High-resolution Spectral Line Indices Useful for the Analysis of Stellar Populations

The well-known age–metallicity-attenuation degeneracy does not permit unique and good estimates of basic parameters of stars and stellar populations. The effects of dust can be avoided using spectral line indices, but current methods have not been able to break the age–metallicity degeneracy. Here we show that using at least two new spectral line indices defined and measured on high-resolution (R = 6000) spectra of a signal-to-noise ratio (S/N) ≥ 10, one gets unambiguous estimates of the age and metallicity of intermediate to old stellar populations. Spectroscopic data retrieved with new astronomical facilities, e.g., X-shooter, MEGARA, and MOSAIC, can be employed to infer the physical parameters of the emitting source by means of spectral line index and index–index diagram analysis.