1999ApJ...514...96O

We present evolutionary synthesis models applied to near-infrared spectral features observed in the spectra of young Magellanic Cloud clusters and starburst galaxies. The temporal evolution of the first and second overtones of CO at 2.29 µm (2-0 band head) and 1.62 µm (6-3 band head) and of the U-B, B-V, and J-K colors are investigated. We find that the current evolutionary tracks of massive stars with subsolar chemical composition in the red supergiant phase are not reliable for any synthesis of the temporal evolution of infrared stellar features. The high sensitivity of the selected infrared features to the atmospheric parameters of cool stars allows us to place 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 spectrophotometric models by adjusting the red supergiant parameters so that the properties of the observed templates are reproduced.