Modelling chemical abundance anticorrelations on globular cluster spectra

TL;DR

This study models how the second generation of stars with specific abundance patterns in globular clusters influences integrated spectra, affecting age and metallicity indicators and potentially biasing age estimates.

Contribution

It introduces stellar population synthesis models that incorporate second-generation star effects, revealing their impact on spectral indices and age determinations.

Findings

Second generation stars have minimal impact on age-sensitive indices.

Enhanced helium affects Balmer lines, biasing age estimates.

Spectral indices are largely insensitive to second-generation populations.

Abstract

It is widely accepted that individual Galactic globular clusters harbor two coeval generations of stars, the first one born with the `standard' alpha-enhanced metal mixture observed in field Halo objects, the second one characterized by an anticorrelated CN-ONa abundance pattern overimposed on the first generation, alpha-enhanced metal mixture. We have investigated with appropriate stellar population synthesis models how this second generation of stars affects the integrated spectrum of a typical metal rich Galactic globular cluster, like 47Tuc. Our main conclusions are: 1) the age-sensitive Balmer line, Fe line and the [MgFe] indices widely used to determine age, Fe and total metallicity of extragalactic systems are largely insensitive to the second generation population; 2) enhanced He in second generation stars affects the Balmer line indices of the integrated spectra, through the change of the turn off temperature and the horizontal branch morphology of the underlying isochrones, which translate into a bias towards slightly younger ages.