Super Slowly Spinning Stars in Close Binaries

TL;DR

This paper investigates the dynamics of stars in triple systems, revealing a population with slow spins and misalignments caused by complex tidal interactions and orbital evolution, providing insights into stellar and orbital physics.

Contribution

It identifies a new class of stars in triple systems with slow, misaligned spins due to Cassini state dynamics and models the effects of inertial wave tidal dissipation.

Findings

Discovered stars with slow rotation rates and high obliquities in triple systems.

Orbital evolution models confirm the formation of Cassini states in these systems.

Tidal dissipation via inertial waves can explain the observed slow spins.

Abstract

Stars in short-period binaries typically have spins that are aligned and synchronized with the orbit of their companion. In triple systems, however, the combination of spin and orbital precession can cause the star's rotation to evolve to a highly misaligned and sub-synchronous equilibrium known as a Cassini state. We identify a population of recently discovered stars that exhibit these characteristics and which are already known to have tertiary companions. These third bodies have a suitable orbital period to allow the inner binary to evolve into the sub-synchronous Cassini state, which we confirm with orbital evolution models. We also compute the expected stellar obliquity and spin period, showing that the observed rotation rates are often slower than expected from equilibrium tidal models. However, we show that tidal dissipation via inertial waves can alter the expected spin-orbit misalignment angle and rotation rate, potentially creating the very slow rotation rates in some systems. Finally, we show how additional discoveries of such systems can be used to constrain the tidal physics and orbital evolution histories of stellar systems.