18,094 research outputs found
Critical behavior of long straight rigid rods on two-dimensional lattices: Theory and Monte Carlo simulations
The critical behavior of long straight rigid rods of length (-mers) on
square and triangular lattices at intermediate density has been studied. A
nematic phase, characterized by a big domain of parallel -mers, was found.
This ordered phase is separated from the isotropic state by a continuous
transition occurring at a intermediate density . Two analytical
techniques were combined with Monte Carlo simulations to predict the dependence
of on , being . The first involves
simple geometrical arguments, while the second is based on entropy
considerations. Our analysis allowed us also to determine the minimum value of
(), which allows the formation of a nematic phase on a
triangular lattice.Comment: 23 pages, 5 figures, to appear in The Journal of Chemical Physic
Diversity Of Short Gamma-Ray Burst Afterglows From Compact Binary Mergers Hosting Pulsars
Short gamma-ray bursts (sGRBs) are widely believed to result from the mergers
of compact binaries. This model predicts an afterglow that bears the
characteristic signatures of a constant, low density medium, including a smooth
prompt-afterglow transition, and a simple temporal evolution. However, these
expectations are in conflict with observations for a non-negligible fraction of
sGRB afterglows. In particular, the onset of the afterglow phase for some of
these events appears to be delayed and, in addition, a few of them exhibit
late- time rapid fading in their lightcurves. We show that these peculiar
observations can be explained independently of ongoing central engine activity
if some sGRB progenitors are compact binaries hosting at least one pulsar. The
Poynting flux emanating from the pulsar companion can excavate a bow-shock
cavity surround- ing the binary. If this cavity is larger than the shock
deceleration length scale in the undisturbed interstellar medium, then the
onset of the afterglow will be delayed. Should the deceleration occur entirely
within the swept-up thin shell, a rapid fade in the lightcurve will ensue. We
identify two types of pulsar that can achieve the conditions necessary for
altering the afterglow: low field, long lived pulsars, and high field pulsars.
We find that a sizable fraction (~20-50%) of low field pulsars are likely to
reside in neutron star binaries based on observations, while their high field
counterparts are not. Hydrodynamical calculations motivated by this model are
shown to be in good agreement with observations of sGRB afterglow lightcurves.Comment: Accepted to ApjL. Direct comparison to observed X-Ray afterglows now
included. 5 Figure
Gas and dust from solar metallicity AGB stars
We study the asymptotic giant branch (AGB) evolution of stars with masses
between . We focus on stars with a solar chemical
composition, which allows us to interpret evolved stars in the Galaxy. We
present a detailed comparison with models of the same chemistry, calculated
with a different evolution code and based on a different set of physical
assumptions. We find that stars of mass experience hot
bottom burning at the base of the envelope. They have AGB lifetimes shorter
than yr and eject into their surroundings gas contaminated
by proton-capture nucleosynthesis, at an extent sensitive to the treatment of
convection. Low mass stars with become
carbon stars. During the final phases the C/O ratio grows to . We find
a remarkable agreement between the two codes for the low-mass models and
conclude that predictions for the physical and chemical properties of these
stars, and the AGB lifetime, are not that sensitive to the modelling of the AGB
phase. The dust produced is also dependent on the mass: low-mass stars produce
mainly solid carbon and silicon carbide dust, whereas higher mass stars produce
silicates and alumina dust. Possible future observations potentially able to
add more robustness to the present results are also discussed.Comment: 27 pages, 24 figures; accepted for publication in MNRA
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