20,560 research outputs found
Gamma Limit for Transition Paths of Maximal Probability
Chemical reactions can be modelled via diffusion processes conditioned to
make a transition between specified molecular configurations representing the
state of the system before and after the chemical reaction. In particular the
model of Brownian dynamics - gradient flow subject to additive noise - is
frequently used. If the chemical reaction is specified to take place on a given
time interval, then the most likely path taken by the system is a minimizer of
the Onsager-Machlup functional. The Gamma limit of this functional is
determined in the case where the temperature is small and the transition time
scales as the inverse temperatur
Earth-atmosphere evolution based on the new determination of Devonian atmosphere Ar isotopic composition
The isotopic composition of the noble gases, in particular Ar, in samples of ancient atmosphere trapped in rocks and minerals provides the strongest constraints on the timing and rate of Earth atmosphere formation by degassing of the Earth's interior. We have re-measured the isotopic composition of argon in the Rhynie chert from northeast Scotland using a high precision mass spectrometer in an effort to provide constraints on the composition of Devonian atmosphere. Irradiated chert samples yield 40Ar/36Ar ratios that are often below the modern atmosphere value. The data define a 40Ar/36Ar value of 289.5±0.4 at K/36Ar = 0. Similarly low 40Ar/36Ar are measured in un-irradiated chert samples. The simplest explanation for the low 40Ar/36Ar is the preservation of Devonian atmosphere-derived Ar in the chert, with the intercept value in 40Ar–39Ar–36Ar space representing an upper limit. In this case the Earth's atmosphere has accumulated only 3% (5.1±0.4×1016 mol) of the total 40Ar inventory since the Devonian. The average accumulation rate of 1.27±0.09×108 mol40Ar/yr overlaps the rate over the last 800 kyr. This implies that there has been no resolvable temporal change in the outgassing rate of the Earth since the mid-Palaeozoic despite the likely episodicity of Ar degassing from the continental crust. Incorporating the new Devonian atmosphere 40Ar/36Ar into the Earth degassing model of Pujol et al. (2013) provides the most precise constraints on atmosphere formation so far. The atmosphere formed in the first ∼100 Ma after initial accretion during a catastrophic degassing episode. A significant volume of 40Ar did not start to accumulate in the atmosphere until after 4 Ga which implies that stable K-rich continental crust did not develop until this time
Extreme Ultraviolet Emission in the Fornax Cluster of Galaxies
We present studies of the Extreme Ultraviolet (EUV) emission in the Fornax
cluster of galaxies; a relatively nearby well-studied cluster with X-ray
emitting cluster gas and a very large radio source. We examine both the
large-scale (~size of the X-ray emitting cluster gas), and the small-scale
(<arcmin) emission.
We find that this cluster has large-scale diffuse EUV emission. However, at
the sensitivity level of the existing EUVE data, this emission is due entirely
to the low energy tail of the X-ray emitting gas. We have also examined
small-scale structures in raw EUVE images of this cluster. We find that
small-scale irregularities are present in all raw Deep Survey images as a
result of small-scale detector effects. These effects can be removed by
appropriate flat-fielding. After flat-fielding, the Fornax cluster still shows
a few significant regions of small-scale EUV enhancement. We find that these
are emission from stars and galaxies in the field. We find that at existing
levels of sensitivity, there is no excess EUV emission in the cluster on either
large or small scales.Comment: 6 pages, 3 eps figures, aastex5, Accepted to ApJ
Space Station Freedom coupling tasks: An evaluation of their space operational compatibility
The development of the Space Station Freedom tasks that are compatible with both telerobotic as well as extravehicular activity is a necessary redundancy in order to insure successful day to day operation. One task to be routinely performed aboard Freedom will be the changeout of various quick disconnect fluid connectors. In an attempt to resolve these potentially contradictory issues of compatibility, mock-ups of couplings suitable to both extravehicular as well as telerobotic activity were designed and built. An evaluation performed at the Remote Operator Interaction Laboratory at NASA's Johnson Space Center is discussed, which assessed the prototype couplings as well as three standard coupling designs. Data collected during manual and telerobotic manipulation of the couplings indicated that the custom coupling was in fact shown to be faster to operate and generally preferred over the standard coupling designs
A Magnetic Model of the Tetragonal-Orthorhombic Transition in the Cuprates
It is shown that a quasi two dimensional (layered) Heisenberg antiferromagnet
with fully frustrated interplane couplings ({\it e.g.} on a body-centered
tetragonal lattice) generically exhibits two thermal phase transitions with
lowering temperature -- an upper transition at (``order from disorder
without order'') in which the lattice point-group symmetry is spontaneously
broken, and a lower N\'{e}el transition at at which spin-rotation
symmetry is broken. Although this is the same sequence of transitions observed
in LaCuO, in the Heisenberg model (without additional lattice degrees
of freedom) is much smaller than is observed. The model may
apply to the bilayer cuprate LaCaCuO, in which the transitions are
nearly coincident.Comment: 10 pages, 4 figure
Stability of Filters for the Navier-Stokes Equation
Data assimilation methodologies are designed to incorporate noisy
observations of a physical system into an underlying model in order to infer
the properties of the state of the system. Filters refer to a class of data
assimilation algorithms designed to update the estimation of the state in a
on-line fashion, as data is acquired sequentially. For linear problems subject
to Gaussian noise filtering can be performed exactly using the Kalman filter.
For nonlinear systems it can be approximated in a systematic way by particle
filters. However in high dimensions these particle filtering methods can break
down. Hence, for the large nonlinear systems arising in applications such as
weather forecasting, various ad hoc filters are used, mostly based on making
Gaussian approximations. The purpose of this work is to study the properties of
these ad hoc filters, working in the context of the 2D incompressible
Navier-Stokes equation. By working in this infinite dimensional setting we
provide an analysis which is useful for understanding high dimensional
filtering, and is robust to mesh-refinement. We describe theoretical results
showing that, in the small observational noise limit, the filters can be tuned
to accurately track the signal itself (filter stability), provided the system
is observed in a sufficiently large low dimensional space; roughly speaking
this space should be large enough to contain the unstable modes of the
linearized dynamics. Numerical results are given which illustrate the theory.
In a simplified scenario we also derive, and study numerically, a stochastic
PDE which determines filter stability in the limit of frequent observations,
subject to large observational noise. The positive results herein concerning
filter stability complement recent numerical studies which demonstrate that the
ad hoc filters perform poorly in reproducing statistical variation about the
true signal
Comparative Analysis of Molecular Clouds in M31, M33 and the Milky Way
We present BIMA observations of a 2\arcmin field in the northeastern spiral
arm of M31. In this region we find six giant molecular clouds that have a mean
diameter of 5713 pc, a mean velocity width of 6.51.2 \kms, and a mean
molecular mass of 3.0 1.6 10\Msun. The peak brightness
temperature of these clouds ranges from 1.6--4.2 K. We compare these clouds to
clouds in M33 observed by \citet{wilson90} using the OVRO millimeter array, and
some cloud complexes in the Milky Way observed by \cite{dame01} using the CfA
1.2m telescope. In order to properly compare the single dish data to the
spatially filtered interferometric data, we project several well-known Milky
Way complexes to the distance of Andromeda and simulate their observation with
the BIMA interferometer. We compare the simulated Milky Way clouds with the M31
and M33 data using the same cloud identification and analysis technique and
find no significant differences in the cloud properties in all three galaxies.
Thus we conclude that previous claims of differences in the molecular cloud
properties between these galaxies may have been due to differences in the
choice of cloud identification techniques. With the upcoming CARMA array,
individual molecular clouds may be studied in a variety of nearby galaxies.
With ALMA, comprehensive GMC studies will be feasible at least as far as the
Virgo cluster. With these data, comparative studies of molecular clouds across
galactic disks of all types and between different galaxy disks will be
possible. Our results emphasize that interferometric observations combined with
the use of a consistent cloud identification and analysis technique will be
essential for such forthcoming studies that will compare GMCs in the Local
Group galaxies to galaxies in the Virgo cluster.Comment: Accepted for Publication in the Astrophysical Journa
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