2,611 research outputs found
Dynamics of membranes with immobile inclusions
Cell membranes are anchored to the cytoskeleton via immobile inclusions. We
investigate the effect of such anchors on the in-plane dynamics of a fluid
membrane and mobile inclusions (proteins) embedded in it. The immobile
particles lead to a decreased diffusion coefficient of mobile ones and suppress
the correlated diffusion of particle pairs. Due to the long-range,
quasi-two-dimensional nature of membrane flows, these effects become
significant at a low area fraction (below one percent) of immobile inclusions.Comment: 5 page
On the Role of Initial Data in the Gravitational Collapse of Inhomogeneous Dust
We consider here the gravitational collapse of a spherically symmetric
inhomogeneous dust cloud described by the Tolman-Bondi models. By studying a
general class of these models, we find that the end state of the collapse is
either a black hole or a naked singularity, depending on the parameters of the
initial density distribution, which are , the initial central density
of the massive body, and , the initial boundary. The collapse ends in a
black hole if the dimensionless quantity constructed out of this
initial data is greater than 0.0113, and it ends in a naked singularity if
is less than this number. A simple interpretation of this result can be
given in terms of the strength of the gravitational potential at the starting
epoch of the collapse.Comment: Original title changed, numerical range of naked singularity
corrected. Plain Tex File. 14 pages. To appear in Physical Review
Weighing Super-Massive Black Holes with Narrow Fe K Line
It has been suggested that the narrow cores of the Fe K emission
lines in Active Galactic Nuclei (AGNs) are likely produced in the torus, the
inner radius of which can be measured by observing the lag time between the
and band flux variations. In this paper we compare the virial products of
the infrared time lags and the narrow Fe K widths for 10 type 1 AGNs
with the black hole masses from other techniques. We find the narrow Fe
K line width is in average 2.6 times broader than
expected assuming an isotropic velocity distribution of the torus at the
distance measured by the infrared lags. We propose the thick disk model of the
torus could explain the observed larger line width. Another possibility is the
contamination by emission from the broad line region or the outer accretion
disk. Alternatively, the narrow iron line might originate from the inner most
part of the obscuring torus within the sublimation radius, while the infrared
emission from outer cooler part. We note the correlation between the black hole
masses based on this new technique and those based on other known techniques is
statistically insignificant. We argue that this could be attributed to the
small sample size and the very large uncertainties in the measurements of iron
K line widths. The next generation of X-ray observatories could help verify the
origin of the narrow iron K line and the reliability of this new
technique.Comment: 12 pages, 2 figures, 2 tables, Science China G, in pres
Monitoring SO2 emission at the Soufriere Hills Volcano: implications for changes in erruptive conditions
FLWINinfo:eu-repo/semantics/publishe
Metal Enrichment in the Reionization Epoch
The presence of elements heavier than helium ("metals") is of fundamental
importance for a large number of astrophysical processes occurring in planet,
star and galaxy formation; it also affects cosmic structure formation and
evolution in several ways. Even a small amount of heavy elements can
dramatically alter the chemistry of the gas, opening the path to complex
molecules. Metals might enhance the ability of the gas to radiate away its
thermal energy, thus favoring the formation of gravitationally bound objects;
they can also condensate in a solid phase (dust grains), partly or totally
blocking radiation from luminous sources. Finally, they represent useful
tracers of energy deposition by stars and probe the physical properties of the
environment by absorption or emission lines. Last, but certainly not least,
life -- as we know it on Earth -- is tightly related to the presence of at
least some of the heavy elements. In this pedagogical review I will concentrate
on the connection between early metal enrichment and cosmic reionization. As we
will see these two processes are intimately connected and their joint study
might turn out to be fundamental in understanding the overall evolution of the
Universe during the first billion years after the Big Bang, an epoch
corresponding to redshifts z>6.Comment: Book chapter in Understanding the Epoch of Cosmic Reionization:
Challenges and Progress, Springer International Publishing, Ed. Andrei
Mesinger, ISBN 978-3-319-21956-1. arXiv admin note: text overlap with
arXiv:astro-ph/0007248 by other author
Physical nature of the central singularity in spherical collapse
We examine here the nature of the central singularity forming in the
spherically symmetric collapse of a dust cloud and it is shown that this is
always a strong curvature singularity where gravitational tidal forces diverge
powerfully. An important consequence is that the nature of the naked
singularity forming in the dust collapse turns out to be stable against the
perturbations in the initial data from which the collapse commences.Comment: Latex file, 11 pages, 2 figures, Updated version to match the
published version in PR
Phase Transitions in Rotating Neutron Stars
As rotating neutron stars slow down, the pressure and the density in the core
region increase due to the decreasing centrifugal forces and phase transitions
may occur in the center. We extract the analytic behavior near the critical
angular velocity , where the phase transitions occur in the center of
a neutron star, and calculate the moment of inertia, angular velocity, rate of
slow down, braking index, etc. For a first order phase transition these
quantities have a characteristic behavior, e.g., the braking index diverges as
. Observational consequences for first, second
and other phase transitions are discussed.Comment: 5 pages, one figure included, revtex latex styl
Evolving Einstein's Field Equations with Matter: The ``Hydro without Hydro'' Test
We include matter sources in Einstein's field equations and show that our
recently proposed 3+1 evolution scheme can stably evolve strong-field
solutions. We insert in our code known matter solutions, namely the
Oppenheimer-Volkoff solution for a static star and the Oppenheimer-Snyder
solution for homogeneous dust sphere collapse to a black hole, and evolve the
gravitational field equations. We find that we can evolve stably static,
strong-field stars for arbitrarily long times and can follow dust sphere
collapse accurately well past black hole formation. These tests are useful
diagnostics for fully self-consistent, stable hydrodynamical simulations in 3+1
general relativity. Moreover, they suggest a successive approximation scheme
for determining gravitational waveforms from strong-field sources dominated by
longitudinal fields, like binary neutron stars: approximate quasi-equilibrium
models can serve as sources for the transverse field equations, which can be
evolved without having to re-solve the hydrodynamical equations (``hydro
without hydro'').Comment: 4 postscript figures. Submitted to Phys. Rev. D15 as a Brief Repor
- …