Synthetic Stellar Photometry - I. General considerations and new
  transformations for broad-band systems
  After a pedagogical introduction to the main concepts of synthetic
photometry, colours and bolometric corrections in the Johnson-Cousins, 2MASS,
and HST-ACS/WFC3 photometric systems are generated from MARCS synthetic fluxes
for various [Fe/H] and [alpha/Fe] combinations, and virtually any value of
reddening E(B-V) < 0.7. The successes and failures of model fluxes in
reproducing the observed magnitudes are highlighted. Overall, extant synthetic
fluxes predict quite realistic broad-band colours and bolometric corrections,
especially at optical and longer wavelengths: further improvements of the
predictions for the blue and ultraviolet spectral regions await the use of
hydrodynamic models where the microturbulent velocity is not treated as a free
parameter. We show how the morphology of the colour-magnitude diagram (CMD)
changes for different values of [Fe/H] and [alpha/Fe]; in particular, how
suitable colour combinations can easily discriminate between red giant branch
and lower main sequence populations with different [alpha/Fe], due to the
concomitant loops and swings in the CMD. We also provide computer programs to
produce tables of synthetic bolometric corrections as well as routines to
interpolate in them. These colour-Teff-metallicity relations may be used to
convert isochrones for different chemical compositions to various bandpasses
assuming observed reddening values, thus bypassing the standard assumption of a
constant colour excess for stars of different spectral type. We also show how
such an assumption can lead to significant systematic errors. The MARCS
transformations presented in this study promise to provide important
constraints on our understanding of the multiple stellar populations found in
globular clusters (e.g., the colours of lower main sequence stars are predicted
to depend strongly on [alpha/Fe]) and of those located towards/in the Galactic
bulge.
