Discovery of Fluorine in Cool Extreme Helium Stars

For a long time, the solar system has been the only source of information about the fluorine abundance in the Galaxy (Hall & Noyes 1969: Astrophys. Lett., 4, 143). The astrophysical origin of the solar system's fluorine is not yet identified from the known theories of stellar nucleosynthesis. The major problem with fluorine production is that the element has only one stable, yet rather fragile isotope,¹⁹F. In stellar interiors, it is readily destroyed by hydrogen via ¹⁹F(p,α) ¹⁶0 and helium via ¹⁹F(α,p)²²Ne. The high F abundances that were measured using infrared HF vibration-rotation transitions in the asymptotic giant branch (AGB) stars from the Jorissen et al. (1992: A&A, 261, 164) sample provided us with clues for fluorine production in
AGB stars.
Apart from the third dredge-up in AGB stars, the other two sources of fluorine are Type II supernova (SN) explosions and stellar winds from Wolf-Rayet (W-R) stars. In AGB stars, ¹⁹F is predicted to be produced in the convective He-rich intershell and then dredged up to the surface during the He-burning thermal pulses (Forestini et al. 1992: A&A, 261, 157). The reaction chain for F production in the He-burning environments of AGB and W-R stars is ¹⁴N(α,γ)¹⁸F(β⁺)¹⁸0(p,α)¹⁵N(α,γ)¹⁹F. Protons are provided by the ¹⁴N(n,p)¹⁴C reaction with neutrons liberated from the ¹³C(α,n)¹⁶0. The ¹⁴N and ¹³C nuclei are the result of H burning by the CNO cycle and the initial ¹³C stock acts as a limiting factor for the ¹⁹F yield.

Extreme Helium (Ehe) stars (Pandey et al. 2006: ApJ, 638,454 and the references therein) are suggested to have gone through the AGB phase in their earlier evolution; hence fluorine must be present in their atmospheres. The presence of F in an EHe star's atmosphere can serve as a testbed for fluorine production in AGB stars.
Neutral fluorine (F I) lines are identified in the optical spectra of cool extreme helium (EHe) stars (see Figure). The spectra were obtained from CTIO, McDonald Observatory, and VBO. These are the first F I lines identified in a star's spectrum, and they provide the first measurement of fluorine abundances in EHe stars. The results show that fluorine is overabundant in EHe stars. A sample of cool EHe stars show F enhancements about 100 times solar. Since ¹⁹F is overabundant in cool EHe stars, the reaction ²²Ne(α,n)²⁵Mg is unlikely to be the neutron source in EHe stars (the reaction rate of 19F(α,n) ²²Ne is much higher than that of ²²Ne(α,n) ²⁵Mg).
The work has been published in the 2006 September 10 issue of ApJ Lett. ( G. Pandey )