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

We have determined Halpha equivalent widths and radial velocities with 1sigma accuracies approximately 5 km s-1 for approximately 20 candida te very low mass members of the Hyades and Pleiades clusters. The radi al velocities for the Hyades sample suggest that nearly all of these s tars are indeed highly probable members of the Hyades. The faintest st ars in the Hyades sample have masses of order 0.1-1M.. We also obtaine d radial velocities for four candidate very low mass members of the Pl eiades and two objects that are candidate BD Pleiads. All of these sta rs have apparent V magnitudes fainter than the Hyades stars we observe d, and the resultant radial velocity accuracy is worse. We believe tha t the three brighter stars are indeed likely very low mass stellar mem bers of the Pleiades, whereas the status of the two brown dwarf candid ates is uncertain. The Hyades stars we have observed and the three Ple iades very low mass stars are the lowest mass members of any open clus ter whose membership has been confirmed by radial velocities and whose chromospheric activity has been measured. We see no change in chromos pheric activity at the boundary where stars are expected to become ful ly convective (M congruent-to 0.3M.) in either cluster. In the Pleiade s, however, there may be a decrease in chromospheric activity for star s with (V-I)K>3.5