Spectral evolution of star clusters in the Large Magellanic Cloud : I. blue concentrated clusters in the range 40-300 Myr

Abstract

Aims. Integrated spectroscopy of a sample of 17 blue concentrated Large Magellanic Cloud (LMC) clusters is presented and its spectral evolution studied. The spectra span the range ≈(3600−6800) Å with a resolution of ≈14Å FWHM, being used to determine cluster ages and, in connection with their spatial distribution, to explore the LMC structure and cluster formation history. Methods. Cluster reddening values were estimated by interpolation, using the available extinction maps.We used two methods to derive cluster ages: (i) template matching, in which line strengths and continuum distribution of the cluster spectra were compared and matched to those of template clusters with known astrophysical properties, and (ii) equivalent width (EW) method, in which new age/metallicity calibrations were used together with diagnostic diagrams involving the sum of EWs of selected spectral lines (KCa II, G band (CH), MgI, Hδ, Hγ and Hβ). Results. The derived cluster ages range from 40Myr (NGC2130 and SL 237) to 300 Myr (NGC1932 and SL 709), a good agreement between the results of the two methods being obtained. Combining the present sample with additional ones indicates that cluster deprojected distances from the LMC center are related to age in the sense that inner clusters tend to be younger. Conclusions. Spectral libraries of star clusters are useful datasets for spectral classifications and extraction of parameter information for target star clusters and galaxies. The present cluster sample complements previous ones, in an effort to gather a spectral library with several clusters per age bin.