Fluorine Abundance Variations in Red Giants of the Globular Cluster M4 and Early-Cluster Chemical Pollution

We present fluorine abundances in seven red giant members of the globular cluster M4 (NGC 6121). These abundances were derived from the HF (1-0) R9 line at 2.3357 μm in high-resolution infrared spectra obtained with the Phoenix spectrograph on the Gemini South telescope. Many abundances in the target stars have been studied previously, so their overall abundance distributions are well mapped. The abundance of fluorine is found to vary by more than a factor of 6, with the 19F variations being correlated with the already established oxygen variations and anticorrelated with the sodium and aluminium variations. In this paper we thus add fluorine to the list of elements known to vary in globular cluster stars, and this provides further evidence that H burning is the root cause of the chemical inhomogeneities. The fact that 19F is found to decrease in the M4 stars, as the signature of H burning appears, indicates that the polluting stars must have masses greater than about 3.5 M☉, as less massive stars than this should produce, not destroy, fluorine.