NLTE Stellar Population Synthesis of Globular Clusters using Synthetic Integrated Light Spectra I: Constructing the IL Spectra

TL;DR

This paper investigates the effects of NLTE modeling on the synthesis of integrated light spectra of globular clusters, highlighting differences in spectral features and derived ages compared to LTE models, with implications for stellar population analysis.

Contribution

It introduces a detailed comparison of NLTE and LTE models in globular cluster synthesis, emphasizing the importance of NLTE effects and CMD discretization on spectral and age determinations.

Findings

NLTE spectra are more luminous in UV and optical bands.

Discrepancies between NLTE and LTE spectra are mainly due to light metals and molecular bands.

Age differences from LTE and NLTE spectra range from 0.55 to 2.54 Gyr.

Abstract

We present an investigation of the globular cluster population synthesis method of McWilliam & Bernstein (2008), focusing on the impact of NLTE modeling effects and CMD discretization. Johnson-Cousins-Bessel U-B, B-V, V-I, and J-K colors are produced for 96 synthetic integrated light spectra with two different discretization prescriptions and three degrees of NLTE treatment. These color values are used to compare NLTE and LTE derived population ages. Relative contributions of different spectral types to the integrated light spectra for different wavebands are measured. Integrated light NLTE spectra are shown to be more luminous in the UV and optical than LTE spectra, but show stronger absorption features in the IR. The main features showing discrepancies between NLTE and LTE integrated light spectra may be attributed to light metals, primarily Fe I, Ca I, and Ti I, as well as TiO molecular bands. Main Sequence stars are shown to have negligible NLTE effects at IR wavelengths compared to more evolved stars. Photometric color values are shown to vary at the millimagnitude level as a function of CMD discretization. Finer CMD sampling for the upper main sequence and turnoff, base of the red giant branch, and the horizontal branch minimizes this variation. Differences in ages derived from LTE and NLTE IL spectra are found to range from 0.55 to 2.54 Gyr, comparable to the uncertainty in GC ages derived from color indices with observational uncertainties of 0.01 magnitudes, the limiting precision of the Harris catalog (Harris 1996).