Chemical abundance anticorrelations in globular cluster stars: The effect on cluster integrated spectra

TL;DR

This study uses stellar population models to analyze how second-generation stars with abundance anticorrelations in globular clusters influence integrated spectra, especially affecting certain spectral indices.

Contribution

It demonstrates that only specific spectral indices are significantly affected by abundance anticorrelations, while key age and metallicity indicators remain largely unaffected.

Findings

Ca4227, G4300, CN1, CN2, NaD indices are affected

Age and metallicity indicators are insensitive to second-generation stars

Enhanced helium impacts Balmer line indices through stellar evolution effects

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 47\,Tuc, focusing our analysis on the widely used Lick-type indices. We find that the only indices appreciably affected by the abundance anticorrelations are Ca4227, G4300, ${\rm CN_1}$, ${\rm CN_2}$ and NaD. 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. Enhanced He in second generation stars affects also the Balmer line indices of the integrated spectra, through the change of the turn off temperature and -- in the assumption that the mass loss history of both stellar generations is the same -- the horizontal branch morphology of the underlying isochrones.