Testing tidal theory using Gaia binaries: the red giant branch

TL;DR

This study tests tidal circularization theory in red giant binaries using Gaia data, revealing partial agreement but also notable discrepancies that suggest the need for refined models of tidal interactions.

Contribution

It provides the first detailed comparison of tidal evolution predictions with a large Gaia binary sample, highlighting areas where theory and observations diverge.

Findings

Tidal circularization occurs at smaller separations than predicted for less evolved giants.

Observed binaries show a more extended circularization 'cool island' than models suggest.

Tides can spin up giants, potentially affecting their mass-loss and binary evolution.

Abstract

Tidal interaction is a major ingredient in the theory of binary evolution. Here, we study tidal circularization in binaries with red giant primaries. We compute the tidal evolution for binaries as their primary stars evolve along the red giant branch, under dissipation of dynamical tides in the convective envelope. We then compare this evolution with a sample of ~30,000 red giant binaries reported by Gaia DR3. These binaries clearly show the expected gradual advance of tidal circularization, as the primary expands. But some tension with theory remains. While our calculations always predict a critical separation for tidal circularization at about 3-4 times the stellar radii, binaries with less evolved giants are observed to be circularized out to about twice as far. They also exhibit an overly extended `cool island', a collection of circular orbits that reach a couple times beyond the circularization limit. These discrepancies are reminiscent of, but less severe than, the situation for main-sequence binaries. We also find that tides can spin giant stars up to rotation rates that should affect their mass-loss. Additionally, many binaries may begin mass transfer while still eccentric.