Fluorine Abundances in the Galactic Nuclear Star Cluster

Abstract: Abundances of fluorine ($^{19}$F), as well as isotopic ratios of $^{16}$O/$^{17}$O, are derived in a sample of luminous young ($\sim$10$^{7}$--10$^{8}$ yrs) red giants in the Galactic center (with galactocentric distances ranging from 0.6--30 pc), using high-resolution infrared spectra and vibration-rotation lines of H$^{19}$F near $\lambda$2.3$\mu$m. Five of the six red giants are members of the Nuclear star cluster that orbits the central supermassive black hole. Previous investigations of the chemical evolution of $^{19}$F in Galactic thin and thick disk stars have revealed that the nucleosynthetic origins of $^{19}$F may be rather complex, resulting from two, or more, astrophysical sites; fluorine abundances behave as a primary element with respect to Fe abundances for thick disk stars and as a secondary element in thin disk stars. The Galactic center red giants analyzed fall within the thin disk relation of F with Fe, having near-solar, to slightly larger, abundances of Fe ($<$[Fe/H]$>$=+0.08$\pm$0.04), with a slight enhancement of the F/Fe abundance ratio ($<$[F/Fe]$>$=+0.28$\pm$0.17). In terms of their F and Fe abundances, the Galactic center stars follow the thin disk population, which requires an efficient source of $^{19}$F that could be the winds from core-He burning Wolf Rayet stars, or thermally-pulsing AGB stars, or a combination of both. The observed increase of [F/Fe] with increasing [Fe/H] found in thin disk and Galactic center stars is not predicted by any published chemical evolution models that are discussed, thus a quantitative understanding of yields from the various possible sources of $^{19}$F remains unknown.