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.
