Fake star formation bursts: blue horizontal branch stars masquerade as
  young stars in optical integrated light spectroscopy
  Model color magnitude diagrams of low-metallicity globular clusters usually
show a deficit of hot evolved stars with respect to observations. We
investigate quantitatively the impact of such modelling inaccuracies on the
significance of star formation history reconstructions obtained from optical
integrated spectra. To do so, we analyse the sample of spectra of galactic
globular clusters of Schiavon et al. with STECKMAP (Ocvirk et al.) and the
stellar population models Vazdekis et al. and Bruzual & Charlot, and focus on
the reconstructed stellar age distributions. Firstly, we show that
background/foreground contamination correlates with E(B-V), which allows us to
define a clean subsample of uncontaminated GCs, on the basis of a E(B-V)
filtering.
  We then identify a "confusion zone" where fake young bursts of star formation
pop up in the star formation history although the observed population is
genuinely old. These artifacts appear for 70-100% of cases depending on the
population model used, and contribute up to 12% of the light in the optical.
Their correlation with the horizontal branch ratio indicates that the confusion
is driven by HB morphology: red horizontal branch clusters are well fitted by
old stellar population models while those with a blue HB require an additional
hot component. The confusion zone extends over [Fe/H]=[-2,-1.2], although we
lack the data to probe extreme high and low metallicity regimes. As a
consequence, any young starburst superimposed on an old stellar population in
this metallicity range could be regarded as a modeling artifact, if it weighs
less than 12% of the optical light, and if no emission lines typical of an HII
region are present.
