MODEL ATMOSPHERES FOR VEGA

We compared blanketed LTE models for Vega computed both with the opaci ty distribution function (ODF) method and the opacity-sampling (OS) me thod. In the ODF case we used the ATLAS9 code with the new opacity dat a from Kurucz (1992). Model parameters for Vega depend on the amount o f reddening and on the helium abundance. For E(B-V) ranging from 0.0 t o 0.01, the effective temperature T(eff) is included between 9550K and 9650K. A lower limit for the gravity is log g = 3.95, obtained for T( eff) = 9550K and solar He abundance. The metallicity is [M/H]=-0.5. Th e parameters were fixed by comparing the observed ultraviolet, visual, and near infrared flux distribution with the computed one and by comp aring observed and computed Balmer profiles. A microturbulent velocity xi=2 km s-1 was assumed on the basis of previous spectroscopic works. In the OS case, we computed a model with parameters T(eff) = 9550K, l og g = 3.95, xi = 2 km s-1, but with the real abundances of Vega, as d erived from spectroscopic analyses. We used the ATLAS12 code (Kurucz 1 993b). The comparison of the fluxes and Balmer profiles from the ATLAS 9 and ATLAS12 models computed with the same T(eff), log g, and xi, but with different abundances for some elements has shown that they are a lmost identical, small differences occuring only in the ultraviolet. T herefore ATLAS9 fluxes can be used to predict colors of Vega. The more realistic ATLAS12 models should be used as starting point for abundan ce analyses and for spectrum synthesis.