Synthetic stellar spectra to study multiple populations in globular clusters: an extended grid and the effects on the integrated light

TL;DR

This study provides a comprehensive grid of synthetic stellar spectra to analyze how multiple stellar populations in globular clusters influence their integrated light, revealing detectable spectral variations due to chemical abundance differences.

Contribution

The paper introduces an extended grid of synthetic spectra for different stellar populations in GCs, enabling detailed analysis of their integrated light and spectral index variations.

Findings

Spectroscopic indices are affected by multiple populations, especially for elements like C, N, Ca, Na.

Effects on spectral features vary with metallicity and are detectable despite stochastic fluctuations.

Near-UV and near-IR regions show specific modifications due to multiple populations.

Abstract

Most Galactic Globular Clusters (GCs) harbour multiple populations of stars (MPs), composed of at least two generations: the first characterized by a "standard" $\alpha$-enhanced metal mixture, as observed in field halo stars of the Milky Way, and the second displaying anti-correlated CN--ONa chemical abundance pattern in combination with an enhanced helium fraction. Adequate collections of stellar spectra are needed to characterize the effect of such stellar abundance changes on the integrated light of GCs. We present a grid of synthetic stellar spectra covering the atmospheric parameters relevant to old stellar populations at four subsolar metallicities and two abundance patterns, representative of first- and second-generations of stars in GCs. Integrated spectra of populations were computed using our stellar grid and empirical stellar populations, namely, colour-magnitude diagrams from literature for Galactic GCs. The spectra range from 290 to 1000nm, where we measured the effect on several spectrophotometric indices due to the surface abundance variations attributed to MPs. We find non-negligible effects of the MPs on spectroscopic indices sensitive to C, N, Ca, or Na, and on Balmer indices; we also describe how MPs modify specific regions in the near-UV and near-IR that can be measured with narrow or medium photometric passbands. The effects vary with metallicity. A number of these changes remain detectable even when accounting for the stochastic fluctuations due to the finite nature of the stellar population cluster.