Intermediate-band photometry of the Hyades cluster on the Caby system is presented for dwarf stars ranging from spectral type A through late K. A mean hk , b - y relation is constructed using only single stars without anomalous atmospheres and compared to the field stars of the solar neighborhood .
For the F dwarfs , the Hyades relation defines an approximate lower bound in the two-color diagram , consistent with an [ Fe/H ] between +0.10 and +0.15 .
These index-color diagrams follow the common convention of presenting stars with highest abundance at the bottom of the plot although the index values for the metal-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 a color dependence in the slope .
For the G and K dwarfs , the Hyades mean relation crosses the field star distribution in the two-color diagram , defining an approximate upper bound for the local disk stars .
Stars found above the Hyades stars fall in at least one of three categories : [ Fe/H ] below –0.7 , [ Fe/H ] above that of the Hyades , or chromospherically active .
It is concluded that , contrary to the predictions of model atmospheres , the hk index for cool dwarfs at a given color hits a maximum value for stars below solar composition and , with increasing [ Fe/H ] above some critical value , declines .
This trend is consistent , however , with the predictions from synthetic indices based upon much narrower Ca filters where the crossover is caused by the metallicity sensitivity of b - y .