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 .
This work also provides a practical method for constraining horizontal branch morphology from high signal to noise integrated light spectroscopy in the optical .
This will allow post-AGB evolution studies in a range of environments and at distances where resolving stellar populations is impossible with current and planned telescopes .