28,612 research outputs found
Metastable liquid-liquid coexistence and density anomalies in a core-softened fluid
Linearly-sloped or `ramp' potentials belong to a class of core-softened
models which possess a liquid-liquid critical point (LLCP) in addition to the
usual liquid-gas critical point. Furthermore they exhibit thermodynamic
anomalies in the density and compressibility, the nature of which may be akin
to those occurring in water. Previous simulation studies of ramp potentials
have focused on just one functional form, for which the LLCP is
thermodynamically stable. In this work we construct a series of ramp
potentials, which interpolate between this previously studied form and a
ramp-based approximation to the Lennard-Jones (LJ) potential. By means of Monte
Carlo simulation, we locate the LLCP, the first order high density liquid
(HDL)-low density liquid (LDL) coexistence line, and the line of density maxima
for a selection of potentials in the series. We observe that as the LJ limit is
approached, the LLCP becomes metastable with respect to freezing into a
hexagonal close packed crystalline solid. The qualitative nature of the phase
behaviour in this regime shows a remarkable resemblance to that seen in
simulation studies of accurate water models. Specifically, the density of the
liquid phase exceeds that of the solid; the gradient of the metastable LDL-HDL
line is negative in the pressure (p)-temperature (T) plane; while the line of
density maxima in the p-T plane has a shape similar to that seen in water and
extends well into the {\em stable} liquid region of the phase diagram. As such,
our results lend weight to the `second critical point' hypothesis as an
explanation for the anomalous behaviour of water.Comment: 7 pages, 8 figure
Advanced Gas Turbine (AGT): Power-train system development
Technical work on the design and effort leading to the testing of a 74.5 kW (100 hp) automotive gas turbine is described. The general effort was concentrated on building an engine for test starting in July. The buildup progressed with only routine problems and the engine was delivered to the test stand 9 July. In addition to the engine build effort, work continued in selected component areas. Ceramic turbine parts were built and tested. Burst tests of ceramic rotors show strengths are approaching that achieved in test bars; proof testing is required for acceptable strength ceramic vanes. Over 25 hours was accumulated on the combustor rig in three test modes: pilot nozzle only, start nozzle, and main nozzle operation. Satisfactory ignition was achieved for a wide range of starting speeds and the lean blowout limit was as low as 0.06 kg/b (0.14 lb/hr). Lean blowout was more a function of nozzle atomization than fuel/air ratio. A variety of cycle points were tested. Transition from start nozzle flow to main nozzle flow was done manually without difficulty. Regenerator parts were qualification tested without incident and the parts were assembled on schedule. Rig based performance matched first build requirements. Repeated failures in the harmonic drive gearbox during rig testing resulted in that concept being abandoned for an alternate scheme
XMM-Newton Observations of NGC 507: Super-solar Metal Abundances in the Hot ISM
We present the results of the X-ray XMM-Newton observations of NGC 507, a
dominant elliptical galaxy in a small group of galaxies, and report
'super-solar' metal abundances of both Fe and a-elements in the hot ISM of this
galaxy. We find Z_Fe = 2-3 times solar inside the D25 ellipse of NGC 507. This
is the highest Z_Fe reported so far for the hot halo of an elliptical galaxy;
this high Iron abundance is fully consistent with the predictions of stellar
evolution models, which include the yield of both type II and Ia supernovae.
The spatially resolved, high quality XMM spectra provide enough statistics to
formally require at least three emission components: two soft thermal
components indicating a range of temperatures in the hot ISM, plus a harder
component, consistent with the integrated output of low mass X-ray binaries
(LMXBs). The abundance of a-elements (most accurately determined by Si) is also
found to be super-solar. The a-elements to Fe abundance ratio is close to the
solar ratio, suggesting that ~70% of the Iron mass in the hot ISM was
originated from SNe Type Ia. The a-element to Fe abundance ratio remains
constant out to at least 100 kpc, indicating that SNe Type II and Ia ejecta are
well mixed in a scale much larger than the extent of the stellar body.Comment: 29 pages, 6 figures, Accepted in ApJ (v613, Oct. 1, 2004); Minor
revisions after referee's comments; A high-resolution pdf file available at
http://hea-www.harvard.edu/~kim/pap/N507_XMM.pd
A discrete time relativistic Toda lattice
Four integrable symplectic maps approximating two Hamiltonian flows from the
relativistic Toda hierarchy are introduced. They are demostrated to belong to
the same hierarchy and to examplify the general scheme for symplectic maps on
groups equiped with quadratic Poisson brackets. The initial value problem for
the difference equations is solved in terms of a factorization problem in a
group. Interpolating Hamiltonian flows are found for all the maps.Comment: 32 pages, LaTe
Ultraviolet Imaging Polarimetry of the Large Magellanic Cloud. II. Models
Motivated by new sounding-rocket wide-field polarimetric images of the Large
Magellanic Cloud, we have used a three-dimensional Monte Carlo radiation
transfer code to investigate the escape of near-ultraviolet photons from young
stellar associations embedded within a disk of dusty material (i.e. a galaxy).
As photons propagate through the disk, they may be scattered or absorbed by
dust. Scattered photons are polarized and tracked until they escape to be
observed; absorbed photons heat the dust, which radiates isotropically in the
far-infrared, where the galaxy is optically thin. The code produces four output
images: near- UV and far-IR flux, and near-UV images in the linear Stokes
parameters Q and U. From these images we construct simulated UV polarization
maps of the LMC. We use these maps to place constraints on the star + dust
geometry of the LMC and the optical properties of its dust grains. By tuning
the model input parameters to produce maps that match the observed polarization
maps, we derive information about the inclination of the LMC disk to the plane
of the sky, and about the scattering phase function g. We compute a grid of
models with i = 28 deg., 36 deg., and 45 deg., and g = 0.64, 0.70, 0.77, 0.83,
and 0.90. The model which best reproduces the observed polarization maps has i
= 36 +2/-5 degrees and g ~0.7. Because of the low signal-to-noise in the data,
we cannot place firm constraints on the value of g. The highly inclined models
do not match the observed centro-symmetric polarization patterns around bright
OB associations, or the distribution of polarization values. Our models
approximately reproduce the observed ultraviolet photopolarimetry of the
western side of the LMC; however, the output images depend on many input
parameters and are nonunique.Comment: Accepted to AJ. 20 pages, 7 figure
A Natural Solution to the Neutrino Mixing Problem
The combined requirements, of (i) a natural solution to the fermion mass
hierarchy problem and (ii) an explanation of both the atmospheric and solar
neutrino problems, lead to an essentially unique picture of neutrino masses and
mixing angles. The electron and muon neutrinos are quasi-degenerate in mass
with maximal mixing, giving vacuum oscillations. The
overall neutrino mass scale is set by the atmospheric neutrino requirement
eV, implying a mass for and
of order 1 eV in models with a natural mass hierarchy, whilst the tau neutrino
is expected to be much lighter than this and only weakly mixed. We present an
explicit example based on the anti-grand unification model of fermion masses.Comment: 16 pages, LaTeX, uses FeynTeX package for figure
The history of stellar metallicity in a simulated disc galaxy
We explore the chemical distribution of stars in a simulated galaxy. Using simulations of the same initial conditions but with two different feedback schemes (McMaster Unbiased Galaxy Simulations â MUGS â and Making Galaxies in a Cosmological Context â MaGICC), we examine the features of the ageâmetallicity relation (AMR), and the three-dimensional ageâ [Fe/H]â[O/Fe] distribution, both for the galaxy as a whole and decomposed into disc, bulge, halo and satellites. The MUGS simulation, which uses traditional supernova feedback, is replete with chemical substructure. This substructure is absent from the MaGICC simulation, which includes early feedback from stellar winds, a modified initial mass function and more efficient feedback. The reduced amount of substructure is due to the almost complete lack of satellites in MaGICC. We identify a significant separation between the bulge and disc AMRs, where the bulge is considerably more metal-rich with a smaller spread in metallicity at any given time than the disc. Our results suggest, however, that identifying the substructure in observations will require exquisite age resolution, of the order of 0.25 Gyr. Certain satellites show exotic features in the AMR, even forming a âsawtoothâ shape of increasing metallicity followed by sharp declines which correspond to pericentric passages. This fact, along with the large spread in stellar age at a given metallicity, compromises the use of metallicity as an age indicator, although alpha abundance provides a more robust clock at early times. This may also impact algorithms that are used to reconstruct star formation histories from resolved stellar populations, which frequently assume a monotonically increasing AMR
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