The Hα EMISSION OF NEARBY M DWARFS AND ITS RELATION TO STELLAR ROTATION

Abstract

The high-energy emission from low-mass stars is mediated by the magnetic dynamo. Although the mechanisms by which fully convective stars generate large-scale magnetic fields are not well understood, it is clear that, as for solar-type stars, stellar rotation plays a pivotal role. We present 270 new optical spectra of low-mass stars in the Solar Neighborhood. Combining our observations with those from the literature, our sample comprises 2202 measurements or non-detections of Hα emission in nearby M dwarfs. This includes 466 with photometric rotation periods. Stars with masses between 0.1 and 0.6 M e are well-represented in our sample, with fast and slow rotators of all masses. We observe a threshold in the mass-period plane that separates active and inactive M dwarfs. The threshold coincides with the fast-period edge of the slowly rotating population, at approximately the rotation period at which an era of rapid rotational evolution appears to cease. The well-defined active/inactive boundary indicates that Hα activity is a useful diagnostic for stellar rotation period, e.g., for target selection for exoplanet surveys, and we present a mass-period relation for inactive M dwarfs. We also find a significant, moderate correlation between L Hα /L bol and variability amplitude: more active stars display higher levels of photometric variability. Consistent with previous work, our data show that rapid rotators maintain a saturated value of L Hα /L bol . Our data also show a clear power-law decay in L Hα /L bol with Rossby number for slow rotators, with an index of −1.7±0.1.

A Panchromatic View of Brown Dwarf Aurorae

“…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.…”

“…Equivalent width measurements consistent with 0 Å or indicative of absorption are treated as non-detections. In weakly active objects, the emergence of a chromosphere initially manifests as stronger Hα absorption before the line is filled in by stronger emission (Cram & Mullan 1985), although for the coolest M-dwarfs the maximum observed absorption has equivalent width ∼0.075 Å (Newton et al 2017). Consequently, in this analysis, some very weakly active objects are treated as non-detections.…”

An improved age–activity relationship for cool stars older than a gigayear

“…Consequently, calcium emission studies tend to focus on FGK stars and the H α line is widely used as an activity indicator in M dwarfs (e.g. Reiners, Joshi & Goldman 2012;Newton et al 2017).…”

A Physically Motivated and Empirically Calibrated Method to Measure the Effective Temperature, Metallicity, and Ti Abundance of M Dwarfs

“…Measuring the [α/Fe] of an M dwarf can be combined with priors based on kinematics (e.g., Burgasser & Mamajek 2017) to provide a powerful age diagnostic. The ability to measure the ages of field M dwarfs has many applications, including constraining the age-rotation-activity relation of M dwarfs (e.g., Newton et al 2016Newton et al , 2017.…”