Stellar reaction rates and non-r-process nucleosynthesis

6 problems

• The $12\mathrm{C}(\alpha,\gamma)16\mathrm{O}$ rate at helium-burning energies to 10 percent

  Open

  When helium burns in a star, carbon-12 captures a helium nucleus to become oxygen-16 at a rate nobody has measured at the relevant energy. This one rate fixes the carbon-to-oxygen ratio of stars and the masses of the black holes they leave.

• Does deep sub-barrier fusion hindrance set in for 12C+12C?

  Contested

  In many pairs of nuclei, fusion far below the Coulomb barrier (the electric repulsion the nuclei must tunnel through) drops faster than standard models predict, an effect called hindrance. Whether this also happens for two carbon nuclei changes the carbon-burning rate by large factors.

• Resonances in 12C+12C fusion inside the carbon-burning Gamow window

  Contested

  Carbon burning in massive stars and in exploding white dwarfs starts when two carbon-12 nuclei fuse, and the fusion probability may be boosted by narrow resonances (energies where the two nuclei briefly form a long-lived state). Direct and indirect experiments disagree on whether such resonances exist at the stellar energies.

• Why laboratory electron-screening potentials exceed the adiabatic limit

  Open

  The electrons around target atoms partly hide the electric repulsion between colliding nuclei, which raises measured low-energy reaction rates. The measured boost is larger than the maximum that atomic physics seems to allow, and nobody knows why.

• Astrophysical sites of the intermediate neutron-capture (i) process

  Open

  Some old stars show heavy-element patterns that neither the slow nor the rapid neutron-capture process can explain, suggesting an in-between process with medium neutron fluxes. Which kind of star produces this flux is not known.

• Stellar origin of the p-nuclei 92,94Mo and 96,98Ru

  Open

  A few proton-rich isotopes cannot be built by neutron capture, the route that makes most heavy elements, and the leading theory makes them by breaking up heavier nuclei with heat radiation in supernovae. That theory makes far too little molybdenum-92, molybdenum-94, ruthenium-96 and ruthenium-98, which are surprisingly common in the solar system.