Engineering topics

Yahil, A.

Publications and source records attributed to Yahil, A..

5 records

NASA NTRS1993

A generalized Zel'dovich approximation to gravitational instability

The orbits of particles undergoing gravitational instability are parameterized by generalizing the Zel'dovich approximation to a series expansion of arbitrary accuracy in the nonlinear regime. The coefficients of this series are determined from an action principle, or, more generally, from moments of the equation of motion. It is shown that the series is more rapidly convergent than previous nonlinear approximations. The method therefore provides a practical means of determining particle orbits, even for highly nonlinear perturbations. As an alternative, we also outline how the nonlinear dynamics may be computed as a field theory in which the evolution of the density and the velocity is determined in fixed comoving Eulerian coordinates.

Giavalisco, M.↗

NASA NTRS1989

Galaxy clustering and the origin of large-scale flows

Peebles's 'cosmic virial theorem' is extended from its original range of validity at small separations, where hydrostatic equilibrium holds, to large separations, in which linear gravitational stability theory applies. The rms pairwise velocity difference at separation r is shown to depend on the spatial galaxy correlation function xi(x) only for x less than r. Gravitational instability theory can therefore be tested by comparing the two up to the maximum separation for which both can reliably be determined, and there is no dependence on the poorly known large-scale density and velocity fields. With the expected improvement in the data over the next few years, however, this method should yield a reliable determination of omega.

Juszkiewicz, R.↗

NASA NTRS1984

Explosive nucleosynthesis in massive stars - Comparison with the Cassiopeia A fast-moving knots

If the ejecta of a Type II supernova do not undergo extensive mixing, then, based on the explosion of current presupernova models, only a small fraction approximately equal to or less than 0.1 solar mass of the mantle of a massive star can yield abundances similar to those observed in the fast-moving knots of Cas A. This is shown to be independent of the detailed structure of the mantle and the supernova energy. Lack of mixing in Cas A is indicated by strong upper limits on the abundance ratios Ne/O, and Fe/O. If this is confirmed by further observations, then either Cas A is not the result of a standard progenitor of approximately equal to or less than 25 solar masses disrupted by a Type II supernova, or the picture of the last stages of stellar evolution in massive stars needs to be modified substantially.

Johnston, M. D.↗

NASA NTRS1976

A new luminosity limit for spherical accretion onto compact X-ray sources

Preheating of infalling gas by emergent X-rays can suppress accretion on to a compact X-ray source. For spherically symmetric accretion, the effect results in a new luminosity limit, orders of magnitude less than the Eddington limit, above which steady flow is impossible. The model may apply to the globular-cluster X-ray sources.

Ostriker, J. P.↗

NASA NTRS1972

On identifying new kinds of astronomical objects.

The evidence for a new compact X-ray object claimed by the Berkeley group of Margon et al (1971) is called into question by Princeton critics on grounds of inadequate statistical analysis. The presented rebuttal of the Berkeley group upholds the correctness of their analytical procedure, but concedes the need for further observations.

Bahcall, J. N.↗