The Fluorine Destruction in Stars: First Experimental Study of the ¹⁹F(p,α₀)¹⁶O Reaction at Astrophysical Energies, M. La Cognata¹, A. M. Mukhamedzhanov², C. Spitaleri^1,3, I. Indelicato^1,3, M. Aliotta⁴, V. Burjan5, S. Cherubini^1,3, A. Coc⁶, M. Gulino^1,3, Z. Hons⁵, G. G. Kiss¹, V. Kroha⁵, L. Lamia^1,3, J. Mrazek⁵, S. Palmerini⁷, S. Piskor⁵, R. G. Pizzone¹, S. M. R. Puglia^1,3, G. G. Rapisarda^1,3, S. Romano^1,3, M. L. Sergi^1,3, A. Tumino^1,8, ¹ INFN-Laboratori Nazionali del Sud, Catania, Italy; ² Cyclotron Institute-Texas A&M University, College Station, TX, USA; ³ Dipartimento di Fisica e Astronomia-Università di Catania, Catania, Italy; ⁴ School of Physics and Astronomy, University of Edinburgh, Edinburgh, and SUPA-Scottish Universities Physics Alliance, UK; ⁵ Nuclear Physics Institute of ASCR, Rez near Prague, Czech Republic; ⁶ CSNSM CNRS/IN2P3, Université Paris Sud, Orsay, France; ⁷ Dipartimento di Fisica-Università di Perugia, and INFN-Sezione di Perugia, Perugia, Italy; ⁸ Facoltà di Ingegneria e Architettura, Università degli Studi di Enna “Kore,” Enna, Italy − The ¹⁹F(p,α)¹⁶O reaction is an important fluorine destruction channel in the proton-rich outer layers of asymptotic giant branch (AGB) stars and it might also play a role in hydrogen-deficient post-AGB star nucleosynthesis. So far, available direct measurements do not reach the energy region of astrophysical interest (E_cm < 300 keV), because of the hindrance effect of the Coulomb barrier.  Therefore, below E_cm = 460 keV, where data do not exist, a non-resonant contribution is calculated for s-capture and the cross section has been extrapolated assuming this contribution as the dominant one. The Trojan Horse (TH) method was thus used to access this energy region, by extracting the quasi-free contribution to the ²H(¹⁹F,α¹⁶O)n and the ¹⁹F(³He,α¹⁶O)d reactions. A novel approach, the so-called Modified R-matrix, has been developed to analyze the data, aiming to account for the half-off-energy-shell nature of the TH cross section and for the experimental energy resolution. The TH measurement of the α₀ channel, which provides the largest contribution below about 1 MeV, shows the presence of resonant structures not observed before, showing up right at astrophysical energies, which cause an increase of the reaction rate at astrophysical temperatures (about 108 K) up to a factor of 1.7, with potential important consequences for stellar nucleosynthesis.

[1] M. La Cognata, et al., The Astrophysical Journal Letters, 739, L54 (2011).