On the angular momentum evolution of fully-convective stars: rotation periods for field M-dwarfs from the MEarth transit survey

TL;DR

This study measures rotation periods of fully-convective M-dwarfs, revealing a wide range of spin rates and suggesting significant angular momentum loss via stellar winds, especially in older, thick disk stars.

Contribution

It provides the first extensive rotation period measurements for field M-dwarfs near the fully-convective boundary, highlighting mass-dependent spin-down timescales.

Findings

Older M-dwarfs rotate more slowly, with median periods around 92 days.

Younger M-dwarfs have median rotation periods of about 0.7 days.

Spin-down times increase with decreasing stellar mass, becoming comparable to thick disk ages.

Abstract

We present rotation period measurements for 41 field M-dwarfs, all of which have masses inferred (from their parallaxes and 2MASS K-band magnitudes) to be between the hydrogen burning limit and 0.35 Msol, and thus should remain fully-convective throughout their lifetimes. We measure a wide range of rotation periods, from 0.28 days to 154 days, with the latter commensurate with the typical sensitivity limit of our observations. Using kinematics as a proxy for age, we find that the majority of objects likely to be thick disk or halo members (and hence, on average, older) rotate very slowly, with a median period of 92 days, compared to 0.7 days for those likely to be thin disk members (on average, younger), although there are still some rapid rotators in the thick disk sample. When combined with literature measurements for M-dwarfs, these results indicate an increase in spin-down times with decreasing stellar mass, in agreement with previous work, and that the spin-down time becomes comparable to the age of the thick disk sample below the fully-convective boundary. We additionally infer that the spin-down must remove a substantial amount of angular momentum once it begins in order to produce the slow rotators we observe in the thick disk candidates, suggesting that fully-convective M-dwarfs may still experience strong winds.