High-resolution Hα observations of M dwarf stars Implications for stellar dynamo models and stellar kinematic properties at faint magnitudes

We present high-dispersion observations of the Halpha spectral region for a sample of very late M dwarf stars, as obtained with the Multiple Mirror Telescope echelle spectrograph and Reticon detector. Radial velocities and total space motions are derived. The Halpha emission line properties of our sample are thereby examined according to membership in stellar kinematic populations. We find that, contrary to previous assertions, M dwarf stars exist later than spectral type M5.5 which do not exhibit Halpha line emission. In fact, there are comparable numbers showing and not showing Halpha emission for M<SUB>v</SUB> > + 15 in this sample of large proper motion stars. Thus, the onset of full convection along the main sequence does not necessarily imply an enhanced level of magnetic field related chromospheric activity. However, we do find for the small and biased sample considered in this investigation that dMe stars are the dominant class of objects for absolute magnitudes M<SUB>v</SUB> > + 17. The dMe stars tend to exhibit the small total space motions and (U, V, W) velocity dispersions that are indicative of young disk kinematics, while nearly all the M dwarfs lacking Halpha emission are members of either the old disk or halo kinematic populations. The faintest subdwarf included in this investigation has M<SUB>v</SUB> ≈ +14.8. The implications for stellar dynamo theory and for the nature of the stellar luminosity function at faint magnitudes are discussed. In particular, the results constitute corroborative evidence for a rotation-dependent distributed dynamo as a viable model for the generation of magnetic flux in the very low mass stars. Our results also suggest no significant difference between the kinematic properties of the stellar population that is brighter than the likely maximum in the luminosity function and that which is fainter.