A new stellar library in the region of the CO index at 2.3 $\mu$m. New   index definition and empirical fitting functions
  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$_{\rm CO}$, 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$_{\rm eff}$, $\log g$ and [Fe/H]), showing a detailed quantitative metallicity dependence. 
E. M\'armol-Queralt\'o, N. Cardiel, A. J. Cenarro, A. Vazdekis, J.   Gorgas, S. Pedraz, R.F. Peletier, P. S\'anchez-Bl\'azquez