New rotation period measurements for M dwarfs in the southern hemisphere: an abundance of slowly rotating, fully convective stars

TL;DR

This study provides new rotation period measurements for 234 southern hemisphere M dwarfs, revealing many are slowly rotating and fully convective, with implications for stellar aging and magnetic activity.

Contribution

We extended rotation period measurements to southern M dwarfs using ground-based photometry, improving data coverage and accuracy for key exoplanet host stars.

Findings

66% recovery rate for high-quality datasets

Longest detected periods around 140 days

Most stars with long periods are as old as 9 Gyr

Abstract

Stellar rotation periods are valuable both for constraining models of angular momentum loss and for under- standing how magnetic features impact inferences of exoplanet parameters. Building on our previous work in the northern hemisphere, we have used long-term, ground-based photometric monitoring from the MEarth Observatory to measure 234 rotation periods for nearby, southern hemisphere M dwarfs. Notable examples include the exoplanet hosts GJ 1132, LHS 1140, and Proxima Centauri. We find excellent agreement between our data and K2 photometry for the overlapping subset. Amongst the sample of stars with the highest quality datasets, we recover periods in 66%; as the length of the dataset increases, our recovery rate approaches 100%. The longest rotation periods we detect are around 140 days, which we suggest represent the periods that are reached when M dwarfs are as old as the local thick disk (about 9 Gyr).