The Rotation and Galactic Kinematics of Mid M Dwarfs in the Solar Neighborhood

Abstract

Rotation is a directly observable stellar property, and it drives magnetic field generation and activity through a magnetic dynamo. Main-sequence stars with masses below approximately 0.35  M (mid-to-late M dwarfs) are fully convective, and are expected to have a different type of dynamo mechanism than solar-type stars. Measurements of their rotation rates provide insight into these mechanisms, but few rotation periods are available for these stars at field ages. Using photometry from the MEarth Project, we measure rotation periods for 387 nearby, mid-to-late M dwarfs in the northern hemisphere, finding periods from 0.1 to 140 days. The typical rotator has stable, sinusoidal photometric modulations at a semi-amplitude of 0.5%-1%. We find no period-amplitude relation for stars below 0.25  M and an anticorrelation between period and amplitude for higher-mass M dwarfs. We highlight the existence of older, slowly rotating stars without Hα emission that nevertheless have strong photometric variability. We use parallaxes, proper motions, radial velocities, photometry, and near-infrared metallicity estimates to further characterize the population of rotators. The Galactic kinematics of our sample is consistent with the local population of G and K dwarfs, and rotators have metallicities characteristic of the solar neighborhood. We use the W space velocities and established age-velocity relations to estimate that stars with P<10 days have ages of on average <2 Gyr, and that those with P>70 days have ages of about 5 Gyr. The period distribution is dependent on mass: as the mass decreases, the slowest rotators at a given mass have longer periods, and the fastest rotators have shorter periods. We find a lack of stars with intermediate rotation periods, and the gap between the fast and slow rotators is larger for lower masses. Our data are consistent with a scenario in which these stars maintain rapid rotation for several gigayears, then spin down quickly, reaching periods of around 100 days by a typical age of 5 Gyr.

Rotation of Late-type Stars in Praesepe with K2

“…Particularly in this last regime, primarily the M star regime, there is a bimodal distribution of periods, with few between ∼2 and ∼10 days. We interpret this to mean that once M stars start to slow down, they do so rapidly; this is likely a predecessor of the bimodal distribution of M star rotation rates found in much older field stars (e.g., Kado-Fong et al 2016;Newton et al 2016). The distribution of P versus -( ) V K s exhibits significant changes between the Pleiades (∼125 Myr) and Praesepe (∼790 Myr).…”

“…The bimodal period distribution of M stars in Praesepe (but not the Pleiades) is interesting because field M stars are found to have a bimodal P distribution in the Kepler field (e.g., McQuillan et al 2013;Davenport 2016) and in nearby M field stars (e.g., Kado-Fong et al 2016;Newton et al 2016). These field stars are older, on average, than Praesepe, and the locations of the two peaks are slower (∼19 and ∼33 days) compared to Praesepe with ∼1 and ∼17 days for the ∼500 Praesepe stars with -( ) V K s 3.79, the color corresponding to M0.…”

“…However, it is interesting that both distributions are bimodal. It may be that the mechanism that causes some M stars to rapidly spin down (e.g., Brown 2014; Newton et al 2016) has started to operate in some of the Praesepe stars. We note, of course, that our data cannot constrain the behavior of M stars later than about M5 or M6.…”

“…For the stars with -( ) V K s 4, these could be examples of systems where one star has spun down and the other has not; the difference in periods in these binaries has significantly increased since the age of the Pleiades. These pairs could perhaps shed light on the mechanism that causes some M stars to rapidly spin down (see, e.g., Newton et al 2016). …”

Synergies between exoplanet surveys and variable star research

“…However, it is important to note that several other projects also made very significant contributions. For example, MEarth 6 (M dwarf stars in search of new Earth-like exoplanets, see [47]) has provided an important contribution to the variability survey of M dwarfs ( [67]), and made the first discoveries of a transiting Super-Earth/Mini-Neptune ( [24]) and an Earth-sized planet from the ground ( [15]). Yet another, highly significant discovery comes from the TRAPPIST 7 (TRAnsiting Planets and PlanetesImals Small Telescope) project.…”