric prescription for a lateral mass- and energy-transfer stream that travels from the hotter primary to the cooler secondary. We allow this stream to have a variable heat capacity. We fit a system from the Kepler sample with a strong O'Connell effect, and show that a low-heat capacity stream can explain the unequal maxima. This suggests that, in such systems, surface flows can play a significant role in transferring heat between components. Our methods can be used on larger samples of contact binaries from OGLE, Kepler, or TESS to advance our understanding of contact binary structure and evolution.