Integrated K-band spectra of old and intermediate-age globular clusters in the Large Magellanic Cloud

Current stellar population models have arguably the largest uncertainties in the near-IR wavelength range, partly due to a lack of large and well calibrated empirical spectral libraries. In this paper we present a project whose aim it is to provide the first library of luminosity weighted integrated near-IR spectra of globular clusters to be used to test the current stellar population models and serve as calibrators for future ones. Our pilot study presents spatially integrated K-band spectra of three old (>= 10 Gyr) and metal poor ([Fe/H] similar to -1.4), and three intermediate age (1-2 Gyr) and more metal rich ([Fe/H] similar to -0.4) globular clusters in the LMC. We measured the line strengths of the Na I, CaI and (CO)-C-12(2-0) absorption features. The Na I index decreases with increasing age and decreasing metallicity of the clusters. The D-CO index, used to measure the (CO)-C-12(2-0) line strength, is significantly reduced by the presence of carbon-rich TP-AGB stars in the globular clusters with age similar to 1 Gyr. This is in contradiction to the predictions of the stellar population models of Maraston (2005, MNRAS, 362, 799). We find that this disagreement is due to the different CO absorption strength of carbon-rich Milky Way TP-AGB stars used in the models and the LMC carbon stars in our sample. For globular clusters with age >= 2 Gyr we find D-CO index measurements consistent with the model predictions.