Among the most distinctive and important observables from supernova events are their nucleosynthesis and prodigious neutrino emission. As the source of most intermediate mass elements from oxygen to calcium, perhaps half of the iron-peak elements (including all such nuclei found in young galaxies) and the most probable site for the r-process that produces half of the species heavier than iron, core collapse supernovae are the preeminent foundry of our nuclear heritage. As many supernova and supernova remnant observations are dependent on the composition and distribution of the thermonuclear products in the ejecta, nucleosynthesis provides simultaneously the most important output from and constraint upon the models we are pursuing. Since much of our present understanding of supernova nucleosynthesis is based on models which decouple the nucleosynthesis from the central engine, much remains to be learned. Recent discoveries of the changes wrought by matter made proton-rich by neutrino interactions in the inner portions of the ejecta provide a hint of the changes that come when the nucleosynthesis is coupled to the central engine.
We collaborated with Carla Froehlich, Gabriel Martinez-Pindeo, Matthias Liebendoerfer and Friedel Thielemann on the first investigation of the proton-rich ejecta that is a universal result of spectral neutrino transport models for core collapse supernovae. These studies reveal that proper inclusion of neutrino effects on the nucleosynthesis result in noticeable improvements in the iron peak composition of the ejecta when compared to observations, with a notable reduction in the historically troublesome abundances of neutron-rich Iron and Nickel isotopes and noticeable improvements in the predicted abundances of Chromium, Copper and Zinc. We also discovered a new nucleosynthetic process, the nu-p process, driven by neutrons created by anti-neutrino capture on free protons, that could provide a supernova source for light p-process nuclei. (Publications:1,2,3)
In order to carry studies of supernova nucleosynthesis to our current multi-dimensional supernova models, Ching-Tsai Lee, for his Ph.D. dissertation, developed a tracer particle algorithm for use in CHIMERA, which initially include small thermonuclear reaction networks due to computational limitations. This will facilitate post-processing nucleosynthesis studies of Lagrangian mass elements that would not otherwise be possible in these Eulerian supernova simulations. Preliminary results of post-processing nucleosynthesis using the tracer data showed clear evidence of the nu-p process, but more weakly than Froehlich et al’s results indicate. Subsequent analysis revealed that the strength of the nu-p process is weakly dependent on the entropy of the matter but strongly dependent on the expansion rate. Since the preliminary analysis requires significant extrapolation in time, firm conclusions on the strength of the nu-p process must await completion of the current set of models. We are currently working on implementing the tracer tracking in the 3D version of CHIMERA and improving the efficiency of the 2D implementation.
While post-processing calculations based on tracer particles are an often used technique to explore nucleosynthesis, it has significant drawbacks. The primary limitation in a post-processing approach is the accuracy of the energy generation rate provided by the approximation included within the hydrodynamics. For oxygen, neon and carbon burning in the outer layers of core, the nuclear energy release is a noticeable fraction (10-20%) of the energy imparted by the shock. For the innermost supernova ejecta, the nuclear energy released by the recombination of alpha-particles into iron is even more important, thus there is significant feedback between the rate of this nuclear recombination and the temperature evolution which is driving the recombination and ultimately determining the nucleosynthesis. To improve the fidelity of our nucleosynthesis predictions, and therefore improve our understanding of supernova nucleosynthesis, we need to replace the small reaction networks currently used in supernova simulations with larger, more complete networks. Our work on thermonuclear kinetics is vital to this effort. As part of his Ph.D. dissertation, Austin Chertkow is working on implementing these developments in the CHIMERA code to enable detailed, self-consistent nucleosynthesis in these models that will ultimately allow us to follow-up recent discoveries of the impact that neutrinos have on the supernova ejecta with fully self-consistent simulations.
R-process (Hix with Surman, McLaughlan, Beun, Ruffert, Janka)
We have also collaborated with Rebecca Surman, Gail McLaughlin and others on several investigations of the r-process. One of these studies revealed that for certain species, notably 130Sn, variation of the (n,gamma) reaction rate produced significant changes for a wide range of species in the r-process. This is strong contrast to the result for most alterations of (n,gamma) reaction rates, relatively small, local changes. We have determined that for a neutron capture on a given species to be globally influential requires three factors; a prominent place for that isotope on the r-process path, a long beta decay half-life and a large Q value for the neutron capture. In the case of 130Sn, the first two result in a significant abundance of 130Sn at late times in the r-process while the last causes this reaction to drop out of (n,gamma)(gamma,n) equilibrium early enough that significant numbers of neutron captures can occur out of equilibrium. We are currently expanding our survey to find more reactions whose rate can globally influence the r-process, excellent candidates for measurement at radioactive ion beam facilities. (Publications: 1, 2)
This collaboration also published simulations of the effects of fission cycling on supernova r-process nucleosynthesis, finding that fission cycling can effectively couple the second and third r-process peaks, in rough agreement with the observed correlation of these abundances in metal-poor stars. We have also improved our previous study of the nucleosynthesis produced in the wind from rapidly accreting disks, showing that models with improved neutrino transport allow r-process species to be made in these winds. (Publications: 1, 2, 3)