A ug 2 00 4 Color transformations and bolometric corrections for Galactic halo stars : α − enhanced vs 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, metal content) covered by Galactic globular cluster stars. A comparison of updated and homogeneous ATLAS 9 UBV RIJHKL 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 [M/H], and the differences increase with increasing metallicity and decreasing Teff . A preliminary analysis of the contribution of the various α elements to the stellar colors shows that the magnesium abundance (and to lesser extent oxygen and silicon) is the main responsible for these differences. On the contrary, the bolometric correction to the V band and more infrared colors predicted by α−enhanced transformations are well reproduced by scaled-solar results, due to their weak dependence on the metal content. Key parameters like the Turn Off 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 logL/L⊙-Teff plane. We find that, for both scaled solar and α-enhanced metal mixtures, the integration of a solar T (τ) relationship provides – at least