Fluorine Abundances of Galactic Low-Metallicity Giants

Abstract

With abundances and 2σ upper limits of fluorine (F) in seven metal-poor field giants, nucleosynthesis of stellar F at low metallicity is discussed. The measurements are derived from the HF(1-0) R9 line at 23358 Å using nearinfrared K-band high-resolution spectra obtained with CRIRES at the Very Large Telescope. The sample reaches lower metallicities than previous studies on F of field giants, ranging from [Fe/H] = −1.56 down to −2.13. Effects of three-dimensional model atmospheres on the derived F and O abundances are quantitatively estimated and shown to be insignificant for the program stars. The observed F yield in the form of [F/O] is compared with two sets of Galactic chemical evolution models, which quantitatively demonstrate the contribution of Type II supernova (SN II) ν-process and asymptotic giant branch/Wolf-Rayet stars. It is found that at this low-metallicity region, models cannot well predict the observed distribution of [F/O], while the observations are better fit by models considering an SN II ν-process with a neutrino energy of E ν = 3 × 10 53 erg. Our sample contains HD 110281, a retrograde orbiting low-α halo star, showing a similar F evolution as globular clusters. This supports the theory that such halo stars are possibly accreted from dwarf galaxy progenitors of globular clusters in the halo.

Chemical evolution of fluorine in the bulge

“…6 we present for the first time the combination of the bulge model as described in Kobayashi et al (2011b) with the ν-process as modeled in Kobayashi et al (2011a). If we ignore for the moment the light-green values for the re-analyzed Cunha et al (2008) stars, we find, just like Li et al (2013) did for the solar neighborhood, that the fluorineoxygen abundance trend in the bulge is best described with the model that includes AGB stars and E ν = 3 × 10 53 erg, but that the models do not to reproduce the trend of the lower-metallicity stars: there seems to be a steeper slope in the observed data than in the models, see left panel of Fig. 6.…”

“…Within the Gaia-ESO collaboration (Gilmore et al 2012) it has also been developed to handle NLTE for many iron lines Bergemann et al 2012). We have no knowledge of estimated 3D-effects on the fluorine line used in the analysis for our stellar parameters, but Li et al (2013) have calculated 3D-corrections for more metal-poor stars and showed that the corrections are small.…”

“…Kobayashi et al (2011a) showed models that included the ν-process in SNe II, with two different neutrino energies, but only for the solar neighborhood. The two energies chosen were E ν = 3 × 10 53 erg (the energy estimated by Hartmann et al 1991 and best reproducing the values of Li et al 2013 in the solar neighborhood) and E ν = 9 × 10 53 erg (the theoretically highest possible value). In Fig.…”

“…The models agree well with field stars of higher metallicities. At lower metallicities the models cannot reproduce the observations of Li et al (2013), but the model that fits best still includes the ν-process with E ν = 3 × 10 53 erg. The abundance of fluorine in stars is difficult to measure because of a paucity of suitable spectral lines.…”

The Abundance of Fluorine in Normal G and K Stars of the Galactic Thin Disk

“…The star will not be considered further here. Li et al (2013) have considered deviations from radiative equilibrium in the formation of the HF R9 line, specifically due to 3D effects in the stellar atmospheres. The investigations of Li et al apply in the low metallicity regime near [M/H] = -2.0.…”