The Impact of Systematic Uncertainties in Stellar Parameters on Integrated Spectra of Stellar Populations

In this paper, we investigate a hitherto unexplored source of potentially significant error in stellar population synthesis (SPS) models, caused by systematic uncertainties associated with the three fundamental stellar atmospheric parameters: effective temperature T eff , surface gravity g , and iron abundance [Fe/H]. All SPS models rely on calibrations of T eff , log g , and [Fe/H] scales, which are implicit in stellar models, isochrones, and synthetic spectra, and are explicitly adopted for empirical spectral libraries. We assess the effect of a mismatch in scales between isochrones and spectral libraries (the two key components of SPS models) and quantify the effects on 23 commonly used diagnostic line indices. We find that typical systematic offsets of 100 K in T eff , 0.15 dex in [Fe/H] and/or 0.25 dex in log g significantly alter inferred absolute ages of simple stellar populations (SSPs) and that in some circumstances, relative ages also change. Offsets in T eff , log g , and [Fe/H] scales for a scaled-solar SSP produce deviations from the model which can mimic the effects of altering abundance ratios to non-scaled-solar chemical compositions, and could also be spuriously interpreted as evidence for a more complex population, especially when multiple-index or full spectral energy distribution fitting methods are used. We stress that the behavior we find can potentially affect any SPS models, whether using full integrated spectra or fitting functions to determine line strengths. We present measured offsets in 23 diagnostic line indices and urge caution in the overinterpretation of line-index data for stellar populations.