Integrated spectral properties of blue concentrated star clusters of the Large Magellanic Cloud

Extract

Integrated spectra of 17 blue Large Magellanic Cloud (LMC) clusters were obtained in the (3600-6800 Å) range using the CASLEO (Argentina) 2.15 m telescope. The typical resolution and dispersion were 12 Å and 3.5 Å/pixel, respectively. Cluster ages were derived by means of two methods: the template matching, in which the observed spectra are compared and matched to template spectra with well-known determined properties, and the equivalent width (EW) method, in which diagnostic diagrams involving the sum of EWs of selected spectral lines were employed together with their calibrations with age and metallicity given by Santos & Piatti (2004), hereafter SP. The spectra were normalized to F_λ = 1 at ∼ 5870 Å. The EWs of H Balmer, KCaII, G band and MgI were measured within the spectral windows defined by Bica & Alloin (1986). We then obtained the sum of EWs of the 3 metallic lines (S_m) and of the 3 Balmer lines Hβ, Hγ and Hδ (S_h). As a first approach to get cluster ages, we used the diagnostic diagrams defined by SP. The clusters were then age-ranked according to the SP's calibrations. We used S_m to get a first age estimate using: log t(Gyr) = a₀ + a₁S_m + a₂S_m², where a₀ = −2.18 ± 0.38, a₁ = 0.188 ± 0.080 and a₂ = −0.0030 ± 0.0032. We then used S_h to get a second age estimate guided by the previous S_m estimate, since from S_h two solutions are possible: log t(Gyr) = { − b ± [b² − 4a(c − S_h)]^1/2}/2a, where a = −6.35 ± 0.18, b = −8.56 ± 0.35 and c = 23.32 ± 0.20. The average of these two estimates is listed in column 7 of Table 1.