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Title: POSSIBLE RESONANCES IN THE {sup 12}C + {sup 12}C FUSION RATE AND SUPERBURST IGNITION

Abstract

Observationally inferred superburst ignition depths are shallower than models predict. We address this discrepancy by reexamining the superburst trigger mechanism. We first explore the hypothesis of Kuulkers et al. that exothermic electron captures trigger superbursts. We find that all electron capture reactions are thermally stable in accreting neutron star oceans and thus are not a viable trigger mechanism. Fusion reactions other than {sup 12}C + {sup 12}C are infeasible as well since the possible reactants either deplete at much shallower depths or have prohibitively large Coulomb barriers. Thus, we confirm the proposal of Cumming and Bildsten and Strohmayer and Brown that {sup 12}C + {sup 12}C triggers superbursts. We then examine the {sup 12}C + {sup 12}C fusion rate. The reaction cross section is experimentally unknown at astrophysically relevant energies, but resonances exist in the {sup 12}C + {sup 12}C system throughout the entire measured energy range. Thus it is likely, and in fact has been predicted, that a resonance exists near the Gamow peak energy E {sup pk} {approx} 1.5 MeV. For such a hypothetical 1.5 MeV resonance, we derive both a fiducial value and upper limit to the resonance strength ({omega}{gamma}){sub R} and find that such a resonancemore » could decrease the theoretically predicted superburst ignition depth by up to a factor of 4; in this case, observationally inferred superburst ignition depths would accord with model predictions for a range of plausible neutron star parameters. Said differently, such a resonance would decrease the temperature required for unstable {sup 12}C ignition at a column depth 10{sup 12} g cm{sup -2} from 6 x 10{sup 8} K to 5 x 10{sup 8} K. A resonance at 1.5 MeV would not strongly affect the ignition density of Type Ia supernovae, but it would lower the temperature at which {sup 12}C ignites in massive post-main-sequence stars. Determining the existence of a strong resonance in the Gamow window requires measurements of the {sup 12}C + {sup 12}C cross section down to a center-of-mass energy near 1.5 MeV, which is within reach of the proposed DUSEL facility.« less

Authors:
 [1];
  1. Kavli Institute for Theoretical Physics, University of California, Santa Barbara, CA 93106 (United States)
Publication Date:
OSTI Identifier:
21333793
Resource Type:
Journal Article
Journal Name:
Astrophysical Journal
Additional Journal Information:
Journal Volume: 702; Journal Issue: 1; Other Information: DOI: 10.1088/0004-637X/702/1/660; Country of input: International Atomic Energy Agency (IAEA); Journal ID: ISSN 0004-637X
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ABUNDANCE; CARBON 12; COULOMB FIELD; CROSS SECTIONS; ELECTRON CAPTURE; MAIN SEQUENCE STARS; MASS; MESONS; MEV RANGE 01-10; NEUTRON STARS; NUCLEOSYNTHESIS; SUPERNOVAE

Citation Formats

Cooper, Randall L, Steiner, Andrew W, and Brown, Edward F. POSSIBLE RESONANCES IN THE {sup 12}C + {sup 12}C FUSION RATE AND SUPERBURST IGNITION. United States: N. p., 2009. Web. doi:10.1088/0004-637X/702/1/660; COUNTRY OF INPUT: INTERNATIONAL ATOMIC ENERGY AGENCY (IAEA).
Cooper, Randall L, Steiner, Andrew W, & Brown, Edward F. POSSIBLE RESONANCES IN THE {sup 12}C + {sup 12}C FUSION RATE AND SUPERBURST IGNITION. United States. https://doi.org/10.1088/0004-637X/702/1/660; COUNTRY OF INPUT: INTERNATIONAL ATOMIC ENERGY AGENCY (IAEA)
Cooper, Randall L, Steiner, Andrew W, and Brown, Edward F. 2009. "POSSIBLE RESONANCES IN THE {sup 12}C + {sup 12}C FUSION RATE AND SUPERBURST IGNITION". United States. https://doi.org/10.1088/0004-637X/702/1/660; COUNTRY OF INPUT: INTERNATIONAL ATOMIC ENERGY AGENCY (IAEA).
@article{osti_21333793,
title = {POSSIBLE RESONANCES IN THE {sup 12}C + {sup 12}C FUSION RATE AND SUPERBURST IGNITION},
author = {Cooper, Randall L and Steiner, Andrew W and Brown, Edward F.},
abstractNote = {Observationally inferred superburst ignition depths are shallower than models predict. We address this discrepancy by reexamining the superburst trigger mechanism. We first explore the hypothesis of Kuulkers et al. that exothermic electron captures trigger superbursts. We find that all electron capture reactions are thermally stable in accreting neutron star oceans and thus are not a viable trigger mechanism. Fusion reactions other than {sup 12}C + {sup 12}C are infeasible as well since the possible reactants either deplete at much shallower depths or have prohibitively large Coulomb barriers. Thus, we confirm the proposal of Cumming and Bildsten and Strohmayer and Brown that {sup 12}C + {sup 12}C triggers superbursts. We then examine the {sup 12}C + {sup 12}C fusion rate. The reaction cross section is experimentally unknown at astrophysically relevant energies, but resonances exist in the {sup 12}C + {sup 12}C system throughout the entire measured energy range. Thus it is likely, and in fact has been predicted, that a resonance exists near the Gamow peak energy E {sup pk} {approx} 1.5 MeV. For such a hypothetical 1.5 MeV resonance, we derive both a fiducial value and upper limit to the resonance strength ({omega}{gamma}){sub R} and find that such a resonance could decrease the theoretically predicted superburst ignition depth by up to a factor of 4; in this case, observationally inferred superburst ignition depths would accord with model predictions for a range of plausible neutron star parameters. Said differently, such a resonance would decrease the temperature required for unstable {sup 12}C ignition at a column depth 10{sup 12} g cm{sup -2} from 6 x 10{sup 8} K to 5 x 10{sup 8} K. A resonance at 1.5 MeV would not strongly affect the ignition density of Type Ia supernovae, but it would lower the temperature at which {sup 12}C ignites in massive post-main-sequence stars. Determining the existence of a strong resonance in the Gamow window requires measurements of the {sup 12}C + {sup 12}C cross section down to a center-of-mass energy near 1.5 MeV, which is within reach of the proposed DUSEL facility.},
doi = {10.1088/0004-637X/702/1/660; COUNTRY OF INPUT: INTERNATIONAL ATOMIC ENERGY AGENCY (IAEA)},
url = {https://www.osti.gov/biblio/21333793}, journal = {Astrophysical Journal},
issn = {0004-637X},
number = 1,
volume = 702,
place = {United States},
year = {Tue Sep 01 00:00:00 EDT 2009},
month = {Tue Sep 01 00:00:00 EDT 2009}
}