Moderate Resolution Spectroscopy of VLM and BD Candidate Members of the Pleiades and Hyades

Abstract

We have obtained optical spectra of about twenty of the faintest stars from the Hambly, Hawkins and Jameson Pleiades proper motion survey. Analysis of these spectra confirms that essentially all of these stars are likely to be members of the Pleiades. However, contrary to the conclusions of Steele, Jameson and Hambly, we believe that these stars are slightly above the hydrogen burning mass limit rather than brown dwarfs and that their photometry is consistent with their being coeval. Ha equivalent widths obtained for a large sample of faint Pleiades and Hyades members shows that the chromospheric activity of the members of the 70 Myr old Pleiades cluster is, on average, considerably greater than for members of the 600 Myr Hyades cluster for stars with masses greater than 0. 3M _⊙. For masses less than 0. 2M _⊙, the Ha equivalent width distributions in the two clusters are indistinguishable, indicating that it may not be possible to use Ha as an age indicator for the lowest mass stars.