A new stellar library in the region of the CO index at 2.3 $\mu$m. New index definition and empirical fitting functions

Abstract

Context: The analysis of unresolved stellar populations demands evolutionary synthesis models with realistic physical ingredients and extended wavelength coverage. Aims: To obtain a quantitative description of the first CO bandhead at 2.3 $\mu$m, to allow stellar population models to provide improved predictions in this wavelength range. Methods: We have observed a new stellar library with a better coverage of the stellar atmospheric parameter space than preceding works. We have performed a detailed analysis of the robustness of previous CO index definitions with spectral resolution, wavelength calibration, signal-to-noise ratio, and flux calibration. Results: We define a new line-strength index for the first CO bandhead at 2.3 $\mu$m, D${}_{\mathrm{CO}}$, better suited for stellar population studies than previous index definitions. We compute empirical fitting functions for the CO feature as a function of the stellar parameters (T${}_{\mathrm{eff}}$, $\log g$ and [Fe/H]), showing a detailed quantitative metallicity dependence.

26 pages, 15 figures, accepted for publication in A&A. Typos in Eq. 9, 11 and 12 fixed

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