New Improved Indirect Measurement of the F-19(p, alpha)O-16 Reaction at Energies of Astrophysical Relevance

Fluorine abundance determination is of great importance in stellar physics to understand s-elements production and mixing processes in asymptotic giant branch (AGB) stars. Up to now, theoretical models overproduce F abundances in AGB stars with respect to the observed values, thus calling for further investigation of the reactions involving fluorine. In particular, the F-19(p, alpha)O-16 reaction is the main destruction channel of fluorine at the bottom of the convective envelope in AGB stars, an H-rich environment where it can experience temperatures high enough to determine its destruction, owing to additional mixing processes. In this paper the Trojan horse method (THM) was used to extract the F-19(p, alpha(0))O-16 S-factor in the energy range of astrophysical interest (E-cm approximate to 0-1 MeV). This is the most relevant channel at the low temperatures (few 10(7) K) characterizing the bottom of the convective envelope, according to current knowledge. A previous indirect experiment using the THM has observed three resonances in the energy regions below E-cm approximate to 450 keV. These energies correspond to typical AGB temperatures, thus implying a significant increase in the reaction rate. Statistics are scarce for performing an accurate separation between resonances, preventing one from drawing a quantitative conclusion about their total widths and spin parities. Before THM measurement, only extrapolations were available below about 500 keV, showing a non-resonant behavior that sharply contradicts the trend of the astrophysical factor at higher energies. A new experiment has been performed to verify the measured TH astrophysical factor and to perform more accurate spectroscopy of the involved resonances.