UM ESTUDO SOBRE A ORIGEM DO FLÚOR NA VIA LÁCTEA E NA GRANDE NUVEM DE MAGALHÃES

Abstract

The astrophysical origin of fluorine is not yet well known, but it is suggested that this element might be produced in four sites: stars on the asymptotic giant branch, via neutron and proton captures during He-burning thermal pulses; spallation of a proton of the 20Ne element after being excited by a neutrino, this process occurs in the Type II supernova phase; in core He-burning inWolf-Rayet stars experiencing high rates of mass loss; and mass loss from low-metallicity rapidly-rotating massive stars. In this work we present results for fluorine abundances, as well as carbon and sodium, in red giant samples from the globular clusters M4 and ω Cen, fluorine and iron abundances in a sample of Galactic disc red giants, and fluorine abundances in Large Magellanic Cloud red giants. The chemical abundances were calculated from spectrum synthesis in Local Thermodynamic Equilibrium using the Turbospectrum code, which is compatible with the MARCS spherical atmospheric models adopted in this work. The high-resolution stellar spectra analyzed here were obtained in the infrared region of the K band with the spectrographs Phoenix on the 8.1 meter Gemini South telescope, IGRINS on the 2.7- meter Harlan J. Smith telescope at the McDonald Observatory, CRIRES on the 8.2-meter Very Large Telescope of ESO, the FTS on the 4.0-meter telescope at the Kitt Peak National Observatory, Phoenix on the 2.1-meter telescope, also at the Kitt Peak National Observatory, and iSHELL on the 3.0-meter Infrared Telescope Facility on Maunakea. Additionally, we analyzed high-resolution optical spectra obtained with the SES spectrograph on the 2.1-meter Struve Telescope at the McDonald Observatory. Fluorine abundances in the samples of stars were derived using the (1-0)R9 rotational-vibrational line of the HF molecule, and whenever possible we also measured the R13, R14, R15 and R16 lines, all close to λ2.3μm. Our results find a significant reduction in fluorine abundances compared to previous literature values, due to a downward revision of the excitation potential of the HF R9 line used in this analysis. Fluorine abundances obtained for the M4 red giants show an anti-correlation with Na abundances, following the typical pattern of abundance variations seen in globular clusters between distinct star populations; our results indicate that as the abundance of Na increases by ∼ +0.4 dex, the fluorine abundance decreases by -0.2 dex. A comparison of the determined abundances with model predictions indicate that the stars responsible for the anti-correlation between the fluorine and sodium abundances observed in M4 are massive AGB stars, with masses M ≥ 6M�. The analysis of red giant stars from the Galactic disk covers a significant range in metallicity ([Fe/H]∼-1,2 – 0,0 dex); these stars are probable members of the thin and thick disk / halo, two of them likely members of the Monoceros over-density. Our results indicate that at low metallicity, -1.2 < [Fe/H] < -0.4 dex, the fluorine abundance varies as vi a primary element with respect to the Fe abundance, maintaining a constant subsolar value of [F/Fe] between ∼ -0.3 and -0.4 dex. In the higher metallicities, however, the [F/Fe] ratio decreases rapidly with [Fe/H], exhibiting an almost secondary behavior with respect to Fe. One of the chemical evolutionary models from the literature that includes neutrino nucleosynthesis, describes approximately the observed [F/Fe] plateau at lower metallicities, while the [F/Fe] ratio in this model increases to an approximately solar value at higher metallicities, as found for our thin disc sample. The probable stars from the thick disk (|Z| > 300 pc) spans Galactocentric distances from Rg ∼ 6–13.7 kpc. These stars exhibit an approximately constant values of [F/Fe], indicating a nearly flat fluorine abundance gradient across a significant portion of the Galaxy at distances of |Z|> 300 pc from the Galactic mean plane. The stars from the Large Magellanic Cloud are from the field and the populous cluster NGC 2203. Our results indicate that the stars of NGC 2203 follow approximately the primary behavior of fluorine with respect to iron as found for Galactic thick disk / halo disk stars. The values of [F/Fe] for stars in the Large Magellanic Cloud field are significantly lower (∼0.6 dex) than the values for stars in our Galaxy over the same range of metallicity. Additionally, there is a tendency for a moderate decrease in the [F/O] ratio with the oxygen abundance, which is also generally predicted from models that include fluorine production in Type II supernovae in the Galaxy.