Fluorine Abundances in the Large Magellanic Cloud and Centauri: Evidence for Neutrino Nucleosynthesis?

Abstract

The behavior of fluorine with metallicity has not yet been probed in any stellar population. In this work, we present the first fluorine abundances measured outside of the Milky Way from a sample of red giants in the Large Magellanic Cloud (LMC), as well the Galactic globular cluster ω Centauri. The fluorine abundances are derived from vibration-rotation transitions of HF using infrared spectra obtained with the Phoenix spectrograph on the Gemini South 8.1 m telescope. It is found that the abundance ratio of F/O declines as the oxygen abundance decreases. The values of F/O are especially low in the two ω Cen giants; this very low value of F/O probably indicates that 19F synthesis in asymptotic giant branch (AGB) stars is not the dominant source of fluorine in stellar populations. The observed decline in F/O with lower O abundances is in qualitative agreement with what is expected if 19F is produced via H- and He-burning sequences in very massive stars, with this fluorine then ejected in high mass-loss rate Wolf-Rayet winds. A quantitative comparison of observations with this process awaits results from more detailed chemical evolution models incorporating the yields from Wolf-Rayet winds. Perhaps of more significance is the quantitative agreement between the Galactic and LMC results with predictions from models in which 19F is produced from neutrino nucleosynthesis during core collapse in supernovae of Type II. The very low values of F/O in ω Cen are also in agreement with neutrino nucleosynthesis models if the peculiar star formation history of ω Cen, with two to four distinct episodes of star formation, is considered.