New Grids of ATLAS9 Model Atmospheres

New opacity distribution functions (ODFs) for several metallicities have been computed. The main improvements upon previous ODFs computed by Kurucz (1990) are: (1) the replacement of the solar abundances from Anders & Grevesse (1989) with those from Grevesse & Sauval (1998); (2) the replacement of the TiO lines provided by Kurucz (1993) with the TiO lines from Schwenke (1998), as distributed by Kurucz (1999a); (3) the addition of the H2O lines from Partridge & Schwenke (1997), as distributed by Kurucz (1999b); (4) the addition of the H I-H I and H I-H quasi-molecular absorptions near 1600 Å and 1400 Å computed according to Allard et al. (1998). Other minor improvements are related with some changes in a few atomic and molecular data. New grids of ATLAS9 model atmospheres for Teff from 3500 K to 50000 K and log g from 0.0 dex to 5.0 dex have been computed for several metallicities with the new ODFs. Preliminary comparisons of results from the old and new models have shown differences in the energy distributions of stars cooler than 4500 K, in the ultraviolet energy distribution of metal-poor A-type stars, in the U−B and u−b color indices for Teff≤6750 K and in all the color indices for Teff≤4000 K.