B Stars as a Diagnostic of Star Formation at Low and High Redshift

We have extended the evolutionary synthesis models by Leitherer et al.(1999b) by including a new library of B stars generated from the IUEhigh-dispersion spectra archive. We present the library and show how thestellar spectral properties vary according to luminosity classes and spectraltypes. We have generated synthetic UV spectra for prototypical young stellarpopulations varying the IMF and the star formation law. Clear signs of ageeffects are seen in all models. The contribution of B stars in the UV linespectrum is clearly detected, in particular for greater ages when O stars haveevolved. With the addition of the new library we are able to investigate thefraction of stellar and interstellar contributions and the variation in thespectral shapes of intense lines. We have used our models to date the spectrumof the local super star cluster NGC1705-1. Photospheric lines of CIII1247,SiIII1417, and SV1502 were used as diagnostics to date the burst of NGC 1705-1at 10 Myr. We have selected the star-forming galaxy 1512-cB58 as a firstapplication of the new models to high-z galaxies. This galaxy is at z=2.723, itis gravitationally lensed, and its high signal-to-noise Keck spectrum showfeatures typical of local starburst galaxies, such as NGC 1705-1. Models withcontinuous star formation were found to be more adequate for 1512-cB58 sincethere are spectral features typical of a composite stellar population of O andB stars. A model with Z =0.4Z_solar and an IMF with alpha=2.8 reproduces thestellar features of the 1512-cB58 spectrum.