Integrated light stellar population synthesis of globular clusters using non-local thermodynamic equilibrium modelling

Abstract

We present an investigation of McWilliam & Bernstein's 2008 globular cluster (GC) stellar population synthesis method, focusing on the impact of non-local thermodynamic equilibrium (NLTE) modeling effects. For this purpose, we have generated comprehensive, fully NLTE libraries of individual stellar spectra and GC integrated light (IL) spectra. The stellar library spans large ranges in T_eff, log g, and [M/H], reproduced for 0.5 and 1 M_☉ and two degrees of α-enhancement. The IL library spans 9 to 15 Gyr in age, and -1.790 to -0.253 in [M/H]. The IL spectral library is used to investigate Johnson-Cousins-Bessel UBV IJK IL colours, sensitivity of IL spectral features to cluster age or metallicity, and deriving the ages and metallicities of 11 GCs for which IL spectra are acquired from Colucci, Bernstein & McWilliam (2017). The IL colours confirm previously reported trends of GC reddening with increasing age or metallicity, and demonstrate that NLTE colours are bluer than LTE by up to a few tens of millimagnitudes, as are α-enhanced colours. We find a dependence of a few millimagnitudes on the discretization resolution of population CMDs when using the 25-30 boxes suggested in the literature. This dependence is minimized when the number of boxes increased to 40-50. We find 240 spectral features sensitive to either cluster age or metallicity, of which 209 are newly identified as potential GC diagnostic features. These features represent 19 different species, including Fe I, Fe II, Ca I, and Ti I. Chi-squared minimization is used to determine the best fit to the observed GC IL spectra, deriving ages for six of the 11 clusters, and metallicities for all of them. The uncertainties of both the ages and metallicities are reduced by a factor of two to three times when fit with NLTE IL spectra when compared with those from fitting LTE spectra.