Far-ultraviolet spectra of B stars near the ecliptic

Spectra of B stars in the wavelength range of 911-1100 Angstrom have been o btained with the Espectrografo Ultravioleta de Radiacion Difusa (EURD) spec trograph on board the Spanish satellite MINISAT-01 with similar to5 Angstro m spectral resolution. International Ultraviolet Explorer (IUE) spectra of the same stars have been used to normalize Kurucz models to the distance, r eddening, and spectral type of the corresponding star. The comparison of ei ght main-sequence stars studied in detail (alpha Vir, lambda Tau, lambda Ta u, tau Tau, alpha Leo, zeta Lib, theta Oph, and sigma Sgr) shows agreement with Kurucz models, but observed fluxes are 10%-40% higher than the models in most cases. The difference in flux between observations and models is hi gher in the wavelength range between Ly alpha and Ly beta. We suggest that Kurucz models underestimate the far-ultraviolet (FUV) flux of main-sequence B stars between these two Lyman lines. Computation of flux distributions o f line-blanketed model atmospheres including non-LTE effects suggests that this flux underestimate could be due to departures from LTE, although other causes cannot be ruled out. We found that the common assumption of solar m etallicity for young disk stars should be made with care, since small devia tions can have a significant impact on FUV model fluxes. Two peculiar stars (rho Leo and epsilon Aqr) and two emission-line stars (epsilon Cap and pi Aqr) were also studied. Of these, only a Aqr has a flux in agreement with t he models. The rest have strong variability in the IUE range and/or uncerta in reddening, which makes the comparison with models difficult.