Ultraviolet spectra forλBoo (HD 125162) computed with H$_{\mathsf 2}$ opacities and Lyman-αH-H and H-H+opacities

Abstract

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) ODFs is the addition to the line blanketing of the Lyman- H{H and H{H + quasi-molecular absorptions near 1600 A and 1400 A. New-ODF fluxes are expected to reproduce the ultraviolet observations of 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 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(Nelem=Ntot) equal to 3.85, 3.99, and 3.11, respectively, were adopted. We selected Boo in order to compare results from the new-ODFs with those from Allard et al. (1998a, 1998b), 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 Boo shortward 1600 A. When both atomic and molecular lines are considered, the slope of the observed energy distribution is well reproduced in the whole region 1300{3000 A by the new-ODF model, but the H{H quasi-molecular absorption at 1600 A is computed about 10% too strong. The t of the low-resolution IUE image SWP17872 to a small grid of new-ODF models gives parameters Te = 8650 K, log g =4 :0, while the t of the high-resolution image SWP42081, rebinned at the low resolution wavelength step size, gives parameters Te = 8500 K, log g =4 :0. These last parameters are in close agreement with Te = 8550 K, log g =4 :1 obtained by tting the visible energy distribution. The dierent IUE images are discussed.