The Site of the rp-Process

Since the rp-process requires the capture of charged particles (protons and alpha particles, primarily) there is a Coulomb barrier and it can only take place at temperatures of order 1 billion K. It is thought that the likely possible sites are nova outbursts, X-ray bursters, and supernova explosions.

Calculations indicate that a supernova probably will not produce a very strong rp-process, so most attention has been focused on novae and X-ray bursts. There is consierable doubt that novae are hot enough, at least for average ones. X-ray bursts produce hot enough conditions, but there is considerable doubt in this case that the proton-rich elements produced could escape the strong gravitational field of the neutron star.

The r-Process (3) ...

What about the isotopes that lie to the left of the stability valley? These are called proton-rich nuclei, because they have more protons relative to neutrons than do nuclei of similar mass in the stability valley. The production of proton-rich nuclei is not as well understood as the production of the stable and neutron-rich nuclei.

The Rapid Proton Capture
or rp-Process
A promising mechanism for producing many of the proton-rich nuclei is called the rp-process. The rp-process is similar to the r-process except that a seed nucleus captures protons (and sometimes alpha particles) rapidly instead of neutrons, and that the relevant beta decays are beta plus decays that convert a proton to a neutron. The expected path of the rp-process is illustrated in the figure shown below (which also gives the s-process and r-process paths). Notice that the rp-process runs very close to the proton drip line, just as the r-process runs very close to the neutron drip line.

As noted in the above right box, the likely environment for an rp-process is a thermonuclear runaway on a white dwarf in a nova, or a similar runaway on a neutron star in an X-ray burst. In both cases, there are unresolved issues in our understanding. Also unresolved at this time is how high in mass the rp-process can go. Here is an animation illustrating the rp-process path in the chart of the nuclides.