Tidal instability in systems' evolution

TL;DR

This paper introduces tidal (elliptical) instability as a new factor influencing the evolution of extrasolar planetary systems, with potential effects on stellar rotation axes and system stability.

Contribution

It proposes a theoretical framework for tidal instability in rotating stars, estimates growth rates for hot-Jupiter systems, and presents a stability diagram based on observational data.

Findings

Most systems are in the 'forbidden zone' where instability cannot grow.

Some systems exhibit rapid growth of tidal instability.

Implications for stellar axis stability in misaligned transiting systems.

Abstract

A new element is proposed to play a role in the evolution of extrasolar planetary systems: the tidal (or elliptical) instability. It comes from a parametric resonance and takes place in any rotating fluid whose streamlines are (even slightly) elliptically deformed. Based on theoretical, experimental and numerical works, we estimate the growth rate of the instability for hot-jupiter systems, when the rotation period of the star is known. We present the physical process, its application to stars, and preliminary results obtained on a few dozen systems, summarized in the form of a stability diagram. Most of the systems are trapped in the so-called "forbidden zone", where the instability cannot grow. In some systems, the tidal instability is able to grow, at short timescales compared to the system evolution. Implications are discussed in the framework of mis-aligned transiting systems, as the rotational axis of the star would be unstable in systems where this elliptical instability grows.