B stars as a diagnostic of star formation at low and high redshift

We have extended the evolutionary synthesis models by Leitherer et al. by including a new library of B stars generated from the IUE high-dispersion spectra archive. We present the library and show how the stellar spectral properties vary according to luminosity classes and spectral types. We have generated synthetic UV spectra for prototypical young stellar populations varying the IMF and the star formation law. Clear signs of age effects are seen in all models. The contribution of B stars in the UV line spectrum is clearly detected, in particular for greater ages when O stars have evolved. With the addition of the new library we are able to investigate the fraction of stellar and interstellar contributions and the variation in the spectral shapes of intense lines. We have used our models to date the spectrum of the local super-star cluster NGC 1705-1. Photospheric lines of C III;lambda 1247, Si III lambda 1417, and S v lambda 1502 were used as diagnostics to date the burst of NGC 1705-1 at 10 Myr. Interstellar lines are clearly seen in the NGC 1705-1 spectrum. Broadening and blueshifts of several resonance lines are stronger in the galaxy spectrum than in our models and are confirmed to be intrinsic of the galaxy. Si II pi 1261 and Al II pi 1671 were found to be pure interstellar lines with an average blueshift of 78 km s(-1) owing to a directed outflow of the interstellar medium. We have selected the star-forming galaxy 1512-cB58 as a first application of the new models to high-z galaxies. This galaxy is at z = 2.723, it is gravitationally lensed, and its high signal-to-noise ratio Keck spectrum shows features typical of local starburst galaxies, such as NGC 1705-1. Models with continuous star formation were found to be more adequate for 1512-cB58 since there are spectral features typical of a composite stellar population of O and B stars. A model with Z = 0.4 Z. and an IMF with alpha = 2.8 reproduces the stellar features of the 1512-cB58 spectrum.