A different view on light element anti-correlations in globular clusters: Fluorine abundances in NGC 6656 (M22)

Abstract

Observed chemical (anti)correlations in the light elements C, N, O, Na, Mg, and Al among globular cluster (GC) stars are presently recognised as the signature of self-pollution from previous generations of stars. This defines the multiple population scenario (MPS). Since fluorine is involved in the complete CNO cycle, determining its abundance in GCs provides new and complementary clues regarding the nature of these previous generations. In fact, along with the Na-O and Mg-Al anti-correlations, the MPS also predicts an anti-correlation between F and Na and a positive correlation between F and O. Furthermore, theoretical models suggest that low-mass (≲5Mȯ) asymptotic giant branch (AGB) stars are F producers (and are also responsible for C+N+O and s-process element variations). We present our results on near-infrared CRIRES spectroscopic observations of six giant stars in the metal-poor GC NGC 6656 (M22): we aim at inferring the F content and its (possible) variation in this GC, which exhibits a rather peculiar abundance pattern, with variations in both light (C, N, O, Na, Mg, Al) and heavy (Fe-peak and s-process) elements.