Blue magellanic clusters : near-infrared spectral evolution

Abstract

We present new integrated spectra in the range 5600-10000 Å for 28 LMC and 3 SMC young star clusters. We measure the equivalent widths (W) of prominent features and the continuum distribution. The analysis, supplemented by 8 additional LMC clusters from our previous studies, indicates that the red supergiant phase is indeed very time-peaked, occuring from 7 to 12 Myr. In addition to the previous case of NGC 2004, we find that NGC 1805, NGC 1994, NGC 2002, NGC 2098 and NGC 2100 as well as NGC 2011 to a lesser extent, are undergoing this phase. The red supergiant phase is clearly denoted by strong TiO bands and Ca II triplet as well as a flat continuum or in extreme cases a continuum with positive slope for λ> 6000 Å. In the SMC clusters, the molecular bands are weak, owing to metal deficiency, but we can show nevertheless that NGC 299 clearly belongs to the red supergiant phase as demonstrated by its spectral slope and enhanced Ca II. We also find new evidence of another red phase at t ≈ 100 Myr, possibly produced by AGB stars, in which the clusters NGC 2031 and NGC 2134 exhibit strong molecular bands like NGC 1866, a previously known case. Clusters between 10 and 50 Myr have strong Hα emission arising from Be stars, a phenomenon which occurs also in Galactic open clusters. From the LMC data set, it has been possible to identify 8 stages in the cluster spectral evolution from 5 to 500 Myr. We derive the corresponding average spectra which shall be useful for population synthesis of galaxies in which recent bursts of star formation have occurred.