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

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 Hα emission is quite common. We used the cross-correlation technique to quantify the rotational broadening; six of the stars in our sample have v sin i ≥ 3.5 km s –1 . Our most significant and perplexing result is that three of these stars do not exhibit Hα 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.