Fluorine variations in the globular cluster NGC 6656 (M22): implications
  for internal enrichment timescales
  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 5 Msun 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.
