Possible Resonances in the 12C + 12C 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 12C + 12C 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 & Bildsten and Strohmayer & Brown that 12C + 12C triggers superbursts. We then examine the 12C + 12C fusion rate. The reaction cross-section is experimentally unknown at astrophysically relevant energies, but resonances exist in the 12C + 12C 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 ~ 1.5 MeV. For such a hypothetical 1.5 MeV resonance, we derive both a fiducial value and upper limit to the resonance strength 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 12C ignition at a column depth 10^12 g/cm^2 from 6 x 10^8 K to 5 x 10^8 K. Determining the existence of a strong resonance in the Gamow window requires measurements of the 12C + 12C cross-section down to a center-of-mass energy near 1.5 MeV, which is within reach of the proposed DUSEL facility.

13 pages, 4 figures; minor improvements, results and conclusions unchanged, accepted to ApJ

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Cited by in corpus (22)

• Microscopic sub-barrier fusion calculations for the neutron star crust
• New measurement of ${}^{12}$C+${}^{12}$C fusion reaction at astrophysical energies
• A simple analytic model for astrophysical S-factors
• Characterizing the Evolving X-Ray Spectral Features During a Superburst from 4U 1636-536
• ${}^{12}\mathbf{C}+{}^{12}\mathbf{C}$ Fusion ${\mathbf{S}}^{\ast }$-factor from a Full-microscopic Nuclear Model
• Calculation of the 12C+12C sub-barrier fusion cross section in an imaginary time-dependent mean field theory
• Carbon Synthesis in Steady-State Hydrogen and Helium Burning On Accreting Neutron Stars
• Impacts of the direct URCA and Superfluidity inside a Neutron Star on Type-I X-Ray Bursts and X-Ray Superbursts
• Impact of the molecular resonances on the 12C+12C fusion reaction rate
• The modified astrophysical S-factor of the ${}^{12}$C+${}^{12}$C fusion reaction at sub-barrier energies
• Massive stars evolution with new C12+C12 nuclear reaction rate -- the core carbon-burning phase
• Cluster model of 12C in density functional theory framework
• Impact of Pycnonuclear Fusion Uncertainties on the Cooling of Accreting Neutron Star Crusts
• Improved Nuclear Physics Near $A=61$ Refines Urca Neutrino Luminosities in Accreted Neutron Star Crusts
• Nuclear fusion in a dense plasma
• Constraining Accreted Neutron Star Crust Shallow Heating with the Inferred Depth of Carbon Ignition in X-ray Superbursts
• Re-examination of fusion hindrance in astrophysical ${}^{12}$C+${}^{12}$C and ${}^{12}$C+${}^{13}$C reactions
• Net reaction rate and neutrino emissivity for the Urca process in departure from chemical equilibrium
• Electron capture rates in a plasma
• A search for the $835\text{Hz}$ superburst oscillation signal in the regular thermonuclear bursts of 4U 1636-536
• Indirect Measurement of the ${\mathrm{S}}^{\ast }(\mathrm{E})$ Factor for ${}^{12}\mathrm{C}({}^{12}\mathrm{C},p){}^{23}\mathrm{Na}$ at Gamow Energies via the Trojan Horse Method with Near-0 degree Spectator Detection
• ${\mathrm{S}}^{\ast }(\mathrm{E})$ measurement of the ${}^{12}\mathrm{C}({}^{12}\mathrm{C},\alpha ){}^{20}\mathrm{Ne}$ reaction at astrophysical energies via the Trojan horse method with ${}^{16}\mathrm{O}$ quasi-free breakup