# Star-planet tidal interaction and the limits of gyrochronology

## TL;DR

This paper explores how close-in massive planets can alter stellar rotation, complicating age estimates, and introduces a new method called tidal-chronology to improve age determination in such star-planet systems.

## Contribution

It proposes the tidal-chronology technique, extending gyrochronology to systems affected by star-planet tidal interactions, especially where planetary engulfment has occurred.

## Key findings

- Tidal interactions can significantly modify stellar rotation rates.
- Gyrochronology can be misleading in systems with close-in massive planets.
- Tidal-chronology offers a potential alternative for age estimation in affected systems.

## Abstract

Age estimation techniques such as gyrochronology and magnetochronology can't be applied to stars that exchanged angular momentum with their close environment. This is especially true for massive close-in planetary companion (with a periods of few days or less), which could have been strongly impacted the rotational evolution of the host star, along the stellar evolution, through the star-planet tidal interaction. We showed that the interaction of a close-in massive planet with its host star can strongly modify the surface rotation rate of this latter, in most of the cases associated to a planetary engulfment. In such cases, a gyrochronology analysis of the star would wrongly make it appear as "rejuvenated", thus preventing us to use this method with confidence. To try overcome this issue, we eventually proposed the proof of concept of a new age determination technique that we call the tidal-chronology, which is based on the observed couple $\rm P_{rot,\star}$-$\rm P_{orb}$ of a given star-planet system. The gyrochronology technique can only be applied to isolated star or star-planet systems outside a specific range of $\rm P_{rot,\star}$-$\rm P_{orb}$. This region tends to expand for increasing stellar and planetary mass. In that forbidden region, or if any planetary engulfment is suspected, gyrochronology should be used with extreme caution while tidal-chronology could be considered. While this technique does not provide a precise age for the system yet, it is already an extension of gyrochronology and could be helpful to determine a more precise range of possible age for planetary system composed of a star between 0.3 and 1.2 $M_{\odot}$ and a planet more massive than 1 $\rm M_{jup}$ initially located at few hundredth au of the host star.