Color Transformations and Bolometric Corrections for Galactic Halo Stars: α-Enhanced versus Scaled-Solar Results

We have performed the first extensive analysis of the impact of an [α/Fe] > 0 metal distribution on broadband colors in the parameter space (surface gravity, effective temperature, and metal content) covered by Galactic globular cluster stars. A comparison of updated and homogeneous ATLAS 9 UBVRIJHKL synthetic photometry, for both α-enhanced and scaled-solar metal distributions, has shown that it is impossible to reproduce α-enhanced (B - V) and (U - B) color transformations with simple rescalings of the scaled-solar ones. At [Fe/H] ~ -2.0, α-enhanced transformations are well reproduced by scaled-solar ones with the same [Fe/H], but this good agreement breaks down at [Fe/H] larger than about -1.6. As a general rule, (B - V) and (U - B) α-enhanced colors are bluer than scaled-solar ones at either the same [Fe/H] or the same [M/H], and the differences increase with increasing metallicity and decreasing T_eff. A preliminary analysis of the contribution of the various α-elements to the stellar colors shows that the magnesium abundance (and to a lesser extent oxygen and silicon) is mainly responsible for these differences. On the contrary, the bolometric correction to the V-band and other infrared colors predicted by α-enhanced transformations are well reproduced by scaled-solar results because of their weak dependence on the metal content. Key parameters, such as the turnoff and zero-age horizontal branch V magnitudes, as well as the red giant branch tip I magnitude obtained from theoretical isochrones, are in general unaffected when using the appropriate α-enhanced transformations in place of scaled-solar ones. We have also studied, for the first time, the effect of boundary conditions obtained from appropriate α-enhanced model atmospheres on the stellar evolutionary tracks in the log L/L_☉-T_eff plane. We find that, for both scaled-solar and α-enhanced metal mixtures, the integration of a solar T(τ) relationship provides—at least for masses larger than 0.5-0.6 M_☉—tracks very similar to those computed using boundary conditions from the appropriate model atmospheres.