p-nuclei

The favored γ-process (see below) in core-collapse supernovae cannot produce all p-nuclei in sufficient amounts, according to current computer simulations.

[7][11] Appropriate combinations of temperature and proton density of a stellar plasma have to be explored in the search of possible production mechanisms for p-nuclei.

Further parameters are the time available for the nuclear processes, and number and type of initially present nuclides (seed nuclei).

[12] Based on its historical meaning, the term p-process is sometimes used for any process synthesizing p-nuclei, even when no proton captures are involved, but this usage is discouraged.

[13] If a sufficiently intensive source of neutrinos is available, nuclear reactions can directly produce certain nuclides, for example 7Li, 11B, 19F, 138La in core-collapse supernovae.

This quickly moves the nucleosynthesis path from the region of stable nuclei to the very proton-rich side of the chart of nuclides.

It is possible to cover the region of the lightest nuclei up to 56Ni within a second because both proton captures and beta decays are fast.

These are nuclides which both have relatively long half-lives (compared to the process timescale) and can only slowly add another proton (that is, their cross section for (p,γ) reactions is small).

If the conditions required for this rapid proton capture are only present for a short time (the timescale of explosive astrophysical events is of the order of seconds), the waiting points limit or hamper the continuation of the reactions to heavier nuclei.

Variations of the main category rapid proton captures are the rp-, pn-, and νp-processes, which will be briefly outlined below.

This results in a considerable reduction of the time required to build heavy elements and allows an efficient production within seconds.

A hot neutron star is made in the center of such a core-collapse supernova and it radiates neutrinos with high intensity.

The neutrinos interact also with the outer layers of the exploding star and cause nuclear reactions which create 138La, among other nuclei.

Because of the short timescale of the explosion and the high Coulomb barrier of the heavier nuclei, such a νp-process could possibly only produce the lightest p-nuclei.

Which nuclei are made and how much of them depends sensitively on many details in the simulations and also on the actual explosion mechanism of a core-collapse supernova, which still is not completely understood.

The accreted matter is rich in hydrogen (protons) and helium (α particles) and becomes hot enough to allow nuclear reactions.

Details of the possible production of p-nuclei in such supernovae depend sensitively on the composition of the matter accreted from the companion star (the seed nuclei for all subsequent processes).

[7] The consensus model of thermonuclear supernovae postulates that the white dwarf explodes after exceeding the Chandrasekhar limit by the accretion of matter because the contraction and heating ignites explosive carbon burning under degenerate conditions.

Then the outermost layers closely beneath the surface of the white dwarf (containing 0.05 solar masses of matter) exhibit the right conditions for a γ-process.

[7] In a subclass of type Ia supernovae, the so-called subChandrasekhar supernova, the white dwarf may explode long before it reaches the Chandrasekhar limit because nuclear reactions in the accreted matter can already heat the white dwarf during its accretion phase and trigger explosive carbon burning prematurely.

To obtain the observed solar relative abundances, a strongly enhanced s-process seed (by factors of 100-1000 or more) has to be assumed which increases the yield of heavy p-nuclei from the γ-process.

Combined hydrogen and helium burning ignites when the accreted layer of degenerate matter reaches a density of 105–106 g/cm3 and a temperature exceeding 0.2 GK.

It will continue until either all free protons are used up or the burning layer has expanded due to the increase in temperature and its density falls below the one required for the nuclear reactions.

[15] It was shown that the properties of X-ray bursts in the Milky Way can be explained by an rp-process on the surface of accreting neutron stars.