Ultraviolet spectra for λ Boo (HD 125162) computed with H$_{\mathsf 2}$ opacities and Lyman-α H-H and H-H+ opacities

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 ([CITE]) ODFs is the addition to the line blanketing of the Lyman-α H-H and H-H+ quasi-molecular absorptions near 1600 Åand 1400 Å. 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 $\rm [M/H]=-2.0$ for all the elements, except CNO. For C, N, and O, abundances $\log(N_{\rm elem}/N_{\rm tot}$) 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. ([CITE], [CITE]), 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 1600 Å. When both atomic and molecular lines are considered, the slope of the observed energy distribution is well reproduced in the whole region 1300-3000 Åby the new-ODF model, but the H-H quasi-molecular absorption at 1600 Åis computed about 10% too strong. The fit of the low-resolution IUE image SWP17872 to a small grid of new-ODF models gives parameters $T_{\rm eff}=8650$ K, $\log g=4.0$, while the fit of the high-resolution image SWP42081, rebinned at the low resolution wavelength step size, gives parameters $T_{\rm eff}=8500$ K, $\log g=4.0$. These last parameters are in close agreement with $T_{\rm eff}=8550$ K, $\log g=4.1$ obtained by fitting the visible energy distribution. The different IUE images are discussed.