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

TL;DR

This paper introduces a new empirical stellar library and a novel CO index at 2.3 μm, improving the modeling of stellar populations by providing more accurate spectral feature measurements and fitting functions.

Contribution

It presents a new stellar library with extensive parameter coverage and defines a new CO index, D_CO, optimized for stellar population analysis, along with empirical fitting functions.

Findings

New stellar library with broad parameter coverage

Defined a new CO index D_CO for better population studies

Provided empirical fitting functions with metallicity dependence

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$_{\rm CO}$, 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.