PopStar I: Evolutionary synthesis models description
  We present new evolutionary synthesis models for Simple Stellar Populations
for a wide range of ages and metallicities. The models are based on the Padova
isochrones. The core of the spectral library is provided by the medium
resolution Lejeune et al. atmosphere models. These spectra are complemented by
NLTE atmosphere models for hot stars that have an important impact in the
stellar cluster's ionizing spectra: O, B and WR stellar spectra at the early
ages, and spectra of post-AGB stars and planetary nebulae, at intermediate and
old ages. At young ages, our models compare well with other existing models but
we find that, the inclusion of the nebular continuum, not considered in several
other models, reddens significantly the integrated colours of very young
stellar populations. This is consistent with the results of spectral synthesis
codes particularly devised for the study of starburst galaxies. At intermediate
and old ages, the agreement with literature model is good and, in particular,
we reproduce well the observed colours of star clusters in LMC. Given the
ability to produce good integrated spectra from the far-UV to the infrared at
any age, we consider that our models are particularly suited for the study of
high redshift galaxies. These models are available on the web site
{http://www.fractal-es.com/SEDmod.htm} and also through the Virtual Observatory
Tools on the PopStar server.
