Engineering topics

Eichler, David

Publications and source records attributed to Eichler, David.

5 records

NASA NTRS1992

Inhibition of electron thermal conduction by electromagnetic instabilities

Heat flux inhibition by electromagnetic instabilities in a hot magnetized plasma is investigated. Low-frequency electromagnetic waves become unstable due to anisotropy of the electron distribution function. The chaotic magnetic field thus generated scatters the electrons with a specific effective mean free path. Saturation of the instability due to wave-wave interaction, nonlinear scattering, wave propagation, and collisional damping is considered. The effective mean free path is found self-consistently, using a simple model to estimate saturation level and scattering, and is shown to decrease with the temperature gradient length. The results, limited to the assumptions of the model, are applied to astrophysical systems. For some interstellar clouds the instability is found to be important. Collisional damping stabilizes the plasma, and the heat conduction can be dominated by superthermal electrons.

Levinson, Amir↗

NASA NTRS1989

Nucleosynthesis, neutrino bursts and gamma-rays from coalescing neutron stars

It is pointed out here that neutron-star collisions should synthesize neutron-rich heavy elements, thought to be formed by rapid neutron capture (the r-process). Furthermore, these collisions should produce neutrino bursts and resultant bursts of gamma rays; the latter should comprise a subclass of observable gamma-ray bursts. It is argued that observed r-process abundances and gamma-ray burst rates predict rates for these collisions that are both significant and consistent with other estimates.

Eichler, David↗

NASA NTRS1988

On black widow evolutionary scenarios for binary neutron stars

The scenario whereby the pulsar 1957 + 20 ablates its companion by soft gamma-ray synchrotron emission (Ruderman et al., 1988) is critically examined, with particular regard to how the outflowing material, beginning at photospheric temperatures, is heated through the cooling barrier to coronal temperatures. Assuming the conductivity to be at most the Spitzer value, this consideration is found to constrain the mass flux more than two orders of magnitude more severely than merely considering cooling near the sonic point. This would imply that the ablation scenario fails by a large margin, even if the emission from the pulsar is beamed along the orbital plane.

Eichler, David↗

NASA NTRS1988

Energized winds

The general theory of winds whose energy flux increases along the flow trajectories, but without momentum deposition, is developed. It is shown that the sonic point is generally within the region of significant energy deposition or at its boundary. Assuming that the energy is deposited into a thin layer, it is shown that the jump conditions across the layer imply that the fluid jumps to a Mach number of one just downstream of the deposition layer if the deposited energy per unit area exceeds some critical amount. This result is used to estimate the incident flux needed to excite wind from a stellar surface in terms of the star's mass and the penetration grammage of the incident flux. The expected mass flux from the star bombarded by a 'supercritical' energy flux is calculated, and the results are compared to the measured P-dot/P of Cyg X-3.

Eichler, David↗

NASA NTRS1985

Distributed reacceleration of cosmic rays

A model is developed in which cosmic rays, in addition to their initial acceleration by a strong shock, are continuously reaccelerated while propagating through the Galaxy. The equations describing this acceleration scheme are solved analytically and numerically. Solutions for the spectra of primary and secondary cosmic rays are given in a closed analytic form, allowing a rapid search in parameter space for viable propagation models with distributed reeacceleration included. The observed boron-to-carbon ratio can be reproduced by the reacceleration theory over a range of escape parameters, some of them quite different from the standard leaky-box model. It is also shown that even a very modest amount of reacceleration by strong shocks causes the boron-to-carbon ratio to level off at sufficiently high energies.

Wandel, Amri↗