Evolutionary synthesis modeling of red supergiant features in the near-infrared

We present evolutionary synthesis models applied to near-infrared spectral features observed in the spectra of young Magellanic Cloud clusters and sta rburst galaxies. The temporal evolution of the first and second overtones o f CO at 2.29 mu m (2-0 band head) and 1.62 mu m (6-3 band head) and of the U-B, B-V, and J-K colors are investigated. We find that the current evoluti onary tracks of massive stars with subsolar chemical composition in the red supergiant phase are not reliable for any synthesis of the temporal evolut ion of infrared stellar features. The high sensitivity of the selected infr ared features to the atmospheric parameters of cool stars allows us to plac e constraints on the temperature and the fraction of time spent in the red part of the Hertzsprung-Russell diagram by massive stars during their core helium burning phase. We derive a set of empirically calibrated spectrophot ometric models by adjusting the red supergiant parameters so that the prope rties of the observed templates are reproduced.