RADIAL-VELOCITIES OF VERY-LOW MASS STARS AND CANDIDATE BROWN DWARF MEMBERS OF THE HYADES AND PLEIADES .2.

We have determined Hα equivalent widths and radial velocities with 1σ accuracies of ∼5 km s-1 for ∼20 candidate very low mass members of the Pleiades cluster and for a few proposed very low mass members of the Hyades. Most of the Pleiades targets were selected from the recent Hambly, Hawkins, and Jameson proper motion survey, where they were identified as probable Pleiades brown dwarfs with an age spread from 3 to 70 Myr. Our spectroscopic data and a reinterpretation of the photometric data confirm that these objects are indeed likely Pleiades members; however, we believe that they more likely have masses slightly above the hydrogen burning mass limit and that there is no firm evidence for an age spread amongst these stars. All of the very low mass Pleiades and Hyades members show Hα in emission. However, the ratio of Hα flux to bolometric flux in the Pleiades shows a maximum near MBol ≃9.5 (script M sign ≃0.3 script M sign⊙) and a sharp decrease to lower masses. This break occurs at the approximate mass where low mass stars are expected to become fully convective, and it is tempting to assume that the decrease in Hα flux is caused by some change in the behavior of stellar dynamos at this mass. We do not see a similar break in activity at this mass in the Hyades. We discuss possible evolutionary explanations for this difference in the Hα activity between the two clusters.