Rp-process

The rp-process (rapid proton capture process) consists of consecutive proton captures onto seed nuclei to produce heavier elements[1]. It is a nucleosynthesis process and, along with the s process and the r process, may be responsible for the generation of many of the heavy elements present in the universe. However, it is notably different from the other processes mentioned in that it occurs on the proton-rich side of stability as opposed to on the neutron-rich side of stability. The end point of the rp-process (the highest mass element it can create) is not yet well established, but recent research has indicated that in neutron stars it cannot progress beyond tellurium.[2] The rp-process is inhibited by alpha decay, which puts an upper limit on the end point at ¹⁰⁵Te, the lightest observed alpha decaying nuclei[3], though lighter isotopes of tellurium are also believed to be stable and alpha decaying.

Conditions

The process has to occur in very high temperature environments (above 1 x 10⁹ kelvins or 1 gigakelvin) so that the protons can overcome the large coulomb barrier for charged particle reactions. A hydrogen rich environment is also a prerequisite due to the large proton flux needed. The seed nuclei needed for this process to occur are thought to be formed during breakout reactions from the hot CNO cycle. Typically proton capture in the rp-process will compete with (α,p) reactions, as most environments with a high flux of hydrogen are also rich in helium. The time-scale for the rp-process is set by β⁺ decays at or near the proton drip line, because the weak interaction is notoriously slower than the strong interaction and electromagnetic force.

Possible sites

Sites suggested for the rp-process are binary systems where one star is a compact object, either low mass black hole or neutron star. In these systems a star (often a red giant) is feeding material onto its compact partner star. The accreted material is rich in hydrogen and helium because of its origin from the surface layers of the accreting star. Because compact stars have high gravitational fields, the material falls with a high velocity towards the compact star, usually colliding with other accreted material en route, forming an accretion disk. In the case accretion onto a neutron star, as this material slowly builds up on the surface, it will have a high temperature, typically around 1 x 10⁸ K, and it is thus electron degenerate. Eventually, it is believed that thermal instabilities arise in this hot atmosphere, and because of the electron degeneracy of the matter, increases in temperature do not lead to a notable increase in pressure, and so the temperature will continue to rise until it leads to a runaway thermonuclear explosion, which we call the rp-process in this case. Observationally, the rp-process in neutron star binaries is seen as an X-ray burst.

References

1. ^ Lars Bildsten, "Thermonuclear Burning on Rapidly Accreting Neutron Stars" in The Many Faces of Neutron Stars, ed. R. Buccheri, J. van Paradijs, & M. A. Alpar (Kluwer), 419 (1998)

2. ^ Schatz, H.; A. Aprahamian, V. Barnard, L. Bildsten, A. Cumming, M. Ouellette, T. Rauscher, F.-K. Thielemann, and M. Wiescher (April 2001). "End Point of the rp Process on Accreting Neutron Stars". Physical Review Letters 86 (16): 3471–3474. doi:10.1103/PhysRevLett.86.3471. Retrieved on 2006-08-24.

3. ^ Tuli, Jagdish K. (2005). Nuclear Wallet Cards, 7th Ed., National Nuclear Data Center. Retrieved on 2007-08-16.