C. Ugalde

The Uncertainties in the 22Ne+α-Capture Reaction Rates and the Production of the Heavy Magnesium Isotopes in Asymptotic Giant Branch Stars of Intermediate Mass

We present new rates for the 22 Ne(� , n) 25 Mg and 22 Ne(� , � ) 26 Mg reactions, with uncertainties that have been considerably reduced compared to previous estimates, and we study how these new rates affect the production of the heavy magnesium isotopes in models of intermediate-mass asymptotic giant branch (AGB) stars of different initial compositions. All the models have deep third dredge-up, hot bottom burning, and mass loss. Calculations have been performed using the two most commonly used estimates of the 22 Ne+� rates as well as the new recommended rates, and with combinations of their upper and lower limits. The main result of the present study is that, with the new rates, uncertainties on the production of isotopes from Mg to P coming from the 22 Ne+� -capture rates have been considerably reduced. We have therefore removed one of the important sources of uncertainty to effect models of AGB stars. We have studied the effects of varying the mass-loss rate on nucleosynthesis and discuss other uncertainties related to the physics employed in the computation of stellar structure, such as the modeling of convection, the inclusion of a partial mixing zone, and the definition of convective borders. These uncertainties are found to be much larger than those coming from 22 Ne+� -capture rates, when using our new estimates. Much effort is needed to improve the situation for AGB models. Subject headingg nuclear reactions, nucleosynthesis, abundances — stars: AGB and post-AGB — stars: evolution — stars: interiors

Charged-particle thermonuclear reaction rates: I. Monte Carlo method and statistical distributions

Abstract A method based on Monte Carlo techniques is presented for evaluating thermonuclear reaction rates. We begin by reviewing commonly applied procedures and point out that reaction rates that have been reported up to now in the literature have no rigorous statistical meaning. Subsequently, we associate each nuclear physics quantity entering in the calculation of reaction rates with a specific probability density function, including Gaussian, lognormal and chi-squared distributions. Based on these probability density functions the total reaction rate is randomly sampled many times until the required statistical precision is achieved. This procedure results in a median (Monte Carlo) rate which agrees under certain conditions with the commonly reported recommended “classical” rate. In addition, we present at each temperature a low rate and a high rate, corresponding to the 0.16 and 0.84 quantiles of the cumulative reaction rate distribution. These quantities are in general different from the statistically meaningless “minimum” (or “lower limit”) and “maximum” (or “upper limit”) reaction rates which are commonly reported. Furthermore, we approximate the output reaction rate probability density function by a lognormal distribution and present, at each temperature, the lognormal parameters μ and σ . The values of these quantities will be crucial for future Monte Carlo nucleosynthesis studies. Our new reaction rates, appropriate for bare nuclei in the laboratory , are tabulated in the second paper of this issue (Paper II). The nuclear physics input used to derive our reaction rates is presented in the third paper of this issue (Paper III). In the fourth paper of this issue (Paper IV) we compare our new reaction rates to previous results.

Experimental evidence for a natural parity state in Mg26 and its impact on the production of neutrons for the s process

We have studied natural parity states in {sup 26}Mg via the {sup 22}Ne({sup 6}Li, d){sup 26}Mg reaction. Our method significantly improves the energy resolution of previous experiments and, as a result, we report the observation of a natural parity state in {sup 26}Mg. Possible spin-parity assignments are suggested on the basis of published {gamma}-ray decay experiments. The stellar rate of the {sup 22}Ne({alpha},{gamma}){sup 26}Mg reaction is reduced and may give rise to an increase in the production of s-process neutrons via the {sup 22}Ne({alpha},n){sup 25}Mg reaction.

Direct Measurement of the

In our Letter [Phys. Rev. Lett. 112, 152701 (2014)] we reported the direct measurement of the 23Naðα; pÞ26Mg reaction cross section at energies relevant for the production of Galactic Al. Our results, which relied on the extracted absolute cross sections given in Table I, have been found to be in error, overestimating the reported cross sections by a factor of 100. In the experiment, protons from the reaction were measured in an annular silicon strip detector placed downstream from a cryogenic He gas target. The cross sections were normalized to the yield of scattered Na ions from a separate Au foil in an upstream monitor detector. The data acquisition system was triggered by a logic “OR” of the proton detector and the “downscaled” monitor detector. The monitor detector rate was downscaled in order to reduce dead time in the data acquisition system. The down-scale factor was n 1⁄4 100, while in the analysis, the factor was mistakenly taken as n 1⁄4 1. Therefore, the cross section numbers given in Table I should be divided by a factor of 100. The stellar rate reported in our Letter should also be down scaled by the same factor of 100, which makes it in agreement, within the experimental uncertainties, with the recommended rate. This problem came to light due to results from recent experiments where the same reaction was studied in regular and inverse kinematics [1,2]. Those studies obtained similar results and were in disagreement with our measurement. A subsequent experiment by our group was carried out with a different technique to verify the results. In this experiment, an active target and detector system measures both the heavy Mg recoils as well as the incoming Na beam, thus avoiding normalization errors [3]. The new results [3] are in agreement with the reported results [1,2] and also with the values in our Letter, within their experimental uncertainties, if the down-scale factor is correctly included.

^{23}

The existence of ^{26}Al (t_{1/2}=7.17×10^{5}  yr) in the interstellar medium provides a direct confirmation of ongoing nucleosynthesis in the Galaxy. The presence of a low-lying 0^{+} isomer (^{26}Al^{m}), however, severely complicates the astrophysical calculations. We present for the first time a study of the ^{26}Al^{m}(d,p)^{27}Al reaction using an isomeric ^{26}Al beam. The selectivity of this reaction allowed the study of ℓ=0 transfers to T=1/2, and T=3/2 states in ^{27}Al. Mirror symmetry arguments were then used to constrain the ^{26}Al^{m}(p,γ)^{27}Si reaction rate and provide an experimentally determined upper limit of the rate for the destruction of isomeric ^{26}Al via radiative proton capture reactions, which is expected to dominate the destruction path of ^{26}Al^{m} in asymptotic giant branch stars, classical novae, and core collapse supernovae.

Na(

In our Letter [Phys. Rev. Lett. 112, 152701 (2014)] we reported the direct measurement of the 23Naðα; pÞ26Mg reaction cross section at energies relevant for the production of Galactic Al. Our results, which relied on the extracted absolute cross sections given in Table I, have been found to be in error, overestimating the reported cross sections by a factor of 100. In the experiment, protons from the reaction were measured in an annular silicon strip detector placed downstream from a cryogenic He gas target. The cross sections were normalized to the yield of scattered Na ions from a separate Au foil in an upstream monitor detector. The data acquisition system was triggered by a logic “OR” of the proton detector and the “downscaled” monitor detector. The monitor detector rate was downscaled in order to reduce dead time in the data acquisition system. The down-scale factor was n 1⁄4 100, while in the analysis, the factor was mistakenly taken as n 1⁄4 1. Therefore, the cross section numbers given in Table I should be divided by a factor of 100. The stellar rate reported in our Letter should also be down scaled by the same factor of 100, which makes it in agreement, within the experimental uncertainties, with the recommended rate. This problem came to light due to results from recent experiments where the same reaction was studied in regular and inverse kinematics [1,2]. Those studies obtained similar results and were in disagreement with our measurement. A subsequent experiment by our group was carried out with a different technique to verify the results. In this experiment, an active target and detector system measures both the heavy Mg recoils as well as the incoming Na beam, thus avoiding normalization errors [3]. The new results [3] are in agreement with the reported results [1,2] and also with the values in our Letter, within their experimental uncertainties, if the down-scale factor is correctly included.

\alpha

Using an array of high-purity Compton-suppressed germanium detectors, we performed an independent measurement of the

,p)

\beta

^{26}

-decay branching ratio from

Mg Reaction Cross Section at Energies Relevant for the Production of Galactic

^{12}\mathrm{B}