14,209 research outputs found
Probing the dark matter profile of hot clusters and the M-T relation with XMM-Newton
We present results based on XMM-Newton observations of a small sample of hot
galaxy clusters. Making a full use of XMM-Newton's spectro-imaging
capabilities, we have extracted the radial temperature profile and gas density
profile, and with this information, calculated the total mass profile of each
cluster (under the assumption of hydrostatic equilibrium and spherical
symmetry). Comparing the individual scaled total mass profiles, we have probed
the Universality of rich cluster mass profiles over a wide range of radii (from
0.01 to 0.7 the virial radius). We have also tested the shape of cluster mass
profiles by comparing with the predicted profiles from numerical simulations of
hierarchical structure formation. We also derived the local mass-temperature
(M-T) scaling relation over a range of temperature going from 4 to 9 keV, that
we compare with theoretical predictions.Comment: 7 pages, 2 figures, Advances in Space Research in press (proceedings
of the COSPAR 2004 Assembly, Paris
Ion Sizes and Finite-Size Corrections for Ionic-Solvation Free Energies
Free energies of ionic solvation calculated from computer simulations exhibit
a strong system size dependence. We perform a finite-size analysis based on a
dielectric-continuum model with periodic boundary conditions. That analysis
results in an estimate of the Born ion size. Remarkably, the finite-size
correction applies to systems with only eight water molecules hydrating a
sodium ion and results in an estimate of the Born radius of sodium that agrees
with the experimental value.Comment: 2 EPS figure
Ion Pair Potentials-of-Mean-Force in Water
Recent molecular simulation and integral equation results alkali-halide ion
pair potentials-of-mean-force in water are discussed. Dielectric model
calculations are implemented to check that these models produce that
characteristic structure of contact and solvent-separated minima for oppositely
charged ions in water under physiological thermodynamic conditions. Comparison
of the dielectric model results with the most current molecular level
information indicates that the dielectric model does not, however, provide an
accurate description of these potentials-of-mean-force. We note that linear
dielectric models correspond to modelistic implementations of second-order
thermodynamic perturbation theory for the excess chemical potential of a
distinguished solute molecule. Therefore, the molecular theory corresponding to
the dielectric models is second-order thermodynamic perturbation theory for
that excess chemical potential. The second-order, or fluctuation, term raises a
technical computational issue of treatment of long-ranged interactions similar
to the one which arises in calculation of the dielectric constant of the
solvent. It is contended that the most important step for further development
of dielectric models would be a separate assessment of the first-order
perturbative term (equivalently the {\it potential at zero charge} ) which
vanishes in the dielectric models but is generally nonzero. Parameterization of
radii and molecular volumes should then be based of the second-order
perturbative term alone. Illustrative initial calculations are presented and
discussed.Comment: 37 pages and 8 figures. LA-UR-93-420
Calibration of the galaxy cluster M_500-Y_X relation with XMM-Newton
The quantity Y_ X, the product of the X-ray temperature T_ X and gas mass M_
g, has recently been proposed as a robust low-scatter mass indicator for galaxy
clusters. Using precise measurements from XMM-Newton data of a sample of 10
relaxed nearby clusters, spanning a Y_ X range of 10^13 -10^15 M_sun keV, we
investigate the M_500-Y_ X relation. The M_500 - Y_ X data exhibit a power law
relation with slope alpha=0.548 \pm 0.027, close to the self-similar value
(3/5) and independent of the mass range considered. However, the normalisation
is \sim 20% below the prediction from numerical simulations including cooling
and galaxy feedback. We discuss two effects that could contribute to the
normalisation offset: an underestimate of the true mass due to the HE
assumption used in X-ray mass estimates, and an underestimate of the hot gas
mass fraction in the simulations. A comparison of the functional form and
scatter of the relations between various observables and the mass suggest that
Y_ X may indeed be a better mass proxy than T_ X or M_g,500.Comment: 4 pages, 2 figures, accepted for publication in A&
Balancing Local Order and Long-Ranged Interactions in the Molecular Theory of Liquid Water
A molecular theory of liquid water is identified and studied on the basis of
computer simulation of the TIP3P model of liquid water. This theory would be
exact for models of liquid water in which the intermolecular interactions
vanish outside a finite spatial range, and therefore provides a precise
analysis tool for investigating the effects of longer-ranged intermolecular
interactions. We show how local order can be introduced through quasi-chemical
theory. Long-ranged interactions are characterized generally by a conditional
distribution of binding energies, and this formulation is interpreted as a
regularization of the primitive statistical thermodynamic problem. These
binding-energy distributions for liquid water are observed to be unimodal. The
gaussian approximation proposed is remarkably successful in predicting the
Gibbs free energy and the molar entropy of liquid water, as judged by
comparison with numerically exact results. The remaining discrepancies are
subtle quantitative problems that do have significant consequences for the
thermodynamic properties that distinguish water from many other liquids. The
basic subtlety of liquid water is found then in the competition of several
effects which must be quantitatively balanced for realistic results.Comment: 8 pages, 6 figure
Quasi-chemical theory with a soft cutoff
In view of the wide success of molecular quasi-chemical theory of liquids,
this paper develops the soft-cutoff version of that theory. This development
has important practical consequences in the common cases that the packing
contribution dominates the solvation free energy of realistically-modeled
molecules because treatment of hard-core interactions usually requires special
purpose simulation methods. In contrast, treatment of smooth repulsive
interactions is typically straightforward on the basis of widely available
software. This development also shows how fluids composed of molecules with
smooth repulsive interactions can be treated analogously to the molecular-field
theory of the hard-sphere fluid. In the treatment of liquid water,
quasi-chemical theory with soft-cutoff conditioning doesn't change the
fundamental convergence characteristics of the theory using hard-cutoff
conditioning. In fact, hard cutoffs are found here to work better than softer
ones.Comment: 5 pages, 2 figure
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