1994A&A...284..749C

We have improved our evolutionary population synthesis models by including the latest set of stellar evolutionary tracks available, computed for five values of metal abundance in the range Z = 2Z_☉ to Z = Z_☉/20. We investigate here the effect of metallicity on the formation and evolution of massive stars, and its influence on a number of observable parameters. The population of Wolf-Rayet and red supergiant stars are the most affected by the metal abundance of the initial gas. WR stars can be dominant in the cluster at high metallicities, but are very scarce in underabundant scenarios, where mainly WN-type stars and almost no WC ones are formed. The population of red supergiants follows the same trend, decreasing their relative fraction an order of magnitude in underabundant regions. As a consequence, the V-K values predicted at low metallicities are significantly bluer than for near-solar metallicity clusters. The slower evolution of massive stars at low metallicities originates a trend towards hotter clusters and larger Hβ equivalent widths the lower the abundance. We have finally calibrated the intensity of the oxygen emission lines as a function of effective temperature, metallicity and number of ionizing stars. We have estimated in this way the effective temperature of a sample of emission lines galaxies, finding a trend towards hotter clusters the lower the metallicity, in accord with the model predictions.