2p or Not 2p: The ¹?O([alpha],2p) ¹?O reaction rate and its implications on nova and X-ray burst nucleosynthesis

by (Luc Lucius), 1973- Dessieux

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 ?-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 ? captures can occur on the CNO waiting point isotopes through the nuclear reactions 14O(?,p) 17F, 15O(?,?) 19 Ne, and 18 Ne(?,?) 21 Na [3]. An additional reaction, 14 O(?,2p) 16 O has been suggested as a possible break out path from the CNO cycle [49]. To investigate the astrophysical significance of the 14O(?,2p)16O 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 14O(?,p) 17F have been varied using a scaling factor from negligible (10-9 of the 14O(?,p) 17 F accepted rate), to a value exceeding the accepted 14 O(?,p) 17 F rate by a factor of 103. 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 14O(?,p) 17F reaction rate by 12 magnitudes (ranging from 10-9 to 103 times the accepted rate) has an insignificant effect on the final abundances produced. When the unknown 14O(?,2p) 16O reaction rate is added into the reaction rate library with a value assumed equal to the accepted 14O(?,p) 17F 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 14O(?,p) 17F reaction rate over 12 magnitudes revealed four distinct groupings of results from the scaled rate. When the rate is reduced by factors between 10-1 and 10-3 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 14O(?,p) 17F 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 14O(?, p) 17F rate is mildly dominant (10-4 to 10-6 of the accepted 14O(?,p) 17F 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 14O(?, p) 17F rate is not dominant (10-7 or less of iv the accepted 14O(?,p) 17F 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. 56Ni 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 14O(?,2p) 16O rate is added to the reaction rate library with a value assumed equal to the accepted 14O(?,p) 17F rate, and than varied in the same manner as the 14O(?,p) 17F rate, we observed three distinct groupings of behavior for X-ray bursts. When the 14O(?,2p) 16rate is reduced by a factor of 10-1 or less, we find no change in nucleosynthesis or energy production when compared to the case using the accepted 14O(?,p) 17F rate. When the 14O(?,2p) 16 14 17 O rate is set equal in strength to the O(?,p) F rate, we observed a depletion of less than 10% in 14O and 16O. The energy production is consistent with that of the accepted rate. At factors greater than unity compared to the accepted 14O(?,p) 17F rate, we observed a depletion of up to 33% and 25%in 14O and 16O respectively. The energy production is consistent with that of the known 14O(?,p) 17F rate . During the X-ray burst the 14O(?,p) 17F reaction is pivotal for energy generation, while the unmeasured 14O(?,2p) 16O reaction has an insignificant effect on energy production and final abundances during the X-ray burst. v