Model atmospheres for Vega.

Abstract

We compared blanketed Local Thermodynamic Equilibrium (LTE) models for Vega computed both with the opacity distribution function (ODF) method and the opacity-sampling (OS) method. In the ODF case we used the ATLAS9 code with the new opacity data from Kurucz (1992). Model parameters for Vega depend on the amount of reddening and on the helium abundance. For E(B-V) ranging from 0.0 to 0.01, the effective temperature T_eff is included between 9550K and 9650K. A lower limit for the graity is log g = 3.95, obtained for T_eff = 9550K and solar He abundance. The metallicity is (M/H) = -0.5. The parameters were fixed by comparing the observed ultraviolet, visual, and near infrared flux distribution with the computed one and by comparing observed and computed Balmer profiles. A microturbulent velocity zeta = 2 km/s was assumed on the basis of previous spectroscopic works. In the OS case, we computed a model with parameters T_eff = 9550K, log g = 3.95, zeta = 2 km/s, but with the real abundances of Vega, as derived from spectroscopic analyses. We used the ATLAS12 code (Kurucz 1993b). The comparison of the fluxes and Balmer profiles from the ATLAS9 and ATLAS12 models computed with the same T_eff, log g, and zeta, but with different abundances for some elements has shown that they are almost identical, small differences occuring only in the ultraviolet. Therefore ATLAS9 fluxes can be used to predict colors of Vega. The more realistic ATLAS12 models should be used as starting point for abundance analyses and for spectrum synthesis.