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