Hydrogen Burning of the Rare Oxygen Isotopes

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  • March 19, 2019
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  • Buckner, Matthew
    • Affiliation: College of Arts and Sciences, Department of Physics and Astronomy
Abstract
  • At the Laboratory for Experimental Nuclear Astrophysics (LENA), two rare oxygen isotope proton capture studies were performed at low energies---<super>18</super>O(p,γ)<super>19</super>F and <super>17</super>O(p,γ)<super>18</super>F. The goal of each study was to improve thermonuclear reaction rates at stellar plasma temperatures relevant to <super>18</super>O and <super>17</super>O destruction, respectively. The stellar nucleosynthesis temperature regime corresponds to very low proton bombarding energies. At these low energies, the Coulomb barrier suppresses the reaction yield in the laboratory, and environmental backgrounds dominate the detected signal, making it difficult to differentiate the γ-cascade from background. At LENA, the electron cyclotron resonance (ECR) ion source produces intense, low-energy proton beam, and these high currents boost the reaction yield. LENA, a "sea-level" facility dedicated to nuclear astrophysics, also has a coincidence detector setup that reduces environmental background contributions and boosts signal-to-noise. The sensitivity afforded by γγ-coincidence and high beam current allowed these rare oxygen isotope reactions to be probed at energies that correspond to stellar plasma temperatures. For stars with masses between 0.8 M<sub>⊙</sub> < M < 8.0 M<sub>⊙</sub>, nucleosynthesis enters its final phase during the asymptotic giant branch (AGB) stage. This is an evolutionary period characterized by grain condensation that occurs in the stellar atmosphere; the star also experiences significant mass loss during this period of instability. Presolar grain production can often be attributed to this unique stellar environment. A subset of presolar oxide grains features dramatic <super>18</super>O depletion that can not be explained by the standard asymptotic giant star burning stages and dredge-up models. An extra mixing process for low-mass asymptotic giant branch stars, known as <italic>cool bottom processing</italic> (CBP), was used in the literature to explain this and other anomalies. Cool bottom processing can also occur during the red giant branch (RGB) phase, but it is not thought to contribute to the <italic>extreme</italic> <super>18</super>O depletion observed within certain stellar environments and within presolar grain samples. However, intense depletion could result from the <super>18</super>O + <italic>p</italic> processes during cool bottom processing in low-mass AGB stars. A portion of this dissertation describes a study of the <super>18</super>O(p,γ)<super>19</super>F reaction at low energies performed at LENA. Based on these new results, it was found that the resonance at E<sub>R</sub> = 95 keV has a negligible effect on the reaction rate at the temperatures associated with cool bottom processing when compared to the (p,α) reaction. It was also observed that the direct capture S-factor is almost a factor of 2 lower than the previously recommended value at low energies. The product of this research is a new thermonuclear reaction rate for <super>18</super>O(p,γ)<super>19</super>F. These results were published in Buckner <italic>et al</italic>. (2012). Classical novae are thought to be among the dominant sources of <super>17</super>O in the Galaxy. These energetic events produce <super>18</super>F that, as it decays to <super>18</super>O, emits positrons that annihilate with electrons producing 511 keV γ-rays. These emissions occur at timescales that correspond to a transparent nova expansion envelope making their observation possible and important for constraining nova stellar models. The importance of the non-resonant component of the <super>17</super>O(p,γ)<super>18</super>F reaction is well established, and numerous studies have been performed to analyze this reaction. The experimental tools available at LENA, in addition to a novel spectral analysis method, allowed the <super>17</super>O(p,γ)<super>18</super>F reaction to be studied within the classical nova Gamow window, and new total S-factors were measured. The lowest energy <italic>in-beam</italic> <super>17</super>O(p,γ)<super>18</super>F measurement ever made was collected during this experiment. A new direct capture S-factor was determined, and it was confirmed that this S-factor is constant at low energies. The E<sub>R</sub> = 193 and 518 keV resonances were also measured, and new resonance strengths were determined. New <super>17</super>O(p,γ)<super>18</super>F thermonuclear reaction rates are reported within this thesis. The direct capture contribution, combined with updated partial widths and strengths from the literature, improved reaction rate uncertainties at low temperatures and may also impact <super>17</super>O overproduction in asymptotic giant branch stellar models. With the improved direct capture S-factor and new resonance strengths, rate uncertainties at classical nova temperatures decreased.
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  • In Copyright
Advisor
  • Clegg, Thomas
  • Cecil, Gerald
  • Champagne, Arthur
  • Heitsch, Fabian
  • Iliadis, Christian
Degree
  • Doctor of Philosophy
Degree granting institution
  • University of North Carolina at Chapel Hill Graduate School
Graduation year
  • 2014
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  • Chapel Hill, NC
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Date uploaded
  • April 22, 2015

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