Synthesis of thorium and uranium in asymptotic giant branch stars

Abstract

Context. The intermediate neutron capture process (i-process) operates at neutron densities between those of the slow and rapid neutron-capture processes. It is believed to be triggered by the ingestion of protons in a convective helium-burning region. One possible astrophysical site is low-mass low-metallicity asymptotic giant branch (AGB) stars. Aims. Although it has been widely believed that actinides, and most particularly Th and U, are exclusively produced by explosive r-process nucleosynthesis, we study here the possibility that actinides may also be significantly synthesized through i-process nucleosynthesis in AGB stars. Methods. We computed a 1 M⊙ model at [Fe/H] = −2.5 with the stellar evolution code STAREVOL. We used a nuclear network of 1160 species from H to Cf coupled to the transport processes. Models of various resolutions (temporal and spatial) that use different nuclear datasets are also considered for the analysis. Results. During the proton ingestion event, the neutron density in our AGB model goes up to ∼1015 cm−3 and is shown to be high enough to give rise to the production of actinides. While most of the nuclear flow cycles in the neutron-rich Pb–Bi–Po region, a non-negligible fraction leaks towards heavier elements and eventually synthesizes actinides. The surface enrichment in Th and U is subject to nuclear and astrophysical model uncertainties that could be lowered in the future, in particular by a detailed analysis of the nuclear inputs that affect the neutron capture rates of neutron-rich isotopes between Pb and Pa, along the i-process path. One stellar candidate that may confirm the production of actinides by the i-process is the carbon-enhanced metal-poor (CEMP) r/s star RAVE J094921.8−161722, which shows Th lines in its spectrum. Its surface abundance is shown to be reasonably well reproduced by our AGB model, though abundances of light N ≃ 50 elements remain underestimated. Combined with cosmochronometry, this finding opens the way to dating the i-process event and thus obtaining a lower limit for the age of CEMP-r/s stars. Such a dating is expected to be accurate only if surface abundances of Th and U can be extracted simultaneously. Conclusions. We show that actinides can be synthesized in low-metallicity low-mass AGB stars through the i-process. This astrophysical site therefore potentially contributes to the Galactic enrichment of Th and U, which demonstrates that the r-process may not be the sole mechanism for the production of U and Th.