The search for faint members of the Pleiades. II - Colors, spectral types, and H-alpha emission line strengths for M dwarfs in the Pleiades, Hyades, and Gliese field

Photoelectric and CCD-based BVRI photometry has been obtained for nearly 100 proper-motion select- ed Pleiades candidates members (Paper I) extending to V ≅ 18. More than three quarters of these stars prove o have positions in a V vs V - I diagram compatible with cluster membership. Low-resolution (∼5 Å) spectra centered on Hα have been obtained for 77 of these proper motion candidates and known (H ii) members of the Pleiades. Additional spectra of K and M dwarf comparison stars, drawn from the Hyades, the Gliese catalog, and MK standard system, were also observed. We derive a relationship between (R - 1)K (Kron system) and spectral type based on the MK system, from which we obtain a mean Pleiades color excess K> = +0.040±0.008, based on 29 M and late-K dwarfs. This is in excellent agreement with previous determinations based on the colors of early type stars. A comparison of the Hα emission strength for the Pleiades M dwarfs with Hyades and field M dwarfs shows that the mean Hα emission strength decreases by nearly a factor of 2 with increasing stellar age. At a given temperature, there appears to be a 10%-20% difference in maximum Hα emission between the Hyades and Pleiades M dwarfs. With a factor of 10 difference in age between the two clusters, the small difference in peak emission suggests that some physical mechanism acts to limit the maximum Hα activity. Within each group, however, there is a wide range of emission line strengths, and even in the Pleiades a few early M dwarfs have quite weak Hα emission. In a CCD-based spectroscopic Hα survey of the Pleiades field at a spectral resolution of ∼5 Å, we estimate that all but 10%-20% of the proper-motion selected sample of dwarfs having spectral types between MO and MS would be recovered.