2,115 research outputs found
A kinetic model describing the processivity of Myosin-V
The precise details of how myosin-V coordinates the biochemical reactions and mechanical motions of its two head elements to engineer effective processive molecular motion along actin filaments remain unresolved. We compare a quantitative kinetic model of the myosin-V walk, consisting of five basic states augmented by two further states to allow for futile hydrolysis and detachments, with experimental results for run lengths, velocities, and dwell times and their dependence on bulk nucleotide concentrations and external loads in both directions. The model reveals how myosin-V can use the internal strain in the molecule to synchronize the motion of the head elements. Estimates for the rate constants in the reaction cycle and the internal strain energy are obtained by a computational comparison scheme involving an extensive exploration of the large parameter space. This scheme exploits the fact that we have obtained analytic results for our reaction network, e.g., for the velocity but also the run length, diffusion constant, and fraction of backward steps. The agreement with experiment is often reasonable but some open problems are highlighted, in particular the inability of such a general model to reproduce the reported dependence of run length on ADP concentration. The novel way that our approach explores parameter space means that any confirmed discrepancies should give new insights into the reaction network model
On the Field Equations of Kaluza's Theory
The field equations of the original Kaluza's theory are analyzed and it is
shown that they lead to modification of Einstein's equations. The appearing
extra energy-momentum tensor is studied and an example is given where this
extra energy-momentum tensor is shown to allow four-dimensional Schwarzschild
geometry to accommodate electrostatics. Such deviation from Reissner-Nordstrom
geometry can account for the interpretation of Schwarzschild geometry as
resulting not from mass only, but from the combined effects of mass and
electric charge, even electric charge alone.Comment: 14 pages, two sections added and title changed. To appear in Physics
Letters
Bondi Accretion in the Presence of Vorticity
The classical Bondi-Hoyle formula gives the accretion rate onto a point
particle of a gas with a uniform density and velocity. However, the Bondi-Hoyle
problem considers only gas with no net vorticity, while in a real astrophysical
situation accreting gas invariably has at least a small amount of vorticity. We
therefore consider the related case of accretion of gas with constant
vorticity, for the cases of both small and large vorticity. We confirm the
findings of earlier two dimensional simulations that even a small amount of
vorticity can substantially change both the accretion rate and the morphology
of the gas flow lines. We show that in three dimensions the resulting flow
field is non-axisymmetric and time dependent. The reduction in accretion rate
is due to an accumulation of circulation near the accreting particle. Using a
combination of simulations and analytic treatment, we provide an approximate
formula for the accretion rate of gas onto a point particle as a function of
the vorticity of the surrounding gas.Comment: 34 pages, 10 figures, accepted for publication in Ap
How Protostellar Outflows Help Massive Stars Form
We consider the effects of an outflow on radiation escaping from the
infalling envelope around a massive protostar. Using numerical radiative
transfer calculations, we show that outflows with properties comparable to
those observed around massive stars lead to significant anisotropy in the
stellar radiation field, which greatly reduces the radiation pressure
experienced by gas in the infalling envelope. This means that radiation
pressure is a much less significant barrier to massive star formation than has
previously been thought.Comment: 4 pages, 2 figures, emulateapj, accepted for publication in ApJ
Letter
Excursion Sets and Non-Gaussian Void Statistics
Primordial non-Gaussianity (NG) affects the large scale structure (LSS) of
the universe by leaving an imprint on the distribution of matter at late times.
Much attention has been focused on using the distribution of collapsed objects
(i.e. dark matter halos and the galaxies and galaxy clusters that reside in
them) to probe primordial NG. An equally interesting and complementary probe
however is the abundance of extended underdense regions or voids in the LSS.
The calculation of the abundance of voids using the excursion set formalism in
the presence of primordial NG is subject to the same technical issues as the
one for halos, which were discussed e.g. in arXiv:1005.1203. However, unlike
the excursion set problem for halos which involved random walks in the presence
of one barrier , the void excursion set problem involves two barriers
and . This leads to a new complication introduced by what
is called the "void-in-cloud" effect discussed in the literature, which is
unique to the case of voids. We explore a path integral approach which allows
us to carefully account for all these issues, leading to a rigorous derivation
of the effects of primordial NG on void abundances. The void-in-cloud issue in
particular makes the calculation conceptually rather different from the one for
halos. However, we show that its final effect can be described by a simple yet
accurate approximation. Our final void abundance function is valid on larger
scales than the expressions of other authors, while being broadly in agreement
with those expressions on smaller scales.Comment: 28 pages (18+appendices), 7 figures; v2 -- minor changes in sec 3.2,
version published in PR
Euclidean wormholes with Phantom field and Phantom field accompanied by perfect fluid
We study the classical Euclidean wormhole solutions for the gravitational
systems with minimally coupled pure Phantom field and minimally coupled Phantom
field accompanied by perfect fluid. It is shown that such solutions do exist
and then the general forms of the Phantom field potential are obtained for
which there are classical Euclidean wormhole solutions.Comment: 15 pages, major revision with perfect flui
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