Signature of Planetary Mergers on Stellar Spins

TL;DR

Planetary mergers with stars can significantly spin up the stars, explaining observed bifurcations in stellar spin periods in young clusters and supporting planetary accretion as a common evolutionary process.

Contribution

This study demonstrates how planetary mergers influence stellar spin periods and aligns theoretical predictions with observed stellar spin bifurcations in young clusters.

Findings

Planetary mergers spin up stars by ~60%.

Spin-up causes a gap in stellar spin periods in young clusters.

Magnetic braking later reduces stellar spin, explaining the disappearance of the bifurcation.

Abstract

One of the predictions of high eccentricity planetary migration is that many planets will end up plunging into their host stars. We investigate the consequence of planetary mergers on their stellar hosts' spin-period. Energy and angular momentum conservation yield that a planet consumption by a star will spin-up the star. We find that our calculations align with the observed bifurcation in the stellar spin-period in young clusters. For example, after a Sun-like star has eaten a Jupiter-mass planet it will spin up by ~60% (i.e., spin-period is reduced by ~60%), causing an apparent gap in the stellar spin period, between stars that consumed a planet and those that did not. The spun-up star will later spin down due to magnetic braking, consistent with the disappearance of this bifurcation in clusters (>300Myr). The agreement between the calculations presented here, and the observed spin-period color diagram of stars in young clusters provides circumstantial evidence that planetary accretion onto their host stars is a generic feature of planetary-system evolution.