11/15/2023 0 Comments Positron and electron captureRecognizing the pivotal role played by capture process, Fuller et al. The competition (and perhaps equilibrium) between positron captures on neutrons and electron captures on protons is an important ingredient of the modeling of Type-II supernovae. In such conditions, a rather high concentration of positron can be reached from an e - + e +« g + g equilibrium which favor the e- e+ pairs. The positron captures are of key importance in stellar core, especially in high temperatures and low density locations. The importance of electron capture for the presupernova collapse is also discussed in Ref. In the late stage of the star evolution, energies of the electrons are high enough to induce transitions to the GT resonance. The electron captures are strongly influenced by the Gamow-Teller (GT+) transitions. This high Fermi energy of the degenerate electron gas leads to enormous electron capture on nuclei and results in the reduction of the electron to baryon ratio Y e. Electron capture on nuclei takes place in very dense environment of the stellar core where the Fermi energy (chemical potential) of the degenerate electron gas is sufficiently large to overcome the threshold energy given by negative Q values of the reactions involved in the interior of the stars. Stars with mass > 8 after passing through all hydrostatic burning stages develop an onion like structure and produce a collapsing core at the end of their evolution and lead to increased nuclear densities in the stellar core. During the stellar core collapse, the entropy of the stellar core decides whether the electron capture occur on heavy nuclei or on free protons produced in the photodisintegration process. Electron capture and beta decay decide the ultimate fate of the star. At this stage the density of the stellar core is much lower than the nuclear matter density and thus the average volume available to a single nucleus is much greater than that of nuclear volume. When the stellar core attains densities close to 10 9 gcm -3, it consists of heavy nuclei imbued in electrically neutral plasma of electrons, with small fraction of drip neutrons and an even smaller fraction of drip protons. The simulation of the core collapse is very much dependent on the electron capture on heavy nuclides. These two quantities are mainly determined by weak interaction processes. This collapse of the stellar core is very sensitive to the core entropy and to the number of lepton to baryon ratio. Electron capture and photodisintegration processes in the stellar interior cost the core energy by reducing the electron density and as a result the collapse of stellar core is accelerated under its own ferocious gravity. The collapse is very sensitive to the entropy and to the number of leptons per baryons, Y e. Weak interactions deleptonize the core of massive star, determine the final electron fraction ( Y e), and the size of the homologous core. These two processes play a vital role in the evolution of stars. Weak interactions and gravity decide the fate of a star. Keywords: Gamow-Teller strength function Electron and positron capture rates pn-QRPA theory 55Co Brinks hypothesis The electron and positron capture rates are calculated over a wide temperature (0.01×10 9 - 30×10 9K) and density (10 - 10 11 gcm -3) grid. Unlike previous calculations the so-called Brinks hypothesis is not assumed leading to a more realistic estimate of the rates. The enhancement is attributed partly to the liberty of selecting a huge model space, allowing consideration of many more excited states in our rate calculations. Our rates are enhanced compared to the reported shell model rates. B(GT) values to low-lying states are calculated microscopically using the pn-QRPA theory. The spectroscopy of electron capture and emitted neutrinos yields useful information on the physical conditions and stellar core composition. Jameel-Un Nabi Muneeb-Ur Rahman Muhammad Sajjadįaculty of Engineering Sciences, Ghulam Ishaq Khan Institute, of Engineering Sciences and Technology, Topi 23640, Swabi, NWFP, Pakistanĥ5Co is not only present in abundance in presupernova phase but is also advocated to play a decisive role in the core collapse of massive stars. Electron and positron capture rates on 55Co in stellar matter
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