Engineering topics

Fushiki, Ikko

Publications and source records attributed to Fushiki, Ikko.

4 records

NASA NTRS1994

Search for important weak interaction nuclei in presupernova evolution

A search is made for the most important electron captures and beta-decays after core silicon burning in massive stars. A nuclear statistical equilibrium code is used to compute isotopic abundances. Electron capture and beta-decay rates are estimated for the 150 most abundant isotopes in a simplified fashion which generally includes the strongest transitions. These estimates are made for nuclei in the fp-shell and use techniques similar to Fuller, Fowler, & Newman (1982a), and are compared to them. The general behavior of the rate of change of Y(sub e) is examined. These methods are then used to follow a typical stellar trajectory, seeking the most important weak interactions during the formation of the iron core. Ranked lists of nuclei are given, to prioritize more detailed studies on individual nuclei. Beta-decays are found to ben an important modification to the evolution below the rate of change of Y(sub e) approx. = 0.46 as the core approaches a state of dynamic equilibrium between electron captures and beta-decays.

Aufderheide, Maurice B.↗

NASA NTRS1992

Thermonuclear flash model for long X-ray tails from Aquila X-1

Attention is given to a thermonuclear flash model for long X-ray tails from the recurrent transient Aql X-1, in which the extended phase of nuclear burning is due to the fact that the envelope is out of thermal equilibrium. Only the first X-ray burst emitted by Aql X-1 during its transient outburst exhibits a long X-ray tail. The properties of subsequent bursts are distinguished by a lack of an X-ray tail reflecting the much smaller accumulated masses which result from the effects of thermal inertia in the neutron star envelope. The characteristics of the latter bursts are similar to those of typical X-ray bursters. For a neutron star characterized by a mass and radius of 1.4 solar mass and 9.1 km, respectively, the occurrence of the long X-ray tail requires that the mass of the accumulated layer be less than 10 exp 23 g and that the envelope temperatures of the neutron star be less than 1.5 x 10 exp 7. This interpretation is found to be consistent with the thermal relaxation of the neutron star envelope during the quiescent state of Aql X-1 and with the mass accretion rates inferred for the transient outburst itself.

Fushiki, Ikko↗

NASA NTRS1987

New insights from a global view of X-ray bursts

It is shown here that the behavior of nuclear burning on accreting neutron stars depends on the mass accretion rate per unit area sigma-dot, the temperature T, and the column density of accreted matter sigma, and is thus intrinsically a three-dimensional problem. The concepts of a nuclear fuel surface and an ignition surface in (sigma-dot, T, sigma) space is introduced. It is shown that certain cuts taken through the ignition surface correspond to particular physical situations. It is found that at high temperatures a succession of hydrogen flashes leads eventually to a pure helium flash. At low temperatures, compressional heating completely determines the burst behavior. Burst sources are expected to show highly erratic behavior if they lie near the boundaries of the H and He ignition surfaces, resolving a major discrepancy between the thermonuclear flash model and observations.

Fushiki, Ikko↗

NASA NTRS1987

S-matrix calculation of the triple-alpha reaction

An S-matrix formalism is developed which can be applied to reactions in which electron screening is important, including three-body reactions and reactions involving weak interactions. The various regimes of the triple-alpha reactions are systematically discussed, and a new nonresonant regime at high densities is identified. Using the S-matrix formalism, an analytic expression is obtained for the screened triple-alpha reactions which is accurate for all temperatures and densities. The results are compared with those of Cameron (1959) and Nomoto et al. (1985), and the latter's expression for the unscreened reaction rate is verified. However, it is shown that the reaction rate in the pycnonuclear regime cannot be obtained from the unscreened rate using a screening factor, and that the results of Nomoto et al. therefore cannot be used in this regime.

Fushiki, Ikko↗