The metallicity dependence of the primary indices of the uvby photometric system for cooler dwarfs ( T _ { e } \sim 6500 K to 5000K ) is investigated .
The data base for the analysis is composed of the overlap between a composite catalog of selected , high-dispersion spectroscopic abundances for 1801 stars on the metallicity scale of Valenti & Fischer ( 77 ) and a merged catalog of high-precision uvby H \beta photometry for over 35,000 stars .
While [ Fe/H ] for F dwarfs is best estimated from m _ { 1 } , with a modest dependence on c _ { 1 } as expected , for hotter G dwarfs the pattern reverses and c _ { 1 } becomes the dominant index .
For cooler G dwarfs and K stars , the c _ { 1 } dominance continues , but a discontinuity appears such that stars between b - y = 0.50 and 0.58 with [ Fe/H ] \geq +0.25 have m _ { 1 } and c _ { 1 } indices that classify them as subgiants , confirming an earlier result based upon a much smaller sample .
The reversal in the sensitivity to m _ { 1 } and c _ { 1 } is traced , in part , to the metallicity sensitivity of the b - y index .
Moreover , b - y grows larger in a non-linear fashion for stars above solar metallicity , leading to an overestimate of the reddening for super-metal-rich stars from some standard intrinsic color relations .
Based upon successful tests using indices from synthetic spectra and the empirical trends among the observations , metallicity calibrations tied to H \beta rather than b - y have been derived for [ Fe/H ] \geq - 1.0 , generating dispersions among the residuals ranging from 0.061 dex to 0.085 dex over the entire temperature range of interest .
The new calibrations have the added advantage of being significantly less sensitive to errors in reddening than previous calibrations .