An abrupt change in the stellar spin-down law at the fully convective boundary

TL;DR

This study reveals a sudden change in how stars lose angular momentum at the boundary between fully convective and partially convective stars, indicating different underlying magnetic dynamo mechanisms.

Contribution

It provides the first observational evidence of an abrupt change in stellar spin-down laws at the fully convective boundary, highlighting differences in magnetic dynamo processes.

Findings

Fully convective stars have a 2.25 times higher angular momentum loss rate.

A larger dipole magnetic field strength is required for fully convective stars.

Mass loss rates are approximately 4.2 times higher in fully convective stars.

Abstract

The importance of the existence of a radiative core in generating a solar-like magnetic dynamo is still unclear. Analytic models and magnetohydrodynamic simulations of stars suggest the thin layer between a star's radiative core and its convective zone can produce shearing that reproduces key characteristics of a solar-like dynamo. However, recent studies suggest fully and partially convective stars exhibit very similar period-activity relations, hinting that dynamos generated by stars with and without radiative cores hold similar properties. Here, using kinematic ages, we discover an abrupt change in the stellar spin-down law across the fully convective boundary. We found that fully convective stars exhibit a higher angular momentum loss rate, corresponding to a torque that is $\sim$ 2.25 times higher for a given angular velocity than partially convective stars around the fully convective boundary. This requires a dipole field strength that is larger by a factor of $\sim$2.5, a mass loss rate that is $\sim$4.2 times larger, or some combination of both of those factors. Since stellar-wind torques depend primarily on large-scale magnetic fields and mass loss rates, both of which derive from magnetic activity, the observed abrupt change in spin-down law suggests that the dynamos of partially and fully convective stars may be fundamentally different