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

Author

Luc L. Dessieux, Jr., University of Tennessee - Knoxville

Date of Award

12-2002

Degree Type

Thesis

Degree Name

Master of Science

Major

Physics

Major Professor

Mike Guidry

Committee Members

Soren Sorensen, Michael Smith, C.C. Shih, Raphael Hix

Abstract

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

¹⁴O(α,p) ¹⁷F, ¹⁵O(α,γ) ¹⁹Ne, and ¹⁸Ne(α,γ) ²¹Na [3]. An additional reaction, ¹⁴O(α,2p)¹⁶O has been suggested as a possible break out path from the CNO cycle [49]. To investigate the astrophysical significance of the ¹⁴O(α,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(α,p) ¹⁷F have been varied using a scaling factor from negligible (10^-9 of the ¹⁴O(α,p) ¹⁷F accepted rate), to a value exceeding the accepted ¹⁴O(α,p) ¹⁷F 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[circle symbol] white dwarf, we found that varying the ¹⁴O(α,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(α,2p) ¹⁶O reaction rate is added into the reaction rate library with a value assumed equal to the accepted ¹⁴O(α,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[circle symbol] ONeMg white dwarf was even smaller, and still smaller for the 1.00 M[circle symbol] C-O white dwarf.

For the X-ray burst case, variation of the ¹⁴O(α,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(α,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(α, p) ¹⁷F rate is mildly dominant (10^-4 to 10^-6 of the accepted ¹⁴O(α,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(α, p) ¹⁷F rate is not dominant (10^-7 or less of the accepted ¹⁴O(α,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(α,2p) ¹⁶O rate is added to the reaction rate library with a value assumed equal to the accepted ¹⁴O(α,p) ¹⁷F rate, and than varied in the same manner as the ¹⁴O(α,p) ¹⁷F rate, we observed three distinct groupings of behavior for X-ray bursts. When the ¹⁴O(α,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(α,p) ¹⁷F rate. When the ¹⁴O(α,2p) ¹⁶O rate is set equal in strength to the ¹⁴O(α,p) ¹⁷F 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(α,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(α,p) ¹⁷F rate . During the X-ray burst the ¹⁴O(α,p) ¹⁷F reaction is pivotal for energy generation, while the unmeasured ¹⁴O(α,2p) ¹⁶O reaction has an insignificant effect on energy production and final abundances during the X-ray burst.

Recommended Citation

Dessieux, Jr., Luc L., "2p or not 2p:The 14O(α,2p) 16O Reaction Rate and Its Implications on Nova and X-ray burst Nucleosynthesis. " Master's Thesis, University of Tennessee, 2002.
https://trace.tennessee.edu/utk_gradthes/2056