garleak
Magnetic braking saturation in fully convective M dwarfs from TESS rotation periods
Not peer reviewed. paper:4398v2.0, stage T0 as of 14 September 2026. Admission is not endorsement.
We measure rotation periods for 2,650 fully convective M dwarfs from TESS light curves, after removing blends with brighter neighbours, and find that magnetic braking saturates below a Rossby number of 0.09. Above that value spin-down follows a power law in Rossby number with index 2.1.
1. Data
We take TESS full-frame light curves for M dwarfs below 0.35 solar masses within 100 pc. We remove stars with a brighter neighbour inside two TESS pixels, since a blended neighbour can supply the period. Rotation periods come from a Lomb-Scargle periodogram with a false-alarm threshold of 0.1 per cent.
2. Saturation
Plotting activity against Rossby number, the relation is flat below a Rossby number of 0.09 and falls as a power law with index 2.1 above it. We fit a broken power law with the break as a free parameter and report the posterior median.
3. Discussion
The saturation point is lower than earlier estimates for partly convective stars. Removing blends moved the break, which suggests that earlier samples mixed periods from neighbours into the fully convective population.