2001A&A...372..260C

New opacity distribution functions (ODF) have been computed for use with the ATLAS9 model atmosphere code of Kurucz. One of the improvements upon the Kurucz (1990, Stellar Atmospheres: Beyond Classical Models, NATO Aso Series, ed. L. Crivellari et al., 441) ODFs is the addition to the line blanketing of the Lyman-α H-H and H-H⁺ quasi-molecular absorptions near 1600Å and 1400Å. New-ODF fluxes are expected to reproduce the ultraviolet observations of λ Boo stars and metal-poor A-type stars in a more realistic way than previous computations did. In this paper we compare low- and high-resolution IUE observations of λ Boo (HD 125162, HR 5351) with fluxes and synthetic spectra based on ATLAS9 models and new-ODFs, which were computed for [M/H]=-2.0 for all the elements, except CNO. For C, N, and O, abundances log(N_elem/N_tot) equal to -3.85, -3.99, and -3.11, respectively, were adopted. We selected λ Boo in order to compare results from the new-ODFs with those from Allard et al. (1998, Proceedings of the Conference Ultraviolet Astrophysics Beyonnd the IUE Final Archive, ESA SP-413, 105, 1998A&A...335.1124A), who tested their semi-classical computations of the H-H and H-H⁺ quasi-molecular absorptions on this star. The analysis of the IUE high-resolution spectrum has shown that lines of H₂ are a very important source of line opacity for λ Boo shortward 1600Å. When both atomic and molecular lines are considered, the slope of the observed energy distribution is well reproduced in the whole region 1300-3000Å by the new-ODF model, but the H-H quasi-molecular absorption at 1600Å is computed about 10% too strong. The fit of the low-resolution IUE image SWP17872 to a small grid of new-ODF models gives parameters T_eff=8650K, logg=4.0, while the fit of the high-resolution image SWP 42081, rebinned at the low resolution wavelength step size, gives parameters T_eff=8500K, logg=4.0. These last parameters are in close agreement with T_eff=8550K, logg=4.1 obtained by fitting the visible energy distribution. The different IUE images are discussed.