Far-Ultraviolet Spectra of B Stars near the Ecliptic

Spectra of B stars in the wavelength range of 911-1100 Å have been obtained with the Espectrógrafo Ultravioleta de Radiación Difusa (EURD) spectrograph on board the Spanish satellite MINISAT-01 with ~5 Å spectral resolution. International Ultraviolet Explorer (IUE) spectra of the same stars have been used to normalize Kurucz models to the distance, reddening, and spectral type of the corresponding star. The comparison of eight main-sequence stars studied in detail (α Vir, ϵ Tau, λ Tau, τ Tau, α Leo, ζ Lib, θ Oph, and σ 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 higher in the wavelength range between Lyα and Lyβ. 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 of 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 metallicity for young disk stars should be made with care, since small deviations can have a significant impact on FUV model fluxes. Two peculiar stars (ρ Leo and ϵ Aqr) and two emission-line stars (ϵ Cap and π Aqr) were also studied. Of these, only ϵ Aqr has a flux in agreement with the models. The rest have strong variability in the IUE range and/or uncertain reddening, which makes the comparison with models difficult.