Stellar rotation periods are valuable both for constraining models of angular momentum loss and for understanding 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). 1.2. The importance of rotation periods to exoplanet research arXiv:1807.09365v1 [astro-ph.SR]

We report the discovery of an excess of main sequence turn-off stars in the direction of the constellations of Eridanus and Phoenix from the first year data of the Dark Energy Survey (DES). The Eridanus-Phoenix (EriPhe) overdensity is centered around l ∼ 285 • and b ∼ −60 • and spans at least 30 • in longitude and 10 • in latitude. The Poisson significance of the detection is at least 9σ. The stellar population in the overdense region is similar in brightness and color to that of the nearby globular cluster NGC 1261, indicating that the heliocentric distance of EriPhe is about d ∼ 16 kpc. The extent of EriPhe in projection is therefore at least ∼ 4 kpc by ∼ 3 kpc. On the sky, this overdensity is located between NGC 1261 and a new stellar stream discovered by DES at a similar heliocentric distance, the so-called Phoenix Stream. Given their similar distance and proximity to each other, it is possible that these three structures may be kinematically associated. Alternatively, the EriPhe overdensity is morphologically similar to the Virgo overdensity and the Hercules-Aquila cloud, which also lie at a similar Galactocentric distance. These three overdensities lie along a polar plane separated by ∼ 120 • and may share a common origin. Spectroscopic follow-up observations of the stars in EriPhe are required to fully understand the nature of this overdensity.

We present an analysis of flares in mid-to-late M dwarfs in the MEarth photometric survey. We search 3985155 observations across 2226 stars, and detect 54 large (∆m ≥ 0.018) flares in total, distributed across 34 stars. We combine our flare measurements with recent activity and rotation period results from MEarth to show that there is an increase in flares per observation from low Rossby number (R o < 0.04, rapid rotators) to intermediate Rossby number (0.04 < R o < 0.44, intermediate rotators) at the 99.85% confidence level. We additionally find an increased flare rate from the high Rossby number population (R o > 0.44, slow rotators) to the intermediate population at the 99.97% level. We posit that the rise in flare rate for intermediate R o could be due to changing magnetic field geometry on the surface of the star.