Article

Rapid neutron capture process in supernovae and chemical element formation

Journal of Astrophysics and Astronomy (Impact Factor: 0.34). 01/2005; 30(3):165-175. DOI:10.1007/s12036-009-0013-x

ABSTRACT The rapid neutron capture process (r-process) is one of the major nucleosynthesis processes responsible for the synthesis
of heavy nuclei beyond iron. Isotopes beyond Fe are most exclusively formed in neutron capture processes and more heavier
ones are produced by the r-process. Approximately half of the heavy elements with mass number A > 70 and all of the actinides in the solar system are believed to have been produced in the r-process. We have studied the
r-process in supernovae for the production of heavy elements beyond A = 40 with the newest mass values available. The supernova envelopes at a temperature >109 K and neutron density of 1024 cm−3 are considered to be one of the most potential sites for the r-process. The primary goal of the r-process calculations is
to fit the global abundance curve for solar system r-process isotopes by varying time dependent parameters such as temperature
and neutron density. This method aims at comparing the calculated abundances of the stable isotopes with observation. We have
studied the r-process path corresponding to temperatures ranging from 1.0 × 109 K to 3.0 × 109 K and neutron density ranging from 1020 cm−3 to 1030 cm−3. With temperature and density conditions of 3.0 × 109 K and 1020 cm−3 a nucleus of mass 273 was theoretically found corresponding to atomic number 115. The elements obtained along the r-process
path are compared with the observed data at all the above temperature and density range.

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Article: New Nuclear Reaction Flow during r-Process Nucleosynthesis in Supernovae: Critical Role of Light Neutron-Rich Nuclei
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ABSTRACT: We study the role of light neutron-rich nuclei during r-process nucleosynthesis in supernovae. Most previous studies of the r-process have concentrated on the reaction flow of heavy unstable nuclei. Although the nuclear reaction network includes a few thousand heavy nuclei, only limited reaction flow through light-mass nuclei near the stability line has been used in those studies. However, in a viable scenario of the r-process in neutrino-driven winds, the initial condition is a high-entropy hot plasma consisting of neutrons, protons, and electron-positron pairs experiencing an intense flux of neutrinos. In such environments light-mass nuclei as well as heavy nuclei are expected to play important roles in the production of seed nuclei and r-process elements. Thus, we have extended our fully implicit nuclear reaction network so that it includes all nuclei up to the neutron drip line for Z $\leq 10$, in addition to a larger network for Z $\geq 10$. In the present nucleosynthesis study, we utilize a wind model of massive SNeII explosions to study the effects of this extended network. We find that a new nuclear-reaction flow path opens in the very light neutron-rich region. This new nuclear reaction flow can change the final heavy-element abundances by as much as an order of magnitude. Comment: 27 pages, 8 figures
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Article: The prompt-shock supernova mechanism. I - The effect of the free-proton mass fraction and the neutrino transport algorithm
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ABSTRACT: Results are presented of an extensive numerical investigation of the prompt-shock supernova mechanism. This paper focuses on the effect of the free-proton mass fraction Xp given by the equation of state (EOS) and of the various levels of standard neutrino physics on the success of the bounce shock in producing a supernova-like explosion in a prompt manner. When neutrino-electron scattering (NES) is turned off and only e-type neutrinos are transported, vigorous explosions are produced for a wide range of values for Xp. The inclusion of NES considerably restricts the possible values of Xp for successful explosions. The additional inclusion of all neutrino flavors further restricts the possible values of Xp. Successful shocks can still be produced (depending on how the shock energy is defined), but only for very small values of Xp.
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Article: Isotopic r-process abundances and nuclear structure far from stability - Implications for the r-process mechanism
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ABSTRACT: The study employs the full isotopic r-process abundances in nature and a unified model for all nuclear properties involved to uniquely deduce the conditions necessary to produce such an abundance pattern. The nature of the steady-flow equilibrium of beta decays between isotopic chains is also investigated. Strong evidence is found to the effect that a steady flow was not global but only local in between neighboring peaks, which requires time scales not much longer than 1 s. The remaining odd-even effects in observed abundances indicate that neutron densities dropped during freeze-out by orders of magnitude on time scales close to 0.04 s. A set of n(n)-T conditions is presented as a test for any astrophysical r-process site. The way remaining deficiencies in the produced abundance pattern can be used to extract nuclear properties far from stability is also shown.
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