We present the N , O , F and Na abundance and ^ { 12 } C/ ^ { 13 } C isotopic ratio measurements or upper limits for a sample of 10 C-rich , metal-poor giant stars , eight enhanced in s-process ( CEMP-s ) elements and two poor in n-capture elements ( CEMP-no ) .
The abundances are derived from IR , K-band , high-resolution CRIRES @ VLT spectra obtained .
The metallicity of our sample ranges from [ Fe/H ] = - 3.4 to - 1.3 .
F abundance could be measured only in two CEMP-s stars .
With [ F/Fe ] = 0.64 , one is mildly F-overabundant , while the other is F-rich , at [ F/Fe ] = 1.44 .
For the remaining eight objects , including both CEMP-no in our sample , only upper limits on F abundance could be placed .
Our measurements and upper limits show that there is a spread in [ F/C+N ] ratio in CEMP-s stars as predicted by theory .
Predictions from nucleosynthetic models for low-mass , low-metallicity Asymptotic Giant Branch stars , account for the derived F abundances , while the upper limits on F content derived for most of the stars are lower than the predicted values .
The measured Na content is accounted for by AGB models in the 1.25 to 1.75 M _ { \odot } range , confirming that the stars responsible for the peculiar abundance pattern observed in CEMP-s stars are low-mass , low-metallicity AGB stars , in agreement with the most accepted astrophysical scenario .
We conclude that the mechanism of F production in current state-of-the-art low-metallicity low-mass AGB models needs further scrutiny and that F measurements in a larger number of metal-poor stars are needed to better constraint the models .