A First Look at Rotation in Inactive Late-Type M Dwarfs

TL;DR

This study investigates the relationship between rotation and magnetic activity in inactive late-type M dwarfs, revealing that rotation does not always correlate with activity, challenging previous assumptions about stellar magnetic behavior.

Contribution

It provides new observational evidence that rotation and activity are not always linked in late-type M dwarfs, suggesting a more complex magnetic activity threshold.

Findings

Some rotating M dwarfs lack H-alpha emission.

Rotation and activity may not be directly correlated in these stars.

Possible dependence on stellar parameters or Rossby number.

Abstract

We have examined the relationship between rotation and activity in 14 late-type (M6-M7) M dwarfs, using high resolution spectra taken at the W.M. Keck Observatory and flux-calibrated spectra from the Sloan Digital Sky Survey. Most were selected to be inactive at a spectral type where strong H-alpha emission is quite common. We used the cross-correlation technique to quantify the rotational broadening; six of the stars in our sample have vsini > 3.5 km/s. Our most significant and perplexing result is that three of these stars do not exhibit H-alpha emission, despite rotating at velocities where previous work has observed strong levels of magnetic field and stellar activity. Our results suggest that rotation and activity in late-type M dwarfs may not always be linked, and open several additional possibilities including a rotationally-dependent activity threshold, or a possible dependence on stellar parameters of the Rossby number at which magnetic/activity "saturation" takes place in fully convective stars.