41,659 research outputs found
Envelope Expansion with Core Collapse. III. Similarity Isothermal Shocks in a Magnetofluid
We explore MHD solutions for envelope expansions with core collapse (EECC)
with isothermal MHD shocks in a quasi-spherical symmetry and outline potential
astrophysical applications of such magnetized shock flows. MHD shock solutions
are classified into three classes according to the downstream characteristics
near the core. Class I solutions are those characterized by free-fall collapses
towards the core downstream of an MHD shock, while Class II solutions are those
characterized by Larson-Penston (LP) type near the core downstream of an MHD
shock. Class III solutions are novel, sharing both features of Class I and II
solutions with the presence of a sufficiently strong magnetic field as a
prerequisite. Various MHD processes may occur within the regime of these
isothermal MHD shock similarity solutions, such as sub-magnetosonic
oscillations, free-fall core collapses, radial contractions and expansions. We
can also construct families of twin MHD shock solutions as well as an
`isothermal MHD shock' separating two magnetofluid regions of two different yet
constant temperatures. The versatile behaviours of such MHD shock solutions may
be utilized to model a wide range of astrophysical problems, including star
formation in magnetized molecular clouds, MHD link between the asymptotic giant
branch phase to the proto-planetary nebula phase with a hot central magnetized
white dwarf, relativistic MHD pulsar winds in supernova remnants, radio
afterglows of soft gamma-ray repeaters and so forth.Comment: 21 pages, 33 figures, accepted by MNRA
Suppressing longitudinal double-layer oscillations by using elliptically polarized laser pulses in the hole-boring radiation pressure acceleration regime
It is shown that well collimated mono-energetic ion beams with a large
particle number can be generated in the hole-boring radiation pressure
acceleration regime by using an elliptically polarized laser pulse with
appropriate theoretically determined laser polarization ratio. Due to the
effect, the double-layer charge separation region is
imbued with hot electrons that prevent ion pileup, thus suppressing the
double-layer oscillations. The proposed mechanism is well confirmed by
Particle-in-Cell simulations, and after suppressing the longitudinal
double-layer oscillations, the ion beams driven by the elliptically polarized
lasers own much better energy spectrum than those by circularly polarized
lasers.Comment: 6 pages, 5 figures, Phys. Plasmas (2013) accepte
Quark deconfinement phase transition in nuclear matter for improved quark mass density-dependent model
The improved quark mass density-dependent (IQMDD) model, which has been
successfully used to describe the properties of both infinite nuclear matter
and finite nuclei, is applied to investigate the properties of quark
deconfinement phase transition. By using the finite-temperature quantum field
theory, we calculate the finite temperature effective potential and extend the
IQMDD model to finite temperature and finite nuclear matter density. The
critical temperature and the critical density of nuclear matter are given and
the QCD phase diagram is addressed. It is shown that this model can not only
describe the saturation properties of nuclear matter, but also explain the
quark deconfinement phase transition successfully
How Stress Can Reduce Dissipation in Glasses
We propose that stress can decrease the internal friction of amorphous
solids, either by increasing the potential barriers of defects, thus reducing
their tunneling and thermal activation that produce loss, or by decreasing the
coupling between defects and phonons. This stress can be from impurities,
atomic bonding constraints, or externally applied stress. Externally applied
stress also reduces mechanical loss through dissipation dilution. Our results
are consistent with the experiments, and predict that stress could
substantially reduce dielectric loss and increase the thermal conductivity.Comment: 9 pages, 7 figure
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