Fluorine production in intermediate-mass stars

The F production during the first dozen thermal pulses of AGB stars with masses M and metallicities Z (M = 3M⊙, Z = 0.02), (M = 6M⊙, Z = 0.02) and (M = 3M⊙, Z = 0.001) is investigated on grounds of detailed stellar models and of revised rates for N(α , γ) F and O(α, γ)Ne. These calculations confirm an early expectation that F is produced in AGB thermal pulses. They also enlarge substantially these previous results by showing that the variations of the level of F production during the evolution is very sensitive to the maximum temperature reached at the base of the pulse. These variations are analyzed in detail, and are shown to result from a subtle balance between different nuclear effects (mainly F production or destruction in a pulse, and N synthesis during the interpulse), possibly superimposed on dilution effects in more or less extended pulse convective tongues. Our calculations, as most others, do not predict the third dredge-up self-consistently. When parametrized, it appears that our models of intermediate-mass AGB stars are able to account only for the lowest F overabundances observed in solar-metallicity MS, S and C stars. That conclusion is expected to hold true for low-mass stars when fluorine production results from secondary C only. Massive AGB stars, on the other hand, are not expected to build up large surface F abundances. Therefore, the large fluorine overabundance reported for the super Li-rich star WZ Cas (where hot bottom burning is supposed to be operating) remains unexplained so far. Our results for the (3M⊙, Z = 0.001) star indicate that fluorine surface overabundances can also be expected in low-metallicity stars provided that third dredge-ups occur after the early cool pulses. The relative increase in the surface F/C ratio is, however, lower in the low-metallicity than in the solar-metallicity star. No observations are reported yet for these stars, and are urgently called for.