Suzanne T. Parete-Koon

A New 17F(p, γ)18Ne Reaction Rate and Its Implications for Nova Nucleosynthesis

Proton capture by 17F plays an important role in the synthesis of nuclei in nova explosions. A revised rate for this reaction, based on a measurement of the 1H(17F, p)17F excitation function using a radioactive 17F beam at Oak Ridge National Laboratorys Holifield Radioactive Ion Beam Facility, is used to calculate the nucleosynthesis in nova outbursts on the surfaces of 1.25 and 1.35 M☉ ONeMg white dwarfs and a 1.00 M☉ CO white dwarf. We find that the new 17F (p, γ)18Ne reaction rate changes the abundances of some nuclides (e.g., 17O) synthesized in the hottest zones of an explosion on a 1.35 M☉ white dwarf by more than a factor of 104 compared to calculations using some previous estimates for this reaction rate, and by more than a factor of 3 when the entire exploding envelope is considered. In a 1.25 M☉ white dwarf nova explosion, this new rate changes the abundances of some nuclides synthesized in the hottest zones by more than a factor of 600, and by more than a factor of 2 when the entire exploding envelope is considered. Calculations for the 1.00 M☉ white dwarf nova show that this new rate changes the abundance of 18Ne by 21% but has negligible effect on all other nuclides. Comparison of model predictions with observations is also discussed.

The QSE-Reduced Nuclear Reaction Network for Silicon Burning

Iron and neighboring nuclei are formed in massive stars shortly before core collapse and during their supernova outbursts, as well as during thermonuclear supernovae. Complete and incomplete silicon burning are responsible for the production of a wide range of nuclei with atomic mass numbers from 28 to 64. Because of the large number of nuclei involved,accuratemodelingofsiliconburningiscomputationallyexpensive.However,examinationofthephysicsof silicon burning has revealed that the nuclear evolution is dominated by large groups of nuclei in mutual equilibrium. We present a new hybrid equilibrium-network scheme which takes advantage of this quasi-equilibrium in order to reduce the number of independent variables calculated. This allows accurate prediction of the nuclear abundance evolution, deleptonization, and energy generation at a greatly reduced computational cost when compared to a conventional nuclear reaction network. During silicon burning, the resultant QSE-reduced network is approximately an order of magnitude faster than the full network it replaces and requires the tracking of less than a third as many abundance variables, without significant loss of accuracy. These reductions in computational cost and the number of species evolved make QSE-reduced networks well suited for inclusion within hydrodynamic simulations, particularly in multidimensional applications. Subject headingg methods: numerical — nuclear reactions, nucleosynthesis, abundances — stars: evolution — supernovae: general

Nova Nucleosynthesis Calculations: Robust Uncertainties, Sensitivities, and Radioactive Ion Beam Measurements

We examine the quantitative impact of nuclear physics uncertainties on predictions of nova models via Monte Carlo simulations wherein, for the first time, the uncertainties of all relevant nuclear reactions are considered simultaneously. We determine uncertainties in predictions of isotope synthesis ‐ including radioisotopes which may be observable tracers of novae ‐ resulting from uncertainties in the input nuclear physics. We also detail the reaction rate sensitivity of radioisotope production, and discuss reactions which need further study. Finally, we examine the influence on nova nucleosynthesis of two new reaction rates ‐ 17F(p,γ)18Ne and 14O(α,2p)16O ‐ that were studied in recent ORNL measurements with radioactive ion beams.

Measurement of the 18F(p,α)15O cross section at nova energies

Abstract Production of the radioisotope 18F in novae is severely constrained by the rate of the 18F(p,α)15O reaction. A resonance at Ec.m. = 330 keV may strongly enhance the 18F(p,α)15O reaction rate, but its strength has been very uncertain. We have determined the strength of this important resonance by measuring the 18F(p,α)15O cross section on- and off-resonance using a radioactive 18F beam at ORNLs Holifield Radioactive Ion Beam Facility. We find that its resonance strength is 1.48 ± 0.46 eV, and that it dominates the 18F(p,α)15O reaction rate over a significant range of temperatures characteristic of nova outbursts occuring on ONeMg white dwarfs.

Study of the 18F(p,α) 15O Reaction at Energies Relevant for 18F Nucleosynthesis in Novae

Production of the radioisotope 18F in novae is severely constrained by the rate of the 18F(p,α)15O reaction. A resonance at Ec.m. = 330 keV may strongly enhance the 18F(p,α)15O reaction rate, but its strength has been very uncertain. We have determined the strength of this important resonance by measuring the 18F(p,α)15O cross section on‐ and off‐ resonance using a radioactive 18F beam at the ORNL Holifield Radioactive Ion Beam Facility. We find that its resonance strength is 1.48 ± 0.46 eV, and that it dominates the 18F(p,α)15O reaction rate over a wide range of temperatures characteristic of novae.