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CNO and F abundances in the globular cluster M22 (NGC 6656)

Recent studies have confirmed the long standing suspicion that M22 shares a metallicity spread and complex chemical enrichment history similar to that observed in Omega Cen. M22 is among the most massive Galactic globular clusters and its colour-magnitude diagram and chemical abundances reveal the existence of sub-populations. To further constrain the chemical diversity of M22, necessary to interpret its nucleosynthetic history, we seek to measure relative abundance ratios of key elements (carbon, nitrogen, oxygen, and fluorine) best studied, or only available, using high-resolution spectra at infrared wavelengths. High-resolution (R = 50,000) and high S/N infrared spectra were acquired of nine red giant stars with Phoenix at the Gemini-South telescope. Chemical abundances were calculated through a standard 1D local thermodynamic equilibrium analysis using Kurucz model atmospheres. We derive [Fe/H] = -1.87 to -1.44, confirming at infrared wavelengths that M22 does present a [Fe/H] spread. We also find large C and N abundance spreads, which confirm previous results in the literature but based on a smaller sample. Our results show a spread in A(C+N+O) of ~ 0.7 dex. Similar to mono-metallic GCs, M22 presents a strong [Na/Fe]-[O/Fe] anticorrelation as derived from Na and CO lines in the K band. For the first time we recover F abundances in M22 and find that it exhibits a 0.6 dex variation. We find tentative evidence for a flatter A(F)-A(O) relation compared to higher metallicity GCs. Our study confirms and expands upon the chemical diversity seen in this complex stellar system. All elements studied to date show large abundance spreads which require contributions from both massive and low mass stars.