Synthetic photometry from ATLAS9 models in the UBV Johnson system

Several grids of synthetic UBV color indices computed in the Johnson system and based on ATLAS9 atmospheric models are compared and discussed. We present synthetic colors which differ from those computed by Kurucz (1995) only for the convective models and for a very small shift of the zero-points. We compare these colors, called C97, with colors computed by Kurucz (1993a), Kurucz (1995) (K95), Bessell et al. (1998) (BCP). The Buser & Kurucz (1992) (BK92) colors, derived from models different from ATLAS9, are also considered. Teff -(U-B), Teff -(B-V), and Teff BCV relations are compared together and are compared with empirical relations from literature. For convective stars (Teff≤ 8750 K), ∆Teff differences related with the different synthetic grids based on the ATLAS9 models are not larger than 100 K for [M/H] = 0.0 and 200 K for [M/H] = -2.0. All the K95, C97, and BCP grids give, in general, Teff larger than those from the empirical Teff -color relations. For dwarfs, at Teff= 5000 K and for [M/H] = 0.0,∆Teff from (U-B) is about 250 K, and∆Teff from (B-V) is on the order of 200 K. For giants, computed and observed Teff -(U-B) data agree well, while, for (B-V), the behaviour is similar to that for dwarfs. These results do not change in a significant way for different metallicities. Computations performed for [M/H] = -2.0 have shown that, for dwarfs, the agreement with the observations improves when models computed for solar scaled abundances, but with α elements enhanced by 0.4 dex and based on the new solar iron abundance log(NFe/Ntot) = −4.53 dex, are used. The dependence of the color indices on Teff , gravity, metallicity, and microturbulent velocity is investigated.