X-Ray Sources in Multiple Stars

TL;DR

This paper explores how short-period binaries in multiple star systems, often undetected, are responsible for strong X-ray emissions, with their evolution driven by dynamical interactions like Kozai cycles leading to mergers or increased activity.

Contribution

It proposes that most luminous X-ray sources in multiple stars are due to undetected short-period binaries formed through dynamical evolution of triple systems.

Findings

Short-period binaries are common in multiple star systems.

Kozai cycling induces high eccentricity, leading to tidal interactions.

Mergers of binaries produce rapidly rotating active stars.

Abstract

Luminous X-ray stars are very often found in visual double or multiple stars. Binaries with periods of a few days possess the highest degree of coronal X-ray activity among regular, non-relativistic stars because of their fast, tidally driven rotation. But the orbital periods in visual double stars are too large for any direct interaction between the companions to take place. We suggest that most of the strongest X-ray components in resolved binaries are yet-undiscovered short-period binaries, and that a few are merged remnants of such binaries. The omnipresence of short-period active stars, e.g. of BY-Dra-type binaries, in multiple systems is explained via the dynamical evolution of triple stars with large mutual inclinations. The dynamical perturbation on the inner pair pumps up the eccentricity in a cyclic manner, a phenomenon known as Kozai cycling. At times of close periapsis, tidal friction reduces the angular momentum of the binary, causing it to shrink. When the orbital period of the inner pair drops to a few days, fast surface rotation of the companions is driven by tidal forces, boosting activity by a few orders of magnitude. If the period drops still further, a merger may take place leaving a rapidly-rotating active dwarf with only a distant companion.