Fluorine abundances and the puzzle of globular cluster chemical history^⋆

Laboratoire Lagrange (UMR 7293), Université de Nice Sophia Antipolis, CNRS, Observatoire de la Côte d’Azur, BP 4229, 06304 Nice Cedex 4, France
e-mail: laverny@oca.eu

Received: 18 March 2013
Accepted: 29 May 2013

Abstract

Aims. A few studies have already shown that the abundance of fluorine in a few galactic globular clusters is strongly varying from star-to-star. These unexpected chemical properties are an additional confirmation of the chemical inhomogeneities already found in several globular clusters, 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 fluorine behaviour in globular cluster stars and to look for new constraints on the formation histories of their multiple stellar populations.

Methods. We have collected near-IR spectra with VLT/CRIRES of 15 red giant branch stars belonging to four different globular clusters spanning a wide range of metallicity: 47 Tuc, M 4, NGC 6397 and M 30. We have estimated the fluorine abundance of these targets from the spectral synthesis of one HF line at 2.336 μm. Sodium and iron abundances have also been determined from two individual lines of FeI and NaI found in the same spectra.

Results. No anticorrelation between fluorine and sodium abundances are found for the most metal-rich cluster of the sample (47 Tuc). In this cluster, 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 M 4, one star of our study confirms the previous Na-F distribution reported by another group in 2005. For the two very metal-poor globular clusters (NGC 6397 and M 30), only upper limits of fluorine abundances have been derived. We show that F-abundances could be estimated (but with high uncertainty) in such metal-poor globular clusters with current telescopes and spectrographs only if unexpected F-rich giants are found and/or exceptional observational conditions are met.

Conclusions. The distribution of the fluorine and sodium abundances in globular clusters reveal that their RGB members seem to belong to two well-separated regions. All the RGB stars analysed so far in the different globular clusters 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 globular clusters.