Tests of model predictions for the response of stellar spectra and
  absorption line indices to element abundance variations
  To analyse stellar populations in galaxies a widely used method is to apply
theoretically derived responses of stellar spectra and line indices to element
abundance variations, hereafter referred to as response functions. These are
applied in a differential way, to base models, in order to generate spectra or
indices with different abundance patterns. In this paper sets of such response
functions for three different stellar evolutionary stages are tested with new
empirical [Mg/Fe] abundance data for the MILES stellar spectral library. Recent
theoretical models and observations are used to investigate the effects of
[Fe/H], [Mg/H] and overall [Z/H] on spectra, via ratios of spectra for similar
stars. Global effects of changes in abundance patterns are investigated
empirically through direct comparisons of similar stars from the MILES library,
highlighting the impact of abundance effects in the blue part of the spectrum,
particularly for lower temperature stars. It is found that the relative
behaviour of iron sensitive line indices are generally well predicted by
response functions, whereas Balmer line indices are not. Other indices tend to
show large scatter about the predicted mean relations. Implications for element
abundance and age studies in stellar populations are discussed and ways forward
are suggested to improve the match with behaviours of spectra and line strength
indices observed in real stars.
