IR spectra of young Magellanic Cloud clusters and starburst galaxies: Constraints on the temperature of red supergiants and new estimates of metallicity in young stellar populations

Infrared spectra of young stellar clusters in the Magellanic Clouds are used to derive information on the red supergiants dominating their 1.6 mu m emission, and to obtain a new and independent estimate of their metallicities. The most striking result is that red supergiants with low metallicity appear to be much cooler than predicted by evolutionary models, and this most probably reflects uncertainties in the calibration of the mixing-length parameter in the outermost layers of the stellar envelopes. The metallicity [Fe/H] can be estimated from the W-lambda (1.62) index which is here calibrated using synthetic stellar spectra, and the new scale is also applied to eight starburst galaxies. The resulting values of [Fe/H] range between -1.3 for the SMC cluster NGC330 (in excellent agreement with previous estimates) to -0.2 for the LMC cluster NGC 1994. Starburst galaxies have metallicities ranging between -1.0 (NGC6230) and -0.5 (NGC7552). The spectra are also used to estimate the Carbon depletion which in MC clusters is found compatible with a 'standard' value of [C/Fe]similar or equal to-0.3. Interestingly, our spec tra show possible evidence of significant variations of Carbon depletion in some starburst galaxies. Finally, the Silicon relative abundance is estimated from the W-lambda(1.59) index. In MC clusters we find [Si/Fe]approximate to+0.5, i.e. values similar to those of old clusters in our galaxy and compatible with primordial Si-enhancement by type II supernovae.