Measurement of Radiative Proton Capture on 18F and Implications for Oxygen-Neon Novae

GammaNovaePPeer ReviewedScience
The rate of the 18F(p,g)19Ne reaction affects the final abundance of the gamma-ray observable radioisotope 18F, produced in novae. However, no successful measurement of this reaction exists and the rate used is
calculated from incomplete information on the contributing resonances. Of the two resonances thought to
play a significant role, one has a radiative width estimated from the assumed analogue state in the mirror
nucleus, 19F. The second does not have an analogue state assignment at all, resulting in an arbitrary
radiative width being assumed. Here, we report the first successful direct measurement of the
18F(p,g)19Ne reaction. The strength of the 665 keV resonance (Ex = 7.076 MeV) is found to be over
an order of magnitude weaker than currently assumed in nova models. Reaction rate calculations show
that this resonance therefore plays no significant role in the destruction of 18F at any astrophysical
Author: C. Akers
Journal: Phys. Rev. C







Novae are cataclysmic variables driven by accretion of H-rich material onto a

white-dwarf (WD) star from its low-mass main-sequence binary companion. New

time-domain observational capabilities, such as the Palomar Transient Factory

and Pan-STARRS, have revealed a diversity of their behaviour that should be

theoretically addressed. Nova outbursts depend sensitively on nuclear physics

data, and more readily available nova simulations are needed in order to effectively

prioritize experimental effort in nuclear astrophysics. In this paper we use

the MESA stellar evolution code to construct multicycle nova evolution sequences

with CO WD cores. We explore a range of WD masses and accretion rates as well

as the effect of different cooling times before the onset of accretion. In addition,

we study the dependence on the elemental abundance distribution of accreted

material and convective boundary mixing at the core-envelope interface. Models

with such convective boundary mixing display an enrichment of the accreted envelope

with C and O from the underlying white dwarf that is commensurate with

observations. We compare our results with the previous work and investigate a

new scenario for novae with the 3He-triggered convection.

Author: Pavel Denissenkov, Falk Herwig, Lars Bildsten and Bill Paxton
Journal: Astrophysical Journal




We present a new framework for 1D numerical simulations of novae explosions and nucleosynthesis. It combines the powers of the state-of-the-art MESA stellar evolution code and NuGrid post-processing nucleosynthesis code. It is designed to be used by any interested researcher or student. To make it easy to run the codes and their visualization tools by a non-expert, the framework will include user- friendly shell scripts and detailed instructions on how to use them. It is shown that our framework allows to compute the evolution of CO novae through several consecutive explosions. The framework will soon be extended to include ONe novae and mixing between the accreted envelope and underlying white dwarf. 

Author: Pavel Denisenkov and Falk Herwig
Journal: In preparation

Direct Measurements of Na22(p,γ)Mg23 Resonances and Consequences for Na22 Production in Classical Novae

GammaNovaePPeer ReviewedScienceSodium 22Target

The radionuclide Na22 is a potential astronomical observable that is expected to be produced in classical novae in quantities that depend on the thermonuclear rate of the Na22(p,γ)Mg23 reaction. We have measured the strengths of low-energy Na22(p,γ)Mg23 resonances directly and absolutely using a radioactive Na22 target. We find the strengths of resonances at Ep=213, 288, 454, and 610 keV to be higher than previous measurements by factors of 2.4-3.2, and we exclude important contributions to the rate from proposed resonances at Ep=198, 209, and 232 keV. The Na22 abundances expected in the ejecta of classical novae are reduced by a factor of ≈2.

Author: Sallaska, A. L.; Wrede, C.; García, A.; Storm, D. W.; Brown, T. A. D.; Ruiz, C.; Snover, K. A.; Ottewell, D. F.; Buchmann, L.; Vockenhuber, C.; Hutcheon, D. A.; Caggiano, J. A.
Journal: Physical Review Letters, vol. 105, Issue 15, id. 152501 (2010)

Absolute determination of the Na22(p,γ)Mg23 reaction rate in novae

GammaNovaePPeer ReviewedScienceSodium 22Target

Gamma-ray telescopes in orbit around the earth are searching for evidence of the elusive radionuclide Na22 produced in novae. Previously published uncertainties in the dominant destructive reaction, Na22(p,γ)Mg23, indicated new measurements in the proton energy range of 150 to 300 keV were needed to constrain predictions. We have measured the resonance strengths, energies, and branches directly and absolutely by using protons from the University of Washington accelerator with a specially designed beam line, which included beam rastering and cold vacuum protection of the Na22 implanted targets. The targets, fabricated at TRIUMF-ISAC, displayed minimal degradation over a ~20 C bombardment as a result of protective layers. We avoided the need to know the absolute stopping power, and hence the target composition, by extracting resonance strengths from excitation functions integrated over proton energy. Our measurements revealed that resonance strengths for Ep=213, 288, 454, and 610 keV are stronger by factors of 2.4-3.2 than previously reported. Upper limits have been placed on proposed resonances at 198, 209, and 232 keV. These substantially reduce the uncertainty in the reaction rate. We have re-evaluated the Na22(p,γ) reaction rate, and our measurements indicate the resonance at 213 keV makes the most significant contribution to Na22 destruction in novae. Hydrodynamic simulations including our rate indicate that the expected abundance of Na22 ejecta from a classical nova is reduced by factors between 1.5 and 2, depending on the mass of the white-dwarf star hosting the nova explosion.

Author: Sallaska, A. L.; Wrede, C.; García, A.; Storm, D. W.; Brown, T. A. D.; Ruiz, C.; Snover, K. A.; Ottewell, D. F.; Buchmann, L.; Vockenhuber, C.; Hutcheon, D. A.; Caggiano, J. A.; José, J.
Journal: Physical Review C, vol. 83, Issue 3, id. 034611 (2011)

Direct measurement of the 18F(p,α)15O reaction at nova temperatures

NovaePeer ReviewedScience

The 18F(p,α)15O reaction rate is crucial for understanding the final abundance of 18F predicted by nova models. The γ-ray emission in the first few hours after a nova outburst is expected to be dominated by 511 keV annihilation photons from the decay of 18F, and so understanding its production can provide important constraints on the conditions during the outburst when compared with observations. Results are presented from the lowest-energy direct measurement to date, performed at the Isotope Separator and Accelerator radioactive beam facility at the TRIUMF laboratory, Canada. Cross section measurements at center-of-mass energies of 250, 330, 453, and 673 keV are obtained and the results compared to previous data and R-matrix calculations. The implications for the overall reaction rate in the context of nova explosions have been discussed.

Author: C. E. Beer, A. M. Laird, A. St. J. Murphy, M. A. Bentley, L. Buchman, B. Davids, T. Davinson, C. A. Diget, S. P. Fox, B. R. Fulton, U. Hager, D. Howell, L. Martin, C. Ruiz, G. Ruprecht, P. Salter, C. Vockenhuber, and P. Walden
Journal: Phys. Rev. C 83, 042801(R) (2011) [4 pages]

First direct measurement of the 23Mg(p,γ)24Al reaction

Magnesium 23NovaePeer ReviewedScience

The lowest-energy resonance in the 23Mg(p,γ)24Al reaction, which is dominant at classical nova temperatures, has been measured directly for the first time using the DRAGON recoil spectrometer. The experiment used a radioactive 23Mg beam (mixed within a significantly stronger 23Na beam) of peak intensity 5×107 s-1, at the ISAC facility at TRIUMF. We extract values of ER=485.7-1.8+1.3keV and ωγ=38-15+21 meV from our data (all values in the center-of-mass frame unless otherwise stated). In addition, the experiment prompted a recalculation of the Q value for this reaction based on a revision of the 24Al mass. The effect on the uncertainties in the quantities of ejected 22Na and 26Al from oxygen-neon classical novae is discussed.

Author: Erikson, L. et al.
Journal: Physical Review C

Nuclear Astrophysics at ISAC with DRAGON

NovaeSupernovaeX-Ray Bursts

In his Nobel Prize acceptance speech in 1984, Prof. Willie Fowler of the California Institute of Technology, and the father of nuclear astrophysics said, "Direct measurements on short-lived radioactive nuclei and the excited states of all nuclei are impossible at the present
time. In this connection the production of radioactive ion beams holds great promise for the future"[Fow84]. It has taken a number of years to reach this point but this future promise is now here. The new ISAC facility combined with the DRAGON and TUDA experimental facilities will help realize this future.

Author: See Paper Document