6,768 research outputs found
An Analytical Model for the Triaxial Collapse of Cosmological Perturbations
We present an analytical model for the non-spherical collapse of overdense
regions out of a Gaussian random field of initial cosmological perturbations.
The collapsing region is treated as an ellipsoid of constant density, acted
upon by the quadrupole tidal shear from the surrounding matter. The dynamics of
the ellipsoid is set by the ellipsoid self-gravity and the external quadrupole
shear. Both forces are linear in the coordinates and therefore maintain
homogeneity of the ellipsoid at all times. The amplitude of the external shear
is evolved into the non-linear regime in thin spherical shells that are allowed
to move only radially according to the mass interior to them. We describe how
the initial conditions can be drawn in the appropriate correlated way from a
random field of initial density perturbations. By considering many random
realizations of the initial conditions, we calculate the distribution of shapes
and angular momenta acquired by objects through the coupling of their
quadrupole moment to the tidal shear. The average value of the spin parameter,
0.04, is found to be only weakly dependent on the system mass, the mean
cosmological density, or the initial power spectrum of perturbations, in
agreement with N-body simulations. For the cold dark matter power spectrum,
most objects evolve from a quasi-spherical initial state to a pancake or
filament and then to complete virialization. Low-spin objects tend to be more
spherical. The evolution history of shapes is primarily induced by the external
shear and not by the initial triaxiality of the objects. The statistical
distribution of the triaxial shapes of collapsing regions can be used to test
cosmological models against galaxy surveys on large scales.Comment: 42 pages, Tex, followed by 10 uuencoded figure
Study made of pneumatic high pressure piping materials /10,000 psi/
Evaluations of five types of steel for use in high pressure pneumatic piping systems include tests for impact strength, tensile and yield strengths, elongation and reduction in area, field weldability, and cost. One type, AISI 4615, was selected as most advantageous for extensive use in future flight vehicles
C.V.D. annual report: November 1965 research project RU27-1 :an analogue method for the determination of potential distributions in semiconductor systems
A general method for the solution of the nonlinear
Shockley-Poisson differential equation which
governs the potential distribution in non-degenerate
semiconductor systems is described which can be applied
to the evaluation of depletion layer widths, carrier
densities and capacitance bias relationships of p-n
junction structures.
The method is based upon the use of a particular
type of resistance network analogue and results obtained
for several one and two dimensional configurations are
discussed
Super-luminous X-ray Emission from the Interaction of Supernova Ejecta with Dense Circumstellar Shells
For supernova powered by the conversion of kinetic energy into radiation due
to the interactions of the ejecta with a dense circumstellar shell, we show
that there could be X-ray analogues of optically super-luminous SNe with
comparable luminosities and energetics. We consider X-ray emission from the
forward shock of SNe ejecta colliding into an optically-thin CSM shell, derive
simple expressions for the X-ray luminosity as a function of the circumstellar
shell characteristics, and discuss the different regimes in which the shock
will be radiative or adiabatic, and whether the emission will be dominated by
free-free radiation or line-cooling. We find that even with normal supernova
explosion energies of 10^51 erg, there exists CSM shell configurations that can
liberate a large fraction of the explosion energy in X-rays, producing
unabsorbed X-ray luminosities approaching 10^44 erg/s events lasting a few
months, or even 10^45 erg/s flashes lasting days. Although the large column
density of the circumstellar shell can absorb most of the flux from the initial
shock, the most luminous events produce hard X-rays that are less susceptible
to photoelectric absorption, and can counteract such losses by completely
ionizing the intervening material. Regardless, once the shock traverses the
entire circumstellar shell, the full luminosity could be available to
observers.Comment: Submitted to MNRAS. 12 pages, 4 figure
The Cosmic Microwave Background and the Ionization History of the Universe
Details of how the primordial plasma recombined and how the universe later
reionized are currently somewhat uncertain. This uncertainty can restrict the
accuracy of cosmological parameter measurements from the Cosmic Microwave
Background (CMB). More positively, future CMB data can be used to constrain the
ionization history using observations. We first discuss how current
uncertainties in the recombination history impact parameter constraints, and
show how suitable parameterizations can be used to obtain unbiased parameter
estimates from future data. Some parameters can be constrained robustly,
however there is clear motivation to model recombination more accurately with
quantified errors. We then discuss constraints on the ionization fraction
binned in redshift during reionization. Perfect CMB polarization data could in
principle distinguish different histories that have the same optical depth. We
discuss how well the Planck satellite may be able to constrain the ionization
history, and show the currently very weak constraints from WMAP three-year
data.Comment: Changes to match MNRAS accepted versio
- ā¦