Fluorine abundances and the puzzle of globular cluster chemical history
  The abundance of fluorine in a few Galactic globular clusters is known to
strongly vary from star-to-star. These unexpected chemical properties are an
additional confirmation of the chemical inhomogeneities already found in
several GC, and probably caused by the first generations of stars formed in
these systems. The aim of this article is to complement our understanding of
the F-behaviour in GC stars and to look for new constraints on the formation
histories of their multiple stellar populations. We have collected near-IR
spectra of 15 RGB stars belonging to GC spanning a wide range of metallicity:
47 Tuc, M4, NGC6397 and M30. F, Na and Fe abundances have been estimated by
spectral synthesis. No anticorrelation between F and Na abundances are found
for the most metal-rich cluster of the sample (47 Tuc). In this GC, RGB stars
indeed exhibit rather small differences in [F/Fe] unlike the larger ones found
for the [Na/Fe] ratios. This reveals a rather inhomogeneous stellar system and
a complex chemical evolution history for 47 Tuc . In M4, one star of our study
confirms the previous Na-F distribution reported by another group in 2005. For
the two very metal-poor GC (NGC6397 and M30), only upper limits of F abundances
have been derived. We show that F abundances could be estimated in such
metal-poor GC with current telescopes and spectrographs only if unexpected
F-rich giants are found and/or exceptional observational conditions are met.
The distribution of the F and Na abundances in GC reveal that their RGB members
seem to belong to two well-separated regions. All the RGB stars analysed so far
in the different GC are indeed found to be either F-rich Na-poor or F-poor
Na-rich. Such well-separated bimodal regimes are consistent with the separate
formation episodes suspected in most galactic GC.
