Non-thermal nucleosynthesis by accelerated particles to account for the surface composition of P-rich stars

Context. Fifteen abnormally phosphorus-rich stars were recently discovered. Their peculiar surface abundance distribution is challenging our present stellar nucleosynthesis theories, because none of the standard thermal nucleosynthesis models are found to explain the observed patterns. Aims. This paper presents an exploration of the extent to which an irradiation process resulting from the interaction of some target material with energetic protons and/or alpha-particles can by itself be at the origin of the chemical pollution observed at the surface of P-rich stars. Methods. In light of our lack of knowledge of the characteristics of the accelerated particles that could be held responsible for this nuclear process, a purely parametric site-independent approach is followed, with the proton and alpha-particle flux amplitude, energy distribution, and fluence taken as free parameters. The irradiated material is assumed to be made either of CNO elements in solar ratio or pure C. Results. Such an irradiation process with energies of no more than a few MeV per nucleon is shown to give rise to rich nucleosynthesis, including significant production of P, as well as Z > 30 heavy elements in relative abundance similar to what the slow neutron-capture process traditionally produced. Conclusions. The final composition obtained by mixing such a non-thermal nucleosynthesis by accelerated particles with nuclearly unaffected material is found to reproduce fairly well the global surface composition of P-rich stars, except for a few species like Al, Si, or Ba.

Automated summary

This paper investigates the origin of chemical pollution on the surface of 15 abnormally phosphorus-rich stars. It suggests that an irradiation process resulting from the interaction of high-energy protons and/or alpha-particles with target material may be responsible for the observed chemical pollution. The study finds that this irradiation process can produce significant amounts of phosphorus and other heavy elements, similar to what the traditional slow neutron-capture process produces, except for a few species that do not match with the overall surface composition of phosphorus-rich stars.