An Activity–rotation Relationship and Kinematic Analysis of Nearby Mid-to-Late-Type M Dwarfs

Abstract

Using spectroscopic observations and photometric light curves of 238 nearby M dwarfs from the MEarth exoplanet transit survey, we examine the relationships between magnetic activity (quantified by Hα emission), rotation period, and stellar age. Previous attempts to investigate the relationship between magnetic activity and rotation in these stars were hampered by the limited number of M dwarfs with measured rotation periods (and the fact that v sin i measurements probe only rapid rotation). However, the photometric data from MEarth allows us to probe a wide range of rotation periods for hundreds of M dwarf stars (from shorter than than one to longer than 100 days). Over all M spectral types that we probe, we find that the presence of magnetic activity is tied to rotation, including for late-type, fully convective M dwarfs. We also find evidence that the fraction of latetype M dwarfs that are active may be higher at longer rotation periods compared to their early-type counterparts, with several active, late-type, slowly rotating stars present in our sample. Additionally, we find that all M dwarfs with rotation periods shorter than 26 days (early-type; M1-M4) and 86 days (late-type; M5-M8) are magnetically active. This potential mismatch suggests that the physical mechanisms that connect stellar rotation to chromospheric heating may be different in fully convective stars. A kinematic analysis suggests that the magnetically active, rapidly rotating stars are consistent with a kinematically young population, while slow-rotators are less active or inactive and appear to belong to an older, dynamically heated stellar population.

A Panchromatic View of Brown Dwarf Aurorae

“…In Figure 6, we show the distribution of UCDs with both Hα measurements and v i sin measurements in the literature (see the Appendix). There is no clear pattern of larger luminosities for faster rotators as has been observed in M-dwarfs (e.g., McLean et al 2012;West et al 2015).…”

“…One commonly invoked dynamo is the 2 a dynamo, which harnesses convective motions and rotation, but models have identified alternate dynamo mechanisms depending on a range of properties, including rotation rate and bolometric luminosity (Browning 2008;Christensen et al 2009;Yadav et al 2015). Interestingly, despite the transition in the internal structure, there does not appear to be an abrupt change in the strength of magnetic activity emission indicators across the fully convective boundary (e.g., West et al 2015;Wright & Drake 2016;Newton et al 2017). This transition coincides with a change in the prevalent magnetic field topologies.…”

The Suppression and Promotion of Magnetic Flux Emergence in Fully Convective Stars

“…Yet M dwarfs on the fully convective side of the 'tachocline divide' ( 0.35M ⊙ ) still effectively build magnetic fields, with this activity increasing in prevalence toward late M spectral types (e.g. West et al 2015). This magnetism is similar to that observed in solar-like stars; i.e., there is still a rotation-activity correlation that plateaus at rapid rotation (e.g.…”

Can brown dwarfs survive on close orbits around convective stars?

“…Second, the absence of a tachoclyne may also affect how the magnetic field is generated in these stars, which could imply a braking law that is not Skumanich-like. Tying age to rotation for late-type M dwarfs is very challenging because both their age and rotation period are difficult to measure, but there is observational evidence that older stars are generally slow rotators for both early-type and late-type M dwarfs (West et al 2015). Leaving aside the complexity of the saturated regime for fast rotators, the magnetic braking law that we used here could also apply to late-type M dwarfs, using an appropriate value of α mb (Reiners & Mohanty 2012).…”