5,182 research outputs found
Critical behavior of the Widom-Rowlinson mixture: coexistence diameter and order parameter
The critical behavior of the Widom-Rowlinson mixture [J. Chem. Phys. 52, 1670
(1970)] is studied in d=3 dimensions by means of grand canonical Monte Carlo
simulations. The finite size scaling approach of Kim, Fisher, and Luijten
[Phys. Rev. Lett. 91, 065701 (2003)] is used to extract the order parameter and
the coexistence diameter. It is demonstrated that the critical behavior of the
diameter is dominated by a singular term proportional to t^(1-alpha), with t
the relative distance from the critical point, and alpha the critical exponent
of the specific heat. No sign of a term proportional to t^(2beta) could be
detected, with beta the critical exponent of the order parameter, indicating
that pressure-mixing in this model is small. The critical density is measured
to be rho*sigma^3 = 0.7486 +/- 0.0002, with sigma the particle diameter. The
critical exponents alpha and beta, as well as the correlation length exponent
nu, are also measured and shown to comply with d=3 Ising criticality
Massive star evolution : rotation, winds, and overshooting vectors in the Mass-Luminosity plane I. A calibrated grid of rotating single star models
We aim to constrain massive star evolution models using the unique testbed
eclipsing binary HD166734 with new grids of MESA stellar evolution models,
adopting calibrated prescriptions of overshooting, mass loss, and rotation. We
introduce a novel tool: the "mass-luminosity plane" or "M-L plane", as an
equivalent to the traditional HR diagram, utilising it to reproduce the testbed
binary HD166734 with newly calibrated MESA stellar evolution models for single
stars. We can only reproduce the Galactic binary system with an enhanced amount
of core overshooting (alpha = 0.5), mass loss, and rotational mixing. We can
utilise the gradient in the M-L plane to constrain the amount of mass loss to
0.5 - 1.5 times the standard Vink et al. 2001 prescriptions, and we can exclude
extreme reduction or multiplication factors. The extent of the vectors in the
M-L plane leads us to conclude that the amount of core overshooting is larger
than is normally adopted in contemporary massive star evolution models. We
furthermore conclude that rotational mixing is mandatory to get the nitrogen
abundance ratios between the primary and secondary components to be correct
(3:1) in our testbed binary system. Our calibrated grid of models, alongside
our new M-L plane approach, present the possibility of a widened main sequence
due to an increased demand for core overshooting. The increased amount of core
overshooting is not only needed to explain the extended main sequence, but the
enhanced overshooting is also needed to explain the location of the
upper-luminosity limit of the red supergiants. Finally, the increased amount of
core overshooting has -- via the compactness parameter -- implications for
supernova explodibility.Comment: Revised version, 14 pages, 19 figures, accepted for publication by
Astronomy & Astrophysic
Simulation and theory of fluid demixing and interfacial tension of mixtures of colloids and non-ideal polymers
An extension of the Asakura-Oosawa-Vrij model of hard sphere colloids and
non-adsorbing polymers, that takes polymer non-ideality into account through a
repulsive stepfunction pair potential between polymers, is studied with grand
canonical Monte Carlo simulations and density functional theory. Simulation
results validate previous theoretical findings for the shift of the bulk fluid
demixing binodal upon increasing strength of polymer-polymer repulsion,
promoting the tendency to mix. For increasing strength of the polymer-polymer
repulsion, simulation and theory consistently predict the interfacial tension
of the free colloidal liquid-gas interface to decrease significantly for fixed
colloid density difference in the coexisting phases, and to increase for fixed
polymer reservoir packing fraction.Comment: 10 pages, 4 figure
Domain formation in membranes with quenched protein obstacles: Lateral heterogeneity and the connection to universality classes
We show that lateral fluidity in membranes containing quenched protein
obstacles belongs to the universality class of the two-dimensional random-field
Ising model. The main feature of this class is the absence of a phase
transition: there is no critical point, and macroscopic domain formation does
not occur. Instead, there is only one phase. This phase is highly
heterogeneous, with a structure consisting of micro-domains. The presence of
quenched protein obstacles thus provides a mechanism to stabilize lipid rafts
in equilibrium. Crucial for two-dimensional random-field Ising universality is
that the obstacles are randomly distributed, and have a preferred affinity to
one of the lipid species. When these conditions are not met, standard Ising or
diluted Ising universality apply. In these cases, a critical point does exist,
marking the onset toward macroscopic demixing.Comment: 10 pages, 10 figure
The Kinematic and Plasma Properties of X-ray Knots in Cassiopeia A from the Chandra HETGS
We present high-resolution X-ray spectra from the young supernova remnant Cas
A using a 70-ks observation taken by the Chandra High Energy Transmission
Grating Spectrometer (HETGS). Line emission, dominated by Si and S ions, is
used for high-resolution spectral analysis of many bright, narrow regions of
Cas A to examine their kinematics and plasma state. These data allow a 3D
reconstruction using the unprecedented X-ray kinematic results: we derive
unambiguous Doppler shifts for these selected regions, with values ranging
between -2500 and +4000 km/s. Plasma diagnostics of these regions, derived from
line ratios of resolved He-like triplet lines and H-like lines of Si, indicate
temperatures largely around 1 keV, which we model as O-rich reverse-shocked
ejecta. The ionization age also does not vary considerably over these regions
of the remnant. The gratings analysis was complemented by the non-dispersed
spectra from the same dataset, which provided information on emission measure
and elemental abundances for the selected Cas A regions. The derived electron
density of X-ray emitting ejecta varies from 20 to 200 cm^{-3}. The measured
abundances of Mg, Si, S and Ca are consistent with O being the dominant element
in the Cas A plasma. With a diameter of 5 arcmin, Cas A is the largest source
observed with the HETGS to date. We, therefore, describe the technique we use
and some of the challenges we face in the HETGS data reduction from such an
extended, complex object.Comment: 26 pages, 16 figures, evised version (minor changes), accepted for
publication in ApJ (Oct 20 2006
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