Abia, C.C.AbiaCRISTALLO, SergioSergioCRISTALLO0000-0001-9683-9406Cunha, K.K.Cunhade Laverny, P.P.de LavernySmith, V. V.V. V.Smith2020-12-032020-12-0320190004-6361http://hdl.handle.net/20.500.12386/28660We present new fluorine abundance measurements for a sample of carbon-rich asymptotic giant branch (AGB) stars and two other metal-poor evolved stars of Ba/CH types. The abundances are derived from IR, K-band, high-resolution spectra obtained using GEMINI-S/Phoenix and TNG/Giano-b. Our sample includes an extragalactic AGB carbon star belonging to the Sagittarius dSph galaxy. The metallicity of our stars ranges from [Fe/H] = 0.0 down to - 1.4 dex. The new measurements, together with those previously derived in similar stars, show that normal (N-type) and SC-type AGB carbon stars of near solar metallicity present similar F enhancements, discarding previous hints that suggested that SC-type stars have larger enhancements. These mild F enhancements are compatible with current chemical-evolution models pointing out that AGB stars, although relevant, are not the main sources of this element in the solar neighbourhood. Larger [F/Fe] ratios are found for lower-metallicity stars. This is confirmed by theory. We highlight a tight relation between the [F/⟨s⟩] ratio and the average s-element enhancement [⟨s⟩/Fe] for stars with [Fe/H] > -0.5, which can be explained by the current state-of-the-art low-mass AGB models assuming an extended <SUP>13</SUP>C pocket. For stars with [Fe/H] < -0.5, discrepancies between observations and model predictions still exist. We conclude that the mechanism of F production in AGB stars needs further scrutiny and that simultaneous F and s-element measurements in a larger number of metal-poor AGB stars are needed to better constrain the models.STAMPAenArticle10.1051/0004-6361/2019352862-s2.0-85073071999000467131000009https://www.aanda.org/articles/aa/full_html/2019/05/aa35286-19/aa35286-19.html2019A&A...625A..40AFIS/05 - ASTRONOMIA E ASTROFISICAERC sectors::Physical Sciences and Engineering::PE9 Universe sciences: astro-physics/chemistry/biology; solar systems; stellar, galactic and extragalactic astronomy, planetary systems, cosmology, space science, instrumentation