Near-infrared spectral features in single-aged stellar populations

Synthetic spectra for single-aged stellar populations of metallicities [M/H] = -0.5, 0.0, and +0.5, ages 3-17 Gyr, and initial mass function indices x = 0.1-2.0 were built in the wavelength range lambda lambda 6000-10200. We have employed the grid of synthetic spectra described in Schiavon & Barbuy, computed for the stellar parameters 2500 less than or equal to T-eff less than or equal to 6000 K, -0.5 less than or equal to log g less than or equal to 5.0, [M/H] = -0.5, 0.0, and +0.5, and [alpha/Fe] = 0.0, together with the isochrones by Bertelli et al. and Baraffe et al. The behavior of the features Na I lambda 8190, Ca II lambda 8662, TiO lambda 6600, and FeH lambda 9900 Angstrom in the integrated spectra of single-aged stellar populations was studied in terms of metallicity, initial mass function (IMF), and age variations. The main conclusions are that the Na I doublet is an IMF-sensitive feature, which is, however, sensitive also to metallicity and age, whereas TiO, Ca II, and FeH are very sensitive to metallicity and essentially insensitive to IMF and age.