Synthetic stellar photometry – I. General considerations and new transformations for broad-band systems

Abstract

After a pedagogical introduction to the main concepts of synthetic\nphotometry, colours and bolometric corrections in the Johnson-Cousins, 2MASS,\nand HST-ACS/WFC3 photometric systems are generated from MARCS synthetic fluxes\nfor various [Fe/H] and [alpha/Fe] combinations, and virtually any value of\nreddening E(B-V) < 0.7. The successes and failures of model fluxes in\nreproducing the observed magnitudes are highlighted. Overall, extant synthetic\nfluxes predict quite realistic broad-band colours and bolometric corrections,\nespecially at optical and longer wavelengths: further improvements of the\npredictions for the blue and ultraviolet spectral regions await the use of\nhydrodynamic models where the microturbulent velocity is not treated as a free\nparameter. We show how the morphology of the colour-magnitude diagram (CMD)\nchanges for different values of [Fe/H] and [alpha/Fe]; in particular, how\nsuitable colour combinations can easily discriminate between red giant branch\nand lower main sequence populations with different [alpha/Fe], due to the\nconcomitant loops and swings in the CMD. We also provide computer programs to\nproduce tables of synthetic bolometric corrections as well as routines to\ninterpolate in them. These colour-Teff-metallicity relations may be used to\nconvert isochrones for different chemical compositions to various bandpasses\nassuming observed reddening values, thus bypassing the standard assumption of a\nconstant colour excess for stars of different spectral type. We also show how\nsuch an assumption can lead to significant systematic errors. The MARCS\ntransformations presented in this study promise to provide important\nconstraints on our understanding of the multiple stellar populations found in\nglobular clusters (e.g., the colours of lower main sequence stars are predicted\nto depend strongly on [alpha/Fe]) and of those located towards/in the Galactic\nbulge.\n