FLUORINE IN ASYMPTOTIC GIANT BRANCH CARBON STARS REVISITED

A re-analysis of the fluorine abundance in three Galactic asymptotic giant branch (AGB) carbon stars (TX Psc, AQ Sgr, and R Scl) has been performed from the molecular HF (1-0) R9 line at 2.3358 mu m. High resolution (R similar to 50,000) and high signal-to-noise spectra obtained with the CRIRES spectrograph and the VLT telescope or from the NOAO archive (for TX Psc) have been used. Our abundance analysis uses the latest generation of MARCS model atmospheres for cool carbon-rich stars. Using spectral synthesis in local thermodynamic equilibrium, we derive for these stars fluorine abundances that are systematically lower by similar to 0.8 dex in average with respect to the sole previous estimates by Jorissen et al. The possible reasons of this discrepancy are explored. We conclude that the difference may rely on the blending with C-bearing molecules (CN and C-2) that were not properly taken into account in the former study. The new F abundances are in better agreement with the prediction of full network stellar models of low-mass AGB stars. These models also reproduce the s-process elements distribution in the sampled stars. This result, if confirmed in a larger sample of AGB stars, might alleviate the current difficulty to explain the largest [F/O] ratios found by Jorissen et al. In particular, it may not be necessary to search for alternative nuclear chains affecting the production of F in AGB stars.