Protostellar spin-down: a planetary lift?

TL;DR

This paper investigates whether early star-planet interactions can explain the low angular momentum of young stars, concluding that such interactions are insufficient compared to star-disk magnetic coupling.

Contribution

The study demonstrates that tidal and magnetic interactions with close-in planets cannot significantly reduce stellar spin compared to star-disk magnetic coupling.

Findings

Star-planet interactions are too weak to prevent stellar spin-up.

Magnetic star-disk coupling remains the primary mechanism for angular momentum loss.

Early planet formation and migration do not significantly influence stellar rotation rates.

Abstract

When they first appear in the HR diagram, young stars rotate at a mere 10\% of their break-up velocity. They must have lost most of the angular momentum initially contained in the parental cloud, the so-called angular momentum problem. We investigate here a new mechanism by which large amounts of angular momentum might be shed from young stellar systems, thus yielding slowly rotating young stars. Assuming that planets promptly form in circumstellar disks and rapidly migrate close to the central star, we investigate how the tidal and magnetic interactions between the protostar, its close-in planet(s), and the inner circumstellar disk can efficiently remove angular momentum from the central object. We find that neither the tidal torque nor the variety of magnetic torques acting between the star and the embedded planet are able to counteract the spin up torques due to accretion and contraction. Indeed, the former are orders of magnitude weaker than the latter beyond the corotation radius and are thus unable to prevent the young star from spinning up. We conclude that star-planet interaction in the early phases of stellar evolution does not appear as a viable alternative to magnetic star-disk coupling to understand the origin of the low angular momentum content of young stars.