  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, logg 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 100K
in T_eff, 0.15 dex in [Fe/H] and/or 0.25 dex in logg significantly alter
inferred absolute ages of simple stellar populations (SSPs) and that in some
circumstances, relative ages also change. Offsets in T_eff, logg 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-SED 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 over-interpretation of line-index data for stellar
populations.
