14,625 research outputs found
Radiative Efficiencies of Continuously Powered Blast Waves
We use general arguments to show that a continuously powered radiative blast
wave can behave self similarly if the energy injection and radiation mechanisms
are self similar. In that case, the power-law indices of the blast wave
evolution are set by only one of the two constituent physical mechanisms. If
the luminosity of the energy source drops fast enough, the radiation mechanisms
set the power-law indices, otherwise, they are set by the behavior of the
energy source itself. We obtain self similar solutions for the Newtonian and
the ultra-relativistic limits. Both limits behave self similarly if we assume
that the central source supplies energy in the form of a hot wind, and that the
radiative mechanism is the semi-radiative mechanism of Cohen, Piran & Sari
(1998). We calculate the instantaneous radiative efficiencies for both limits
and find that a relativistic blast wave has a higher efficiency than a
Newtonian one. The instantaneous radiative efficiency depends strongly on the
hydrodynamics and cannot be approximated by an estimate of local microscopic
radiative efficiencies, since a fraction of the injected energy is deposited in
shocked matter. These solutions can be used to calculate Gamma Ray Bursts
afterglows, for cases in which the energy is not supplied instantaneously.Comment: 28 LaTeX pages, including 9 figures and 3 table
Scaling of Magneto-Quantum-Radiative Hydrodynamic Equations: From Laser-produced Plasmas to Astrophysics
We introduce here the equations of magneto-quantum-radiative hydrodynamics.
By rewriting them in a dimensionless form, we obtain a set of parameters that
describe scale-dependent ratios of all the characteristic hydrodynamic
quantities. We discuss how these dimensionless parameters relate to the scaling
between astrophysical observations and laboratory experiments.Comment: 12 page
Formation of Primordial Protostars
The evolution of collapsing metal free protostellar clouds is investigated
for various masses and initial conditions.
We perform hydrodynamical calculations for spherically symmetric clouds
taking account of radiative transfer of the molecular hydrogen lines and the
continuum, as well as of chemistry of the molecular hydrogen.
The collapse is found to proceed almost self-similarly like Larson-Penston
similarity solution.
In the course of the collapse, efficient three-body processes transform
atomic hydrogen in an inner region of \sim 1 M_{\sun} entirely into molecular
form.
However, hydrogen in the outer part remains totally atomic although there is
an intervening transitional layer of several solar masses, where hydrogen is in
partially molecular form.
No opaque transient core is formed although clouds become optically thick to
H collision-induced absorption continuum, since H dissociation
follows successively.
When the central part of the cloud reaches stellar densities (), a very small hydrostatic core (\sim
5 \times 10^{-3} M_{\sun}) is formed and subsequently grows in mass as the
ambient gas accretes onto it.
The mass accretion rate is estimated to be 3.7 \times 10^{-2} M_{\sun}
{\rm yr^{-1}} (M_{\ast}/M_{\sun})^{-0.37}, where is instantaneous
mass of the central core, by using a similarity solution which reproduces the
evolution of the cloud before the core formation.Comment: 20 pages, 5 Postscript figures, uses AAS LaTe
An interpretation of fluctuations in enzyme catalysis rate, spectral diffusion, and radiative component of lifetimes in terms of electric field fluctuations
Time-dependent fluctuations in the catalysis rate ({delta}k(t)) observed in single-enzyme experiments were found in a particular study to have an autocorrelation function decaying on the same time scale as that of spectral diffusion {delta}{omega}0(t). To interpret this similarity, the present analysis focuses on a factor in enzyme catalysis, the local electrostatic interaction energy (E) at the active site and its effect on the activation free energy barrier. We consider the slow fluctuations of the electrostatic interaction energy ({delta}E(t)) as a contributor to {delta}k(t) and relate the latter to {delta}{omega}0(t). The resulting relation between {delta}k(t) and {delta}{omega}0(t) is a dynamic analog of the solvatochromism used in interpreting solvent effects on organic reaction rates. The effect of the postulated {delta}E(t) on fluctuations in the radiative component ({delta}{gamma}Formula(t)) of the fluorescence decay of chromophores in proteins also is examined, and a relation between {delta}{gamma}Formula(t) and {delta}{omega}0(t) is obtained. Experimental tests will determine whether the correlation functions for {delta}k(t), {delta}{omega}0(t), and {delta}{gamma}Formula are indeed similar for any enzyme. Measurements of dielectric dispersion, {varepsilon}({omega}), for the enzyme discussed elsewhere will provide further insight into the correlation function for {delta}E(t). They also will determine whether fluctuations in the nonradiative component {gamma}Formula of the lifetime decay has a different origin, fluctuations in distance for example
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