937 research outputs found
Promissory Note (fragment), 16 [?] 1839
https://egrove.olemiss.edu/aldrichcorr_a/1084/thumbnail.jp
Inventory Games
AMS classifications: 90D12, 90B05.inventory management;information;cooperative games;proportional division
Current and fluctuation in a two-state stochastic system under non-adiabatic periodic perturbation
We calculate a current and its fluctuation in a two-state stochastic system
under a periodic perturbation. The system could be interpreted as a channel on
a cell surface or a single Michaelis-Menten catalyzing enzyme. It has been
shown that the periodic perturbation induces so-called pump current, and the
pump current and its fluctuation are calculated with the aid of the geometrical
phase interpretation. We give a simple calculation recipe for the statistics of
the current, especially in a non-adiabatic case. The calculation scheme is
based on the non-adiabatic geometrical phase interpretation. Using the Floquet
theory, the total current and its fluctuation are calculated, and it is
revealed that the average of the current shows a stochastic-resonance-like
behavior. In contrast, the fluctuation of the current does not show such
behavior.Comment: 7 pages, 1 figur
Effects of turbulence and rotation on protostar formation as a precursor to seed black holes
Context. The seeds of the first supermassive black holes may have resulted
from the direct collapse of hot primordial gas in K haloes,
forming a supermassive or quasistar as an intermediate stage.
Aims. We explore the formation of a protostar resulting from the collapse of
primordial gas in the presence of a strong Lyman-Werner radiation background.
Particularly, we investigate the impact of turbulence and rotation on the
fragmentation behaviour of the gas cloud. We accomplish this goal by varying
the initial turbulent and rotational velocities.
Methods. We performed 3D adaptive mesh refinement simulations with a
resolution of 64 cells per Jeans length using the ENZO code, simulating the
formation of a protostar up to unprecedentedly high central densities of
cm, and spatial scales of a few solar radii. To achieve this
goal, we employed the KROME package to improve modelling of the chemical and
thermal processes.
Results. We find that the physical properties of the simulated gas clouds
become similar on small scales, irrespective of the initial amount of
turbulence and rotation. After the highest level of refinement was reached, the
simulations have been evolved for an additional ~5 freefall times. A single
bound clump with a radius of AU and a mass of ~ M is formed at the end of each simulation, marking the onset
of protostar formation. No strong fragmentation is observed by the end of the
simulations, regardless of the initial amount of turbulence or rotation, and
high accretion rates of a few solar masses per year are found.
Conclusions. Given such high accretion rates, a quasistar of
M is expected to form within years.Comment: 18 pages, 7 figures, fixed typos, added references and clarified some
details; accepted for publication in A&
A UV flux constraint on the formation of direct collapse black holes
The ability of metal free gas to cool by molecular hydrogen in primordial
halos is strongly associated with the strength of ultraviolet (UV) flux
produced by the stellar populations in the first galaxies. Depending on the
stellar spectrum, these UV photons can either dissociate molecules
directly or indirectly by photo-detachment of as the latter
provides the main pathway for formation in the early universe. In
this study, we aim to determine the critical strength of the UV flux above
which the formation of molecular hydrogen remains suppressed for a sample of
five distinct halos at by employing a higher order chemical solver and a
Jeans resolution of 32 cells. We presume that such flux is emitted by PopII
stars implying atmospheric temperatures of ~K. We performed
three-dimensional cosmological simulations and varied the strength of the UV
flux below the Lyman limit in units of . Our findings show that the
value of varies from halo to halo and is sensitive to the
local thermal conditions of the gas. For the simulated halos it varies from
400-700 with the exception of one halo where .
This has important implications for the formation of direct collapse black
holes and their estimated population at z > 6. It reduces the number density of
direct collapse black holes by almost three orders of magnitude compared to the
previous estimates.Comment: 10 pages, 6 figures, matches the accepted version to ber published in
MNRAS, higher resolution version is available at
http://www.astro.physik.uni-goettingen.de/~mlatif/Jcrit.pd
Nucleation dynamics in 2d cylindrical Ising models and chemotaxis
The aim of our work is to study the effect of geometry variation on
nucleation times and to address its role in the context of eukaryotic
chemotaxis (i.e. the process which allows cells to identify and follow a
gradient of chemical attractant). As a first step in this direction we study
the nucleation dynamics of the 2d Ising model defined on a cylindrical lattice
whose radius changes as a function of time. Geometry variation is obtained by
changing the relative value of the couplings between spins in the compactified
(vertical) direction with respect to the horizontal one. This allows us to keep
the lattice size unchanged and study in a single simulation the values of the
compactification radius which change in time. We show, both with theoretical
arguments and numerical simulations that squeezing the geometry allows the
system to speed up nucleation times even in presence of a very small energy gap
between the stable and the metastable states. We then address the implications
of our analysis for directional chemotaxis. The initial steps of chemotaxis can
be modelled as a nucleation process occurring on the cell membrane as a
consequence of the external chemical gradient (which plays the role of energy
gap between the stable and metastable phases). In nature most of the cells
modify their geometry by extending quasi-onedimensional protrusions (filopodia)
so as to enhance their sensitivity to chemoattractant. Our results show that
this geometry variation has indeed the effect of greatly decreasing the
timescale of the nucleation process even in presence of very small amounts of
chemoattractants.Comment: 27 pages, 6 figures and 2 table
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