615 research outputs found
Conserved Linking in Single- and Double-Stranded Polymers
We demonstrate a variant of the Bond Fluctuation lattice Monte Carlo model in
which moves through cis conformations are forbidden. Ring polymers in this
model have a conserved quantity that amounts to a topological linking number.
Increased linking number reduces the radius of gyration mildly. A linking
number of order 0.2 per bond leads to an eight-percent reduction of the radius
for 128-bond chains. This percentage appears to rise with increasing chain
length, contrary to expectation. For ring chains evolving without the
conservation of linking number, we demonstrate a substantial anti-correlation
between the twist and writhe variables whose sum yields the linking number. We
raise the possibility that our observed anti-correlations may have counterparts
in the most important practical polymer that conserves linking number, DNA.Comment: Revised title, minor changes, updated references. 36 pages, including
14 figures. More formats available at
http://rainbow.uchicago.edu/~plewa/webpaper
Dynamics of gas and dust clouds in active galactic nuclei
We analyse the motion of single optically thick clouds in the potential of a
central mass under the influence of an anisotropic radiation field
~|cos(\theta)|, a model applicable to the inner region of active galactic
nuclei. Resulting orbits are analytically soluble for constant cloud column
densities. All stable orbits are closed, although they have non-trivial shapes.
Furthermore, there exists a stability criterion in the form of a critical
inclination, which depends on the luminosity of the central source and the
column density of the cloud.Comment: 4 pages, 3 figures; language corrections, minor formatting change
Linearized nonequilibrium dynamics in nonconformal plasma
We investigate the behaviour of the lowest nonhydrodynamic modes in a class
of holographic models which exhibit an equation of state closely mimicking the
one determined from lattice QCD. We calculate the lowest quasinormal mode
frequencies for a range of scalar self-interaction potentials and find that the
damping of the quasinormal modes at the phase transition/crossover falls off by
a factor of around two from conformality after factoring out standard conformal
temperature dependence. The damping encoded in the imaginary part of the
frequencies turns out to be correlated with the speed of sound and is basically
independent of the UV details of the model. We also find that the dynamics of
the nonhydrodynamic degrees of freedom remains ultralocal, even to a higher
degree, as we deviate from conformality. These results indicate that the role
of nonhydrodynamic degrees of freedom in the vicinity of the crossover
transition may be enhanced
Spectral Signatures of Gravitationally Confined Thermonuclear Supernova Explosions
We consider some of the spectral and polarimetric signatures of the
gravitational confined detonation scenario for Type Ia supernova explosions. In
this model, material produced by an off-center deflagration (which itself fails
to produce the explosion) forms a metal-rich atmosphere above the white dwarf
surface. Using hydrodynamical simulations, we show that this atmosphere is
compressed and accelerated during the subsequent interaction with the supernova
ejecta. This leads ultimately to the formation of a high-velocity pancake of
metal-rich material that is geometrically detached from the bulk of the ejecta.
When observed at the epochs near maximum light, this absorbing pancake produces
a highly blueshifted and polarized calcium IR triplet absorption feature
similar to that observed in several Type~Ia supernovae. We discuss the
orientation effects present in our model and contrast them to those expected in
other supernova explosion models. We propose that a large sample of
spectropolarimetric observations can be used to critically evaluate the
different theoretical scenarios.Comment: 4 pages, 3 figures. To appear in ApJ Letters. For higher resolution
images and movies see http://panisse.lbl.gov/~dnkasen/gcd.htm
The Physics of Wind-Fed Accretion
We provide a brief review of the physical processes behind the radiative
driving of the winds of OB stars and the Bondi-Hoyle-Lyttleton capture and
accretion of a fraction of the stellar wind by a compact object, typically a
neutron star, in detached high-mass X-ray binaries (HMXBs). In addition, we
describe a program to develop global models of the radiatively-driven
photoionized winds and accretion flows of HMXBs, with particular attention to
the prototypical system Vela X-1. The models combine XSTAR photoionization
calculations, HULLAC emission models appropriate to X-ray photoionized plasmas,
improved models of the radiative driving of photoionized winds, FLASH
time-dependent adaptive-mesh hydrodynamics calculations, and Monte Carlo
radiation transport. We present two- and three-dimensional maps of the density,
temperature, velocity, ionization parameter, and emissivity distributions of
representative X-ray emission lines, as well as synthetic global Monte Carlo
X-ray spectra. Such models help to better constrain the properties of the winds
of HMXBs, which bear on such fundamental questions as the long-term evolution
of these binaries and the chemical enrichment of the interstellar medium.Comment: 9 pages including 5 color encapsulated postscript figures; accepted
for inclusion in the proceedings of "Cool Discs, Hot Flows: The Varying Faces
of Accreting Compact Objects," ed. M. Axelsson (New York: AIP); minor
revision which addresses the referee's comments; added Fig. 1 and removed
Fig. 3 and the associated tex
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