Research Note

Revisited fluorine abundances in the globular cluster M 22

Laboratoire Lagrange (UMR7293), 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: 7 July 2013
Accepted: 4 November 2013

Abstract

Aims. Fluorine is a fairly good tracer of formation histories of multiple stellar populations in globular clusters as already revealed by several studies. Large variations in fluorine abundance in red giant stars of the globular cluster M 22 have been recently reported by two different groups. Futhermore, one of these studies claims that the abundance of fluorine is anti-correlated with sodium abundances in this cluster, leading to strong conclusions on the chemical history of M 22. To validate this important finding, we re-examine the F abundance determinations of some of the previously studied stars.

Methods. We have reanalysed some high-resolution VLT/CRIRES spectra of RGB stars found in M 22 in order to re-estimate their fluorine abundance from the spectral synthesis of the HF line at 2.336 μm.

Results. Unlike what has been previously estimated, we show that only upper limits or doubtful fluorine abundances with large uncertainties in M 22 RGB stars can be derived. This is caused by an incorrect identification of continuum fluctuations as the HF signature combined with a wrong correction of the stellar radial velocity. Such continuum fluctuations could be the consequences of telluric residuals that are still present in the analysed spectra. Therefore, no definitive conclusions on the chemical pollution caused by the M 22 first stellar generation can presently be drawn from the fluorine content of this cluster.