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 , which 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 the future ones .
Our pilot study presents spatially integrated K -band spectra of three old ( \geq 10 Gyr ) and metal poor ( [ Fe/H ] \sim –1.4 ) , and three intermediate age ( 1 – 2 Gyr ) and more metal rich ( [ Fe/H ] \sim –0.4 ) globular clusters in the LMC .
We measured the line strengths of the Na I , Ca I and ^ { 12 } CO ( 2–0 ) absorption features .
The Na I index decreases with the increasing age and decreasing metallicity of the clusters .
The D _ { CO } index , used to measure the ^ { 12 } CO ( 2–0 ) line strength , is significantly reduced by the presence of carbon-rich TP-AGB stars in the globular clusters with age \sim 1 Gyr .
This is in contradiction with the predictions of the stellar population models of Maraston ( 2005 ) .
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 \geq 2 Gyr we find D _ { CO } index measurements consistent with the model predictions .