Arthur E. Champagne

Solar fusion cross-sections

We review and analyze the available information on the nuclear-fusion cross sections that are most important for solar energy generation and solar neutrino production. We provide best values for the low-energy cross-section factors and, wherever possible, estimates of the uncertainties. We also describe the most important experiments and calculations that are required in order to improve our knowledge of solar fusion rates.

The Effects of Thermonuclear Reaction-Rate Variations on Nova Nucleosynthesis: A Sensitivity Study

We investigate the effects of thermonuclear reaction-rate uncertainties on nova nucleosynthesis. One-zone nucleosynthesis calculations have been performed by adopting temperature-density-time profiles of the hottest hydrogen-burning zone (i.e., the region in which most of the nucleosynthesis takes place). We obtain our profiles from seven different, recently published, hydrodynamic nova simulations covering peak temperatures in the range from Tpeak = 0.145 to 0.418 GK. For each of these profiles, we individually varied the rates of 175 reactions within their associated errors and analyzed the resulting abundance changes of 142 isotopes in the mass range below A = 40. In total, we performed ≈7350 nuclear reaction network calculations. We use the most recent thermonuclear reaction-rate evaluations for the mass ranges A = 1-20 and 20-40. For the theoretical astrophysicist, our results indicate the extent to which nova nucleosynthesis calculations depend on currently uncertain nuclear physics input, while for the experimental nuclear physicist, our results represent at least a qualitative guide for future measurements at stable and radioactive ion beam facilities. We find that present reaction-rate estimates are reliable for predictions of Li, Be, C, and N abundances in nova nucleosynthesis. However, rate uncertainties of several reactions have to be reduced significantly in order to predict more reliable O, F, Ne, Na, Mg, Al, Si, S, Cl, and Ar abundances. Results are presented in tabular form for each adopted nova simulation.

Charged-Particle Thermonuclear Reaction Rates: II. Tables and Graphs of Reaction Rates and Probability Density Functions

Abstract Numerical values of charged-particle thermonuclear reaction rates for nuclei in the A = 14 to 40 region are tabulated. The results are obtained using a method, based on Monte Carlo techniques, that has been described in the preceding paper of this issue (Paper I). We present a low rate, median rate and high rate which correspond to the 0.16, 0.50 and 0.84 quantiles, respectively, of the cumulative reaction rate distribution. The meaning of these quantities is in general different from the commonly reported, but statistically meaningless expressions, “lower limit”, “nominal value” and “upper limit” of the total reaction rate. In addition, we approximate the Monte Carlo probability density function of the total reaction rate by a lognormal distribution and tabulate the lognormal parameters μ and σ at each temperature. We also provide a quantitative measure (Anderson–Darling test statistic) for the reliability of the lognormal approximation. The user can implement the approximate lognormal reaction rate probability density functions directly in a stellar model code for studies of stellar energy generation and nucleosynthesis. For each reaction, the Monte Carlo reaction rate probability density functions, together with their lognormal approximations, are displayed graphically for selected temperatures in order to provide a visual impression. Our new reaction rates are appropriate for bare nuclei in the laboratory. The nuclear physics input used to derive our reaction rates is presented in the subsequent paper of this issue (Paper III). In the fourth paper of this issue (Paper IV) we compare our new reaction rates to previous results.

THE EFFECTS OF THERMONUCLEAR REACTION RATE VARIATIONS ON 26Al PRODUCTION IN MASSIVE STARS: A SENSITIVITY STUDY

We investigate the effects of thermonuclear reaction rate variations on 26Al production in massive stars. The dominant production sites in such events were recently investigated by using stellar model calculations: explosive neon-carbon burning, convective shell carbon burning, and convective core hydrogen burning. Post-processing nucleosynthesis calculations are performed for each of these sites by adopting temperature-density-time profiles from recent stellar evolution models. For each profile, we individually multiplied the rates of all relevant reactions by factors of 10, 2, 0.5, and 0.1, and analyzed the resulting abundance changes of 26Al. In total, we performed ≈900 nuclear reaction network calculations. Our simulations are based on a next-generation nuclear physics library, called STARLIB, which contains a recent evaluation of Monte Carlo reaction rates. Particular attention is paid to quantifying the rate uncertainties of those reactions that most sensitively influence 26Al production. For stellar modelers our results indicate to what degree predictions of 26Al nucleosynthesis depend on currently uncertain nuclear physics input, while for nuclear experimentalists our results represent a guide for future measurements. We also investigate equilibration effects of 26Al. In all previous massive star investigations, either a single species or two species of 26Al were taken into account, depending on whether thermal equilibrium was achieved or not. These are two extreme assumptions, and in a hot stellar plasma the ground and isomeric states may communicate via γ-ray transitions involving higher-lying 26Al levels. We tabulate the results of our reaction rate sensitivity study for each of the three distinct massive star sites referred to above. It is found that several current reaction rate uncertainties influence the production of 26Al. Particularly important reactions are 26Al(n,p)26Mg, 25Mg(α,n)28Si, 24Mg(n,γ)25Mg, and 23Na(α,p)26Mg. These reactions should be prime targets for future measurements. Overall, we estimate that the nuclear physics uncertainty of the 26Al yield predicted by the massive star models explored here amounts to about a factor of three. We also find that taking the equilibration of 26Al levels explicitly into account in any of the massive star sites investigated here has only minor effects on the predicted 26Al yields. Furthermore, we provide for the interested reader detailed comments regarding the current status of certain reactions, including 12C(12C,n)23Mg, 23Na(α,p)26Mg, 25Mg(α,n)28Si, 26Al m (p,γ)27Si, 26Al(n,p)26Mg, and 26Al(n,α)23Na.

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.

Low-energy resonances in 25Mg(p, γ)26Al, 26Mg(p, γ)27Al and 27Al(p, γ)28Si☆

Abstract Gamma-ray decay schemes have been measured with bare and Compton-suppressed Ge detectors at low-energy resonances ( E P 25 Mg, 26 Mg and 27 Al. Altogether 58 new decay branches have been observed and a new 26 Mg(p, γ) 27 Al resonance has been found at E P = 154.5 ± 1.0 keV. The new branchings lead to J π ; T determinations (or limitations) for two states in 26 Al and four states in 28 Si. The absolute strengths of the 25 Mg(p, γ) 26 Al and 26 Mg(p, γ) 27 Al resonances have also been obtained, and the uncertainties of the stellar rates, deduced from the available data for both reactions, are significantly reduced. Some astrophysical consequences are discussed.

Charged-particle thermonuclear reaction rates: III. Nuclear physics input

Abstract The nuclear physics input used to compute the Monte Carlo reaction rates and probability density functions that are tabulated in the second paper of this issue (Paper II) is presented. Specifically, we publish the input files to the Monte Carlo reaction rate code RatesMC , which is based on the formalism presented in the first paper of this issue (Paper I). This data base contains overwhelmingly experimental nuclear physics information. The survey of literature for this review was concluded in November 2009.

Reaction rate of 24Mg(p,γ)25Al

Abstract The proton-capture reaction on 24 Mg has been investigated in the bombarding energy range of E p =0.2–1.7 MeV. Resonance properties (strengths, branching ratios and lifetimes) of low-energy resonances have been measured. From the experimental results, accurate proton partial widths, γ -ray partial widths and total widths ( Γ p , Γ γ , and Γ ) have been deduced. The present experimental information establishes the 24 Mg+p reaction rates over the temperature range T =0.02–2.0 GK with statistical uncertainties of 5% to 21%. Our recommended reaction rates deviate from previous estimates by 18% to 45%. Based on our results, we can rule out the recent suggestion that the total width of the E R =223 keV resonance has a significant influence on the reaction rates. We also discuss several effects that might give rise to systematic uncertainties in the reaction rates. The astrophysical implications for hydrogen burning of 24 Mg at low stellar temperatures are presented.

Measurement of 15O(α,γ)19Ne resonance strengths

Abstract States in 19 Ne above the 15 O + α threshold were populated by means of the 19 F( 3 He,t) 19 Ne ∗ reaction, and their alpha-particle decays to the 15 O ground state were measured. The branching ratios Γ α /Γ total for the E c . m . = 850-, 1020-, 1971-, 1183- and 1563- keV resonances in 19 O + α were determined. This information together with alpha-particle and/or gamma-ray partial widths (determined from knowledge of these quantities for the mirror states in 19 F) determines the strengths of these 15 O ( α , γ ) 19 Ne resonances and the 15 O ( α , γ ) 19 Ne reaction rate for temperatures between 7 × 10 8 and3 × 10 9 K.

Particle decays in 28Si: The destruction of 27Al in red giants and novae

Abstract The 27 Al( 3 He, d) 28 Si reaction has been used to populate states near the 27 Al+p threshold and the ensuing proton and alpha decay has been measured. No evidence for new 27 Al(p , α) 24 Mg resonance strength was observed and consequently revised limits have been placed on the thermonuclear reaction rate. As a result, the 27 Al(p , α) 24 Mg reaction is found to be astrophysically unimportant at red-giant and nova temperatures.