The coupled effect of tides and stellar winds on the evolution of compact binaries

TL;DR

This paper investigates how tides and stellar winds jointly influence the evolution of compact binaries, revealing complex behaviors like spin equilibrium and wind-driven Roche-lobe overflow, which are not captured by simple timescale models.

Contribution

It provides a detailed coupled model of tides and stellar winds in compact binaries, highlighting outcomes like spin equilibrium and the limitations of simple timescale approximations.

Findings

Tides coupled with non-massive winds can lead to spin equilibrium.

Massive winds can drive binaries to Roche-lobe overflow even on the main sequence.

Simple timescale models often fail to accurately predict the evolution of these systems.

Abstract

We follow the evolution of compact binaries under the coupled effect of tides and stellar winds until the onset of Roche-lobe overflow. These binaries contain a compact object (either a black-hole, a neutron-star, or a planet) and a stellar component. We integrate the full set of tidal equations, which are based on Hut's model for tidal evolution, and we couple them with the angular momentum loss in a stellar wind. Our aim is twofold. Firstly, we wish to highlight some interesting evolutionary outcomes of the coupling. When tides are coupled with a non-massive stellar wind, one interesting outcome is that in certain types of binaries, the stellar spin tends to reach a quasi-equilibrium state, where the effect of tides and wind are counteracting each other. When tides are coupled with a massive wind, we parametrize the evolution in terms of the decoupling radius, at which the wind decouples from the star. Even for small decoupling radii this \emph{wind braking} can drive systems on the main sequence to Roche-lobe overflow that otherwise would fail to do so. Our second aim is to inspect whether simple timescale considerations are a good description of the evolution of the systems. We find that simple timescale considerations, which rely on neglecting the coupling between tides and stellar winds, do not accurately represent the true evolution of compact binaries. The outcome of the coupled evolution of the rotational and orbital elements can strongly differ from simple timescale considerations, as already pointed out by Barker & Ogilvie 2009 in the case of short-period planetary systems.