The origin of fluorine is a widely debated issue. Nevertheless, the ^{15}N({alpha},{gamma})^{19}F reaction is a common feature among the various production channels so far proposed. Its reaction rate at relevant temperatures is determined by a number
of narrow resonances together with the DC component and the tails of the two broad resonances at E_{c.m.} = 1323 and 1487 keV. Measurement through the direct detection of the 19F recoil ions with the European Recoil separator for Nuclear Astrophysics (ERNA) were performed. The reaction was initiated by a 15N beam impinging onto a 4He windowless gas target. The observed yield of the resonances at Ec.m. = 1323 and 1487 keV is used to determine their widths in the {alpha} and {gamma} channels. We show that a direct measurement of the cross section of the ^{15}N({alpha},{gamma})^{19}F reaction can be successfully obtained with the Recoil Separator ERNA, and the widths {Gamma}_{gamma} and {Gamma}_{alpha} of the two broad resonances have been determined. While a fair agreement is found with earlier determination of the widths of the 1487 keV resonance, a significant difference is found for the 1323 keV resonance {Gamma}_{alpha} . The revision of the widths of the two more relevant broad resonances in the 15N({alpha},{gamma})19F reaction presented in this work is the first step toward a more firm determination of the reaction rate. At present, the residual uncertainty at the temperatures of the ^{19}F stellar nucleosynthesis is dominated by the uncertainties affecting the Direct Capture component and the 364 keV narrow resonance, both so far investigated only through indirect experiments.
The origin of fluorine is a longstanding problem in nuclear astrophysics. It is widely recognized that Asymptotic Giant Branch (AGB) stars are among the most important contributors to the Galactic fluorine production. In general, extant nucleosynthes
is models overestimate the fluorine production by AGB stars with respect to observations. In this paper we review the relevant nuclear reaction rates involved in the fluorine production/destruction. We perform this analysis on a model with initial mass M=2 M$_odot$ and Z=0.001. We found that the major uncertainties are due to the $^{13}$C($alpha$,n)$^{16}$O, the $^{19}$F($alpha$,p)$^{22}$Ne and the $^{14}$N(p,$gamma$)$^{15}$O reactions. A change of the corresponding reaction rates within the present experimental uncertainties implies surface $^{19}$F variations at the AGB tip lower than 10%. For some $alpha$ capture reactions, however, larger variations in the rates of those processes cannot be excluded. Thus, we explore the effects of the variation of some $alpha$ capture rates well beyond the current published uncertainties. The largest $^{19}$F variations are obtained by varying the $^{15}$N($alpha$,$gamma$)$^{19}$F and the $^{19}$F($alpha$,p)$^{22}$Ne reactions. The analysis of some $alpha$ capture processes assuming a wider uncertainty range determines $^{19}$F abundances in better agreement with recent spectroscopic fluorine measurements at low metallicity. In the framework of the latter scenario the $^{15}$N($alpha$,$gamma$)$^{19}$F and the $^{19}$F($alpha$,p)$^{22}$Ne reactions show the largest effects on fluorine nucleosynthesis. The presence of poorly known low energy resonances make such a scenario, even if unlikely, possible. We plan to directly measure these resonances.
Proton captures on Mg isotopes play an important role in the Mg-Al cycle active in stellar H-burning regions. In particular, low-energy nuclear resonances in the $^{25}$Mg(p,$gamma$)$^{26}$Al reaction affect the production of radioactive $^{26}$Al$^{
gs}$ as well as the resulting Mg/Al abundance ratio. Reliable estimations of these quantities require precise measurements of the strengths of low-energy resonances. Based on a new experimental study performed at LUNA, we provide revised rates of the $^{25}$Mg(p,$gamma$)$^{26}$Al$^{gs}$ and the $^{25}$Mg(p,$gamma$)$^{26}$Al$^{m}$ reactions with corresponding uncertainties. In the temperature range 50 to 150 MK, the new recommended rate of the $^{26}$Al$^{m}$ production is up to 5 times higher than previously assumed. In addition, at T$=100$ MK, the revised total reaction rate is a factor of 2 higher. Note that this is the range of temperature at which the Mg-Al cycle operates in an H-burning zone. The effects of this revision are discussed. Due to the significantly larger $^{25}$Mg(p,$gamma$)$^{26}$Al$^{m}$ rate, the estimated production of $^{26}$Al$^{gs}$ in H-burning regions is less efficient than previously obtained. As a result, the new rates should imply a smaller contribution from Wolf-Rayet stars to the galactic $^{26}$Al budget. Similarly, we show that the AGB extra-mixing scenario does not appear able to explain the most extreme values of $^{26}$Al/$^{27}$Al, i.e. $>10^{-2}$, found in some O-rich presolar grains. Finally, the substantial increase of the total reaction rate makes the hypothesis of a self-pollution by massive AGBs a more robust explanation for the Mg-Al anticorrelation observed in Globular-Cluster stars.
The 15N(p,g)16O reaction represents a break out reaction linking the first and second cycle of the CNO cycles redistributing the carbon and nitrogen abundances into the oxygen range. The reaction is dominated by two broad resonances at Ep = 338 keV a
nd 1028 keV and a Direct Capture contribution to the ground state of 16O. Interference effects between these contributions in both the low energy region (Ep < 338 keV) and in between the two resonances (338 <Ep < 1028 keV) can dramatically effect the extrapolation to energies of astrophysical interest. To facilitate a reliable extrapolation the 15N(p,g)16O reaction has been remeasured covering the energy range from Ep=1800 keV down to 130 keV. The results have been analyzed in the framework of a multi-level R-matrix theory and a S(0) value of 39.6 keV b has been found.
The COMPTEL instrument performed the first mapping of the 1.809 MeV photons in the Galaxy, triggering considerable interest in determing the sources of interstellar 26Al. The predicted 26Al is too low compared to the observation, for a better underst
anding more accurate rates for the 25Mg(p; gamma)26Al reaction are required. The 25Mg(p;gamma)26Al reaction has been investigated at the resonances at Er= 745; 418; 374; 304 keV at Ruhr-Universitat-Bochum using a Tandem accelerator and a 4piNaI detector. In addition the resonance at Er = 189 keV has been measured deep underground laboratory at Laboratori Nazionali del Gran Sasso, exploiting the strong suppression of cosmic background. This low resonance has been studied with the 400 kV LUNA accelerator and a HPGe detector. The preliminary results of the resonance strengths will be reported.
The astrophysical S-factor of 14N(p,gamma)15O has been measured for effective center-of-mass energies between E_eff = 119 and 367 keV at the LUNA facility using TiN solid targets and Ge detectors. The data are in good agreement with previous and rece
nt work at overlapping energies. R-matrix analysis reveals that due to the complex level structure of 15O the extrapolated S(0) value is model dependent and calls for additional experimental efforts to reduce the present uncertainty in S(0) to a level of a few percent as required by astrophysical calculations.
The transition between the Main Sequence and the Red Giant Branch in low mass stars is powered by the onset of the CNO burning, whose bottleneck is the $^{14}$N(p,$gamma)^{15}$O. The LUNA collaboration has recently improved the low energy measurement
s of the cross section of this key reaction. We analyse the impact of the revised reaction rate on the estimate of the Globular Clusters ages, as derived from the turnoff luminosity. We found that the age of the oldest Globulars should be increased by about 0.7-1 Gyr with respect to the current estimates.
