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2p or not 2p:The 14O(α,2p) 16O Reaction Rate and Its Implications on Nova and X-ray burst Nucleosynthesis

During explosive hydrogen burning, the catalytic CNO cycle suffers depletion by additional thermonuclear capture reactions on the members of the cycle. This “ break out” from the β-limited CNO cycle has been suggested to occur in novae and X-ray bursts for temperatures greater than 0.4 GK [3,26,28]. At these temperatures α captures can occur on the CNO waiting point isotopes through the nuclear reactions O(α,p) F, O(α,γ) Ne, and Ne(α,γ) Na [3]. An additional reaction, O(α,2p)O has been suggested as a possible break out path from the CNO cycle [49]. To investigate the astrophysical significance of the O(α,2p)O reaction for the isotopes that are synthesized during novae outbursts and X-ray bursts, the unknown rate of this new reaction and the accepted rate of O(α,p) F have been varied using a scaling factor from negligible (10 of the O(α,p) F accepted rate), to a value exceeding the accepted O(α,p) F rate by a factor of 10. If this new rate is found to have significant astrophysical implications, it could be investigated with a radioactive beam at the Oak Ridge National Laboratory’s Holifield Radioactive Beam facility. For a nova occurring on a 1.35 M white dwarf, we found that varying the O(α,p) F reaction rate by 12 magnitudes (ranging from 10 to 10 times the accepted rate) has an insignificant effect on the final abundances produced. When the unknown O(α,2p) O reaction rate is added into the reaction rate library with a value assumed equal to the accepted O(α,p) F rate and the rate varied by 12 magnitudes, no significant effect was observed on the final abundances. The effect on the 1.25 M ONeMg white dwarf was even smaller, and still smaller for the 1.00 M C-O white dwarf. For the X-ray burst case, variation of the O(α,p) F reaction rate over 12 magnitudes revealed four distinct groupings of results from the scaled rate. When the rate is reduced by factors between 10 and 10 an enrichment of 20%-30% is observed in the CNO isotopes and A=20-50 mass region when compared to the case using the accepted O(α,p) F rate. The energy production is consistent with that of the calculation with the accepted rate. At factors greater than unity compared to the accepted rate, we observed a 20%-30% depletion in the CNO isotopes. When the O(α, p) F rate is mildly dominant (10 to 10 of the accepted O(α,p) F rate), we observed an enrichment in both the CNO isotopes and the A=20-50 mass region by a factor up to 7, while the A=51-72 mass region is depleted by a factor up to 7. When the O(α, p) F rate is not dominant (10 or less of iv the accepted O(α,p) F rate), we observed a depletion in both CNO isotopes and the A=20-72 mass region by factors up to 60, due to a one second delay in energy production caused by the reduced rate. Ni is a good candidate for observation as depletion and enrichment occurs in the A=51-72 mass region as a function of the scaled rate. When the O(α,2p) O rate is added to the reaction rate library with a value assumed equal to the accepted O(α,p) F rate, and than varied in the same manner as the O(α,p) F rate, we observed three distinct groupings of behavior for X-ray bursts. When the O(α,2p) rate is reduced by a factor of 10 or less, we find no change in nucleosynthesis or energy production when compared to the case using the accepted O(α,p) F rate. When the O(α,2p) O rate is set equal in strength to the O(α,p) F rate, we observed a depletion of less than 10% in O and O. The energy production is consistent with that of the accepted rate. At factors greater than unity compared to the accepted O(α,p) F rate, we observed a depletion of up to 33% and 25%in O and O respectively. The energy production is consistent with that of the known O(α,p) F rate . During the X-ray burst the O(α,p) F reaction is pivotal for energy generation, while the unmeasured O(α,2p) O reaction has an insignificant effect on energy production and final abundances during the X-ray burst.