Observed chemical ( anti ) correlations in proton-capture elements among globular cluster stars are presently recognized as the signature of self-enrichment from now extinct , previous generations of stars .
This defines the multiple population scenario .
Since fluorine is also affected by proton captures , determining its abundance in globular clusters provides new and complementary clues regarding the nature of these previous generations , and supplies strong observational constraints to the chemical enrichment timescales .
In this paper we present our results on near-infrared CRIRES spectroscopic observations of six cool giant stars in NGC 6656 ( M22 ) : the main objective is to derive the F content and its internal variation in this peculiar cluster , which exhibits significant changes in both light- and heavy- element abundances .
We detected F variations across our sample beyond the measurement uncertainties and found that the F abundances are positively correlated with O and anticorrelated with Na , as expected according to the multiple population framework .
Furthermore , our observations reveal an increase in the F content between the two different sub-groups , s -process rich and s -process poor , hosted within M22 .
The comparison with theoretical models suggests that asymptotic giant stars with masses between 4 and 5M _ { \odot } are responsible for the observed chemical pattern , confirming evidence from previous works : the difference in age between the two sub-components in M22 must be not larger than a few hundreds Myr .