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 selected Pleiades candidates members (Paper I) extending to V approximately-or-equal-to 18. More than three quarters of these stars prove to have positions in a V vs V - I diagram compatible with cluster membership. Low-resolution (approximately 5 angstrom) spectra centered on H-alpha 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 - I)K (Kron system) and spectral type based on the MK system, from which we obtain a mean Pleiades color excess [E(R - I)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 H-alpha emission strength for the Pleiades M dwarfs with Hyades and field M dwarfs shows that the mean H-alpha 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 H-alpha 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 H-alpha 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 H-alpha emission. In a CCD-based spectroscopic H-alpha survey of the Pleiades field at a spectral resolution of approximately 5 angstrom, we estimate that all but 10%-20% of the proper-motion selected sample of dwarfs having spectral types between M0 and M5 would be recovered.