26 research outputs found
Electron-Neutrino Bremsstrahlung in Electro-Weak Theory
The electron-neutrino bremsstrahlung process has been considered in the
framework of electro-weak theory. The scattering cross section has been
calculated in the center of mass frame and approximated to extreme relativistic
as well as non-relativistic case. The rate of energy-loss via this type of
bremsstrahlung process has been obtained both in non-degenerate and degenerate
region. The effect of this electron-neutrino bremsstrahlung process in
different ranges of temperature and density characterizing the late stages of
stellar evolution has been discussed. It is found from our study that this
bremsstrahlung process is highly important in the non-degenerate region,
although it might have some significant effect in the extreme relativistic
degenerate region.Comment: 18 pages including 4 figures and 1 table; Published in J. Phys
Asymptotic self-similar solutions with a characteristic time-scale
For a wide variety of initial and boundary conditions, adiabatic one
dimensional flows of an ideal gas approach self-similar behavior when the
characteristic length scale over which the flow takes place, , diverges or
tends to zero. It is commonly assumed that self-similarity is approached since
in the limit the flow becomes independent of any characteristic
length or time scales. In this case the flow fields must be of the
form with . We show that
requiring the asymptotic flow to be independent only of characteristic length
scales imply a more general form of self-similar solutions,
with , which includes the
exponential () solutions, . We demonstrate that
the latter, less restrictive, requirement is the physically relevant one by
showing that the asymptotic behavior of accelerating blast-waves, driven by the
release of energy at the center of a cold gas sphere of initial density
, changes its character at large : The flow is
described by , , solutions for
solutions with
diverging at finite time () for , and by exponential
solutions for ( depends on the adiabatic index of
the gas, for ). The properties of the new
solutions obtained here for are analyzed, and self-similar
solutions describing the behavior for are also derived.Comment: Minor corrections, Accepted to Ap
Neutrino Bremsstrahlung Process in highly degenerate magnetized electron gas
In this article the neutrino bremsstrahlung process is considered in presence
of strong magnetic field, though the calculations for this process in absence
of magnetic field are also carried out simultaneously. The electrons involved
in this process are supposed to be highly degenerate and relativistic. The
scattering cross sections and energy loss rates for both cases, in presence and
absence of magnetic field, are calculated in the extreme-relativistic limit.
Two results are compared in the range of temperature K K and magnetic field G at a fixed density
, a typical environment during the cooling of magnetized
neutron star. The interpretation of our result is briefly discussed and the
importance of this process during the stellar evolution is speculated.Comment: 12 pages including 2 figures and 1 tabl
On the stability of accelerating relativistic shock waves
We consider the corrugation instability of the self-similar flow with an
accelerating shock in the highly relativistic regime. We derive the correct
dispersion relation for the proper modes in the self-similar regime, and
conclude that this solution is unstable.Comment: 25 pages, 10 figures. Accepted for publication in the Astrophysical
Journa
The early UV/Optical emission from core-collapse supernovae
We derive a simple approximate model describing the early, hours to days,
UV/optical supernova emission, which is produced by the expansion of the outer
<~0.01 solar mass part of the shock-heated envelope, and precedes the optical
emission driven by radioactive decay. Our model includes an approximate
description of the time dependence of the opacity (due mainly to
recombination), and of the deviation of the emitted spectrum from a black body
spectrum. We show that the characteristics of the early UV/O emission constrain
the radius of the progenitor star, its envelope composition, and the ratio of
the ejecta energy to its mass, E/M. For He envelopes, neglecting the effect of
recombination may lead to an over estimate of progenitor radius by more than an
order of magnitude. We also show that the relative extinction at different
wavelengths may be inferred from the light-curves at these wave-lengths,
removing the uncertainty in the estimate of progenitor radius due to reddening
(but not the uncertainty in E/M due to uncertainty in absolute extinction). The
early UV/O observations of the type Ib SN2008D and of the type IIp SNLS-04D2dc
are consistent with our model predictions. For SN2008D we find progenitor
radius to be approx. 10^11 cm, and an indication that the He envelope contains
a significant C/O fraction.Comment: 18 pages, 13 figures. Expanded discussion of diffusio
Supernova Properties from Shock Breakout X-rays
We investigate the potential of the upcoming LOBSTER space observatory (due
circa 2009) to detect soft X-ray flashes from shock breakout in supernovae,
primarily from Type II events. LOBSTER should discover many SN breakout
flashes, although the number is sensitive to the uncertain distribution of
extragalactic gas columns. X-ray data will constrain the radii of their
progenitor stars far more tightly than can be accomplished with optical
observations of the SN light curve. We anticipate the appearance of blue
supergiant explosions (SN 1987A analogs), which will uncover a population of
these underluminous events. We consider also how the mass, explosion energy,
and absorbing column can be constrained from X-ray observables alone and with
the assistance of optically-determined distances. These conclusions are drawn
using known scaling relations to extrapolate, from previous numerical
calculations, the LOBSTER response to explosions with a broad range of
parameters. We comment on a small population of flashes with 0.2 < z < 0.8 that
should exist as transient background events in XMM, Chandra, and ROSAT
integrations.Comment: 14 pages, 9 figures, accepted by MNRAS, presented at AAS 203rd
meetin