Fluorine in the solar neighbourhood: the need for several cosmic sources

The cosmic origin of fluorine is still not well constrained. Several nucleosynthetic channels at different phases of stellar evolution have been suggested, but these must be constrained by observations. For this, the fluorine abundance trend with metallicity spanning a wide range is required. Our aim is to determine stellar abundances of fluorine for -1.1 < [Fe H] < +0.4. We determine the abundances from HF lines in infrared K-band spectra ( 2.3 mm) of cool giants, observed with the IGRINS and Phoenix high-resolution spectrographs. We derive accurate stellar parameters for all our observed K giants, which is important as the HF lines are very temperaturesensitive. We find that [F/Fe] is flat as a function of metallicity at [ F/Fe]0, but increases as the metallicity increases. the fluorine slope shows a clear secondary behavior in this metallicity range. We also find that the [F/ Ce] ratio is relatively flat for -0.6 < [Fe H] < 0, and that for two metal-poor ([Fe H] < - 0.8), s-process element-enhanced giants, we do not detect an elevated fluorine abundance. We interpret all of these observational constraints as indications that several major processes are at play for the cosmic budget of fluorine over time: from those in massive stars at low metallicities, through the asymptotic giant branch star contribution at -0.6 < [Fe H] < 0, to processes with increasing yields with metallicity at supersolar metallicities. the origins of the latter, and whether or not Wolf-Rayet stars and/or novae could contribute at supersolar metallicities, is currently not known. To quantify these observational results, theoretical modeling is required. More observations in the metal-poor region are required to clarify the processes there.
Swedish Research Council, V.R.Swedish Research Council [621-2014-5640]; Royal Physiographic Society in Lund through the Stiftelse Walter Gyllenbergs fond; Crafoord Foundation; Stiftelsen Olle Engkvist ByggmastareSwedish Research Council; Ruth och Nils-Erik Stenbacks stiftelse; US National Science FoundationNational Science Foundation (NSF) [AST-1229522, AST-1702267]; McDonald Observatory of the University of Texas at Austin; Korean GMT Project of KASI
We would like to thank George Meynet for fruitful and enlightening discussions on massive-star yields and the theoretically predicted role of Wolf-Rayet stars for the production of fluorine. N.R. acknowledges support from the Swedish Research Council, V.R. (project numbers 621-2014-5640), and the Royal Physiographic Society in Lund through the Stiftelse Walter Gyllenbergs fond and Marta och Erik Holmbergs donation. H.J. acknowledges support from the Crafoord Foundation, Stiftelsen Olle Engkvist Byggmastare, and Ruth och Nils-Erik Stenbacks stiftelse. This work used the Immersion Grating Infrared spectrograph (IGRINS) that was developed under a collaboration between the University of Texas at Austin and the Korea Astronomy and Space Science Institute (KASI) with the financial support of the US National Science Foundation under grants AST-1229522 and AST-1702267, of the McDonald Observatory of the University of Texas at Austin, and of the Korean GMT Project of KASI. Facilities: McDonald Observatory (IGRINS), Lowell Observatory (IGRINS), KPNO (Phoenix).