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

The rate of the 17F(p,γ)18Ne reaction has a profound effect on the abundances of several isotopes produced during a nova outburst. In 1999 a new rate for 17F(p,γ)18Ne was determined from a measurement of the excitation function for the 1H(17F,p)17F reaction at Oak Ridge National Laboratory\u27s (hereafter ORNL) Holifield Radioactive Ion Beam Facility[1]. This experiment yielded the first definite evidence of a Jπ =3+ state in 18Ne. This state provided a new resonance in the 17F +p capture, which could, depending on its properties, dominate the rate of 17F(p,γ)18Ne at stellar explosive temperatures. The new rate for 17F(p,γ) 18Ne was determined from these parameters and several other resonance parameters that had been previously determined [2]. A nuclear reaction network was used to calculate abundances produced during a nova outburst. The network required the input of an initial abundance profile, a reaction rate library and a set of hydrodynamic trajectories for each nova. The reaction network was run with the new 17F(p,γ)18Ne rate placed in the reaction rate library and also with three previous determination of the rate by Wiescher et al., Sherr et al. and Garcia et al. [3][4][5]. Abundances for 169 isotopes from hydrogen to chromium were calculated. The final abundances produced by each earlier rate were compared to the final abundances produced by the new ORNL rate. This was done for simulations of novae occurring on a 1.35 M ⊙ ONeMg white dwarf, a 1.25 M⊙ ONeMg white dwarf, and a 1.00 M⊙ CO white dwarf. The hotter 1.35 M⊙ white dwarf nova simulation showed the greatest variation in the abundance patterns produced by the four rates. In this simulation, the new ORNL rate changed the abundances of some nuclei, such as 17O, that are synthesized in the hottest zones of the nova by up to 15,000 times, when compared to the network results with the Wiescher rate and up to 4 times, when compared to the network results with the Wiescher rate when all zones of the nova were considered. Similar results were achieved for the ORNL to Wiescher rate comparisons for the l.25 M⊙ WD nova nucleosynthesis calculations, with differences of up to 600 times for the hottest zones and up to 2 times when all zones of the nova were considered. For both the 1.35 M⊙ and 1.25 M⊙ white dwarf nova nucleosynthesis calculations the abundance patterns produced by the networks with the Sherr and Garcia rates were similar to those of the network with the new ORNL rate, with the exception of small differences for a few key isotopes such as 17O and 15N. The 1.00 M⊙ WD nova calculations showed that there was little variation in the abundance patterns produced by the networks with the four rates, even in the hottest zones

The QSE-Reduced Nuclear Reaction Network for Silicon Burning

Iron and neighboring nuclei are formed by silicon burning in massive stars before core collapse and during supernova outbursts. Complete and incomplete silicon burning is responsible for the production of a wide range of nuclei with atomic mass numbers from 28 to 70. Because of the large number of nuclei involved, accurate modeling of these nucleosynthetic stages is computationally expensive. For this reason, hydrodynamic models of supernovae often employ a limited set of nuclei to track the nuclear energy generation until nuclear statistical equilibrium is reached. These limited approximations do not include many of the reaction channels important for the production of iron (Hix & Thielemann, 1996), making them a partial solution at best for energy generation during silicon burning (Timmes et al., 2000). Examination of the physics of silicon burning reveals that the nuclear evolution is dominated by large groups of nuclei in mutual chemical equilibrium before the global Nuclear Statistical Equilibrium (NSE) is reached and after temperatures drop below those needed to maintain NSE during explosive burning (Bodansky, Clayton, & Fowler, 1968). In this work a nuclear reaction network is built which takes advantage of Quasi Statistical Equilibrium (QSE) and NSE at the appropriate temperatures in order to reduce the number of independent variables calculated. This allows accurate prediction of the nuclear abundance evolution, deleptionization, and energy generation. Where conditions apply, the QSE-reduced network runs at least an order of magnitude faster and requires roughly a third as many variables as a standard nuclear reaction network without a significant loss of accuracy. These reductions in computational cost make this network well suited for inclusion within hydrodynamic simulations, particularly in multi-dimensional applications

A New 17F(p,gamma)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 ORNL's Holifield Radioactive Ion Beam Facility, is used to calculate the nucleosynthesis in nova outbursts on the surfaces of 1.25 and 1.35 solar mass ONeMg white dwarfs and a 1.00 solar mass CO white dwarf. We find that the new 17F(p,gamma)18Ne reaction rate changes the abundances of some nuclides (e.g., 17O) synthesized in the hottest zones of an explosion on a 1.35 solar mass white dwarf by more than a factor of 10,000 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 solar mass 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 solar mass 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.Comment: 20 pages, 6 figures, accepted for publication in Ap

The Effects of the pep Nuclear Reaction and Other Improvements in the Nuclear Reaction Rate Library on Simulations of the Classical Nova Outburst

We have continued our studies of the Classical Nova outburst by evolving TNRs on 1.25Msun and 1.35Msun WDs (ONeMg composition) under conditions which produce mass ejection and a rapid increase in the emitted light, by examining the effects of changes in the nuclear reaction rates on both the observable features and the nucleosynthesis during the outburst. In order to improve our calculations over previous work, we have incorporated a modern nuclear reaction network into our hydrodynamic computer code. We find that the updates in the nuclear reaction rate libraries change the amount of ejected mass, peak luminosity, and the resulting nucleosynthesis. In addition, as a result of our improvements, we discovered that the pep reaction was not included in our previous studies of CN explosions. Although the energy production from this reaction is not important in the Sun, the densities in WD envelopes can exceed $10^4$ gm cm$^{-3}$ and the presence of this reaction increases the energy generation during the time that the p-p chain is operating. The effect of the increased energy generation is to reduce the evolution time to the peak of the TNR and, thereby, the accreted mass as compared to the evolutionary sequences done without this reaction included. As expected from our previous work, the reduction in accreted mass has important consequences on the characteristics of the resulting TNR and is discussed in this paper.Comment: Accepted to the Astrophysical Journa

Strength of the 18F(p, α)15O resonance at Ec.m. = 330 keV

The astrophysical rate of the 18F(p,α)15O reaction at nova temperatures is critical to understanding production of the radioisotope 18F, which may be used to constrain nova models via observations with the coming generation of satellite-based γ-ray telescopes. As such, a measurement is made of the strength of this resonance using a radioactive 18F beam at the HRIBF. As a result, it is indicated that the 18F(p,α)15O reaction rate is lower than previous estimates by a factor of ∼2

Description of the 17F(p,gamma)18Ne radiative capture reaction in the continuum shell model

The shell model embedded in the continuum is applied to calculate the astrophysical S-factor and the reaction rate for the radiative proton capture reaction 17F(p,gamma)18Ne. The dominant contribution to the cross-section at very low energies is due to M1 transitions J_i^pi = 2^+ --> J_f^pi = 2_1^+ whose magnitude is controlled by a weakly bound 2_2^+ state at the excitation energy E_x = 3.62 MeV.Comment: 31 pages, latex (uses elsart.cls), 14 figures, submitted to Nuclear Physics

The Practical Obstacles of Data Transfer: Why researchers still love scp General Terms

ABSTRACT The importance of computing facilities is heralded every six months with the announcement of the new Top500 list, showcasing the world's fastest supercomputers. Unfortunately, with great computing capability does not come great long-term data storage capacity, which often means users must move their data to their local site archive, to remote sites where they may be doing future computation or analysis, or back to their home institution, else face the dreaded data purge that most HPC centers employ to keep utilization of large parallel filesystems low to manage performance and capacity. At HPC centers, data transfer is crucial to the scientific workflow and will increase in importance as computing systems grow in size. The Energy Sciences Network (ESnet) recently launched its fifth generation network, a 100 Gbps high-performance, unclassified national network connecting more than 40 DOE research sites to support scientific research and collaboration. Despite the tenfold increase in bandwidth to DOE research sites amenable to multiple data transfer streams and high throughput, in practice, researchers often under-utilize the network and resort to painfully-slow single stream transfer methods such as scp to avoid the complexity of using multiple stream tools such as GridFTP and bbcp, and contend with frustration from the lack of consistency of available tools between sites. In this study we survey and assess the data transfer methods provided at several DOE supported computing facilities, including both leadership-computing facilities, connected through ESnet. We present observed transfer rates, suggested optimizations, and discuss the obstacles the tools must overcome to receive wide-spread adoption over scp