Department of Astronomy & Physics

PhD Thesis Defense: Integrated Light Stellar Population Synthesis of Globular Clusters Using Non-Local Thermodynamic Equilibrium Modelling

Speaker: Mitchell Young
Time: October 18, 2018 - 1:00 AM
Location: Loyola 188


We present an investigation of the globular cluster (GC) stellar population syn-thesis method of McWilliam & Bernstein (2008), focusing on the impact of stellar atmospheric non-local thermodynamic equilibrium (NLTE) modeling effects.  For this purpose, we have generated comprehensive, fully NLTE libraries of both individual stellar spectra and continua and GC integrated light (IL) spectra and continua.  The stellar library spans large ranges in Teff, log g, and [M=H], and is reproduced for 0.5 and 1 M_Sun and two degrees of alpha-enhancement, covering the parameter space of GC population members.  The IL library spans 9 to 15 Gyr in age, and -1.790 to -0.253 in [M=H], covering the full range of Galactic GC ages, and the majority of the Galactic GC metallicity distribution.  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 for deriving the ages and metallicities of 10 Galactic and one extragalactic GC for which IL spectra are acquired from Colucci, Bernstein & McWilliam (2017).  The IL colours cofirm previously reported trends of GC reddening with increasing age or metallicity, and demonstrate that NLTE colours to be bluer than LTE by up to a few tens of millimagnitudes, as does increased alpha-enhancement. We find a dependence of a few millimagnitudes on the discretization resolution of the 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 found 240 spectral features sensitive to either cluster age or metallicity, of which 209 are newly identified as GC diagnostic features.   We 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.


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