The S-Process in Thermally Pulsing Stars

200 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1982.A detailed analysis is undertaken of the synthesis of heavy nuclei by slow neutron capture (the s-process) as it is believed to occur in an astrophysically significant environment: intermediate mass, thermally pulsing stars. A superficial study of the physical conditions of such stars appears to indicate a neutron environment inconsistent with the production of s-process nuclei with the relative abundances which are observed in the Solar System. This analysis is performed in order to confirm or deny this.In order to make a comparison with solar abundances, a detailed reaction network is assembled and time integrated in which 432 nuclear species are simulated. It is found that the discrepancy between the abundance distribution of the manufactured nuclei and that observed in the Solar System disappears when additional effects are incorporated which include the time variability of the neutron density and the possibility that the excited states of a handful of nuclei are not in thermodynaic equilibrium with their ground states.The importance of the time variability of the neutron density means that the basic assumption of constant neutron density, used in the derivation of the classical s-process formalism does not hold in the astrophysically interesting environment found in thermally pulsing stars, thus limiting the use of this formalism in such an environment.This study demonstrates that, when unthermalized nuclear states and a time varying neutron density are included in the calculation of the s-process as it occurs in thermally pulsing stars, the resulting relative abundances are consistent with those found in the Solar System within the (unfortunately large) uncertainties in the input nuclear data; thus indicating that the s-process material in the Solar System may have originated in stars such as these.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD