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Register a new userMeasurement of the reaction O-17(alpha,n)Ne-20 and its impact on the s process in massive stars
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The ratio between the rates of the reactions O-17(alpha,n)Ne-20 and O-17(alpha,gamma)Ne-21 determines whether O-16 is an efficient neutron poison for the s process in massive stars, or if most of the neutrons captured by O-16(n,gamma) are recycled into the stellar environment. This ratio is of particular relevance to constrain the s process yields of fast rotating massive stars at low metallicity. Recent results on the (alpha,gamma) channel have made it necessary to measure the (alpha,n) reaction more precisely and investigate the effect of the new data on s process nucleosynthesis in massive stars. We present a new measurement of the O-17(alpha, n) reaction using a moderating neutron detector. In addition, the (alpha, n_1) channel has been measured independently by observation of the characteristic 1633 keV gamma-transition in Ne-20. The reaction cross section was determined with a simultaneous R-matrix fit to both channels. (alpha,n) and (alpha, gamma) resonance strengths of states lying below the covered energy range were estimated using their known properties from the literature. A new O-17(alpha,n) reaction rate was deduced for the temperature range 0.1 GK to 10 GK. It was found that in He burning conditions the (alpha,gamma) channel is strong enough to compete with the neutron channel. This leads to a less efficient neutron recycling compared to a previous suggestion of a very weak (alpha,gamma) channel. S process calculations using our rates confirm that massive rotating stars do play a significant role in the production of elements up to Sr, but they strongly reduce the s process contribution to heavier elements.
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The breakout reaction $^{15}$O($alpha,gamma$)$^{19}$Ne, which regulates the flow between the hot CNO cycle and the rp-process, is critical for the explanation of the burst amplitude and periodicity of X-ray bursters. We report on the first successful measurement of the critical $alpha$-decay branching ratios of relevant states in $^{19}$Ne populated via $^{19}$F($^3$He,t)$^{19}$Ne. Based on the experimental results and our previous lifetime measurements of these states, we derive the first experimental rate of $^{15}$O($alpha,gamma$)$^{19}$Ne. The impact of our experimental results on the burst pattern and periodicity for a range of accretion rates is analyzed.
The astrophysical $s$-process is one of the two main processes forming elements heavier than iron. A key outstanding uncertainty surrounding $s$-process nucleosynthesis is the neutron flux generated by the ${}^{22}mathrm{Ne}(alpha, n){}^{25}mathrm{Mg}$ reaction during the He-core and C-shell burning phases of massive stars. This reaction, as well as the competing ${}^{22}mathrm{Ne}(alpha, gamma){}^{26}mathrm{Mg}$ reaction, is not well constrained in the important temperature regime from ${sim} 0.2$--$0.4$~GK, owing to uncertainties in the nuclear properties of resonances lying within the Gamow window. To address these uncertainties, we have performed a new measurement of the ${}^{22}mathrm{Ne}({}^{6}mathrm{Li}, d){}^{26}mathrm{Mg}$ reaction in inverse kinematics, detecting the outgoing deuterons and ${}^{25,26}mathrm{Mg}$ recoils in coincidence. We have established a new $n / gamma$ decay branching ratio of $1.14(26)$ for the key $E_x = 11.32$ MeV resonance in $^{26}mathrm{Mg}$, which results in a new $(alpha, n)$ strength for this resonance of $42(11)~mu$eV when combined with the well-established $(alpha, gamma)$ strength of this resonance. We have also determined new upper limits on the $alpha$ partial widths of neutron-unbound resonances at $E_x = 11.112,$ $11.163$, $11.169$, and $11.171$ MeV. Monte-Carlo calculations of the stellar ${}^{22}mathrm{Ne}(alpha, n){}^{25}mathrm{Mg}$ and ${}^{22}mathrm{Ne}(alpha, gamma){}^{26}mathrm{Mg}$ rates, which incorporate these results, indicate that both rates are substantially lower than previously thought in the temperature range from ${sim} 0.2$--$0.4$~GK.
The $^{18}{rm O}(p,alpha)^{15}{rm N}$ reaction is of primary importance in several astrophysical scenarios, including fluorine nucleosynthesis inside AGB stars as well as oxygen and nitrogen isotopic ratios in meteorite grains. Thus the indirect measurement of the low energy region of the $^{18}{rm O}(p,alpha)^{15}{rm N}$ reaction has been performed to reduce the nuclear uncertainty on theoretical predictions. In particular the strength of the 20 and 90 keV resonances have been deduced and the change in the reaction rate evaluated.
Context. Material processed by the CNO cycle in stellar interiors is enriched in 17O. When mixing processes from the stellar surface reach these layers, as occurs when stars become red giants and undergo the first dredge up, the abundance of 17O increases. Such an occurrence explains the drop of the 16O/17O observed in RGB stars with mass larger than 1.5 M_solar. As a consequence, the interstellar medium is continuously polluted by the wind of evolved stars enriched in 17O . Aims. Recently, the Laboratory for Underground Nuclear Astrophysics (LUNA) collaboration released an improved rate of the 17O(p,alpha)14N reaction. In this paper we discuss the impact that the revised rate has on the 16O/17O ratio at the stellar surface and on 17O stellar yields. Methods. We computed stellar models of initial mass between 1 and 20 M_solar and compared the results obtained by adopting the revised rate of the 17O(p,alpha)14N to those obtained using previous rates. Results. The post-first dredge up 16O/17O ratios are about 20% larger than previously obtained. Negligible variations are found in the case of the second and the third dredge up. In spite of the larger 17O(p,alpha)14N rate, we confirm previous claims that an extra-mixing process on the red giant branch, commonly invoked to explain the low carbon isotopic ratio observed in bright low-mass giant stars, marginally affects the 16O/17O ratio. Possible effects on AGB extra-mixing episodes are also discussed. As a whole, a substantial reduction of 17O stellar yields is found. In particular, the net yield of stars with mass ranging between 2 and 20 M_solar is 15 to 40% smaller than previously estimated. Conclusions. The revision of the 17O(p,alpha)14N rate has a major impact on the interpretation of the 16O/17O observed in evolved giants, in stardust grains and on the 17O stellar yields.
Lighter heavy elements beyond iron and up to around silver can form in neutrino-driven ejecta in core-collapse supernovae and neutron star mergers. Slightly neutron-rich conditions favour a weak r-process that follows a path close to stability. Therefore, the beta decays are slow compared to the expansion time scales, and ($alpha$,n) reactions become critical to move matter towards heavier nuclei. The rates of these reactions are calculated with the statistical model and their main uncertainty, at energies relevant for the weak r-process, is the $alpha$+nucleus optical potential. There are several sets of parameters to calculate the $alpha$+nucleus optical potential leading to large deviations for the reaction rates, exceeding even one order of magnitude. Recently the $^{96}$Zr($alpha$,n)$^{99}$Mo reaction has been identified as a key reaction that impacts the production of elements from Ru to Cd. Here, we present the first cross section measurement of this reaction at energies (6.22 MeV $leq$ E$_mathrm{c.m.}$ $leq$ 12.47 MeV) relevant for the weak r-process. The new data provide a stringent test of various model predictions which is necessary to improve the precision of the weak r-process network calculations. The strongly reduced reaction rate uncertainty leads to very well-constrained nucleosynthesis yields for $Z = 44 - 48$ isotopes under different neutrino-driven wind conditions.
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