[ITAL]C[CLC]aby[/CLC][/ITAL] Photometry of the Hyades: Comparisons with the Field Stars

Intermediate-band photometry of the Hyades cluster on the Caby system ispresented for dwarf stars ranging from spectral type A through late K. A meanhk, b-y relation is constructed using only single stars without anomalousatmospheres and compared to the field stars of the solar neighborhood. For theF dwarfs, the Hyades relation defines an approximate LOWER bound in thetwo-color diagram, consistent with an [Fe/H] between +0.10 and +0.15. Theseindex-color diagrams follow the common convention of presenting stars withhighest abundance at the bottom of the plot although the index values for themetal-rich stars are numerically larger. For field F dwarfs in the range [Fe/H]between +0.4 and -1.0, [Fe/H] = -5.6 delta-hk + 0.125, with no evidence for acolor dependence in the slope. For the G and K dwarfs, the Hyades mean relationcrosses the field star distribution in the two-color diagram, defining anapproximate UPPER bound for the local disk stars. Stars found above the Hyadesstars fall in at least one of three categories: [Fe/H] below -0.7, [Fe/H] abovethat of the Hyades, or chromospherically active. It is concluded that, contraryto the predictions of model atmospheres, the hk index for cool dwarfs at agiven color hits a maximum value for stars below solar composition and, withincreasing [Fe/H] above some critical value, declines. This trend isconsistent, however, with the predictions from synthetic indices based uponmuch narrower Ca filters where the crossover is caused by the metallicitysensitivity of b-y.Comment: 13 pages, 9 eps figures, 1 tex table, 1 ascii tabl