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Recent studies have confirmed the long standing suspicion that M22 shares ametallicity spread and complex chemical enrichment history similar to thatobserved in Omega Cen. M22 is among the most massive Galactic globular clustersand its colour-magnitude diagram and chemical abundances reveal the existenceof sub-populations. To further constrain the chemical diversity of M22,necessary to interpret its nucleosynthetic history, we seek to measure relativeabundance ratios of key elements (carbon, nitrogen, oxygen, and fluorine) beststudied, or only available, using high-resolution spectra at infraredwavelengths. High-resolution (R = 50,000) and high S/N infrared spectra wereacquired of nine red giant stars with Phoenix at the Gemini-South telescope.Chemical abundances were calculated through a standard 1D local thermodynamicequilibrium analysis using Kurucz model atmospheres. We derive [Fe/H] = -1.87to -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 previousresults in the literature but based on a smaller sample. Our results show aspread in A(C+N+O) of ~ 0.7 dex. Similar to mono-metallic GCs, M22 presents astrong [Na/Fe]-[O/Fe] anticorrelation as derived from Na and CO lines in the Kband. For the first time we recover F abundances in M22 and find that itexhibits a 0.6 dex variation. We find tentative evidence for a flatterA(F)-A(O) relation compared to higher metallicity GCs. Our study confirms andexpands upon the chemical diversity seen in this complex stellar system. Allelements studied to date show large abundance spreads which requirecontributions from both massive and low mass stars.

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