624 research outputs found
Evaporation of Lennard-Jones Fluids
Evaporation and condensation at a liquid/vapor interface are ubiquitous
interphase mass and energy transfer phenomena that are still not well
understood. We have carried out large scale molecular dynamics simulations of
Lennard-Jones (LJ) fluids composed of monomers, dimers, or trimers to
investigate these processes with molecular detail. For LJ monomers in contact
with a vacuum, the evaporation rate is found to be very high with significant
evaporative cooling and an accompanying density gradient in the liquid domain
near the liquid/vapor interface. Increasing the chain length to just dimers
significantly reduces the evaporation rate. We confirm that mechanical
equilibrium plays a key role in determining the evaporation rate and the
density and temperature profiles across the liquid/vapor interface. The
velocity distributions of evaporated molecules and the evaporation and
condensation coefficients are measured and compared to the predictions of an
existing model based on kinetic theory of gases. Our results indicate that for
both monatomic and polyatomic molecules, the evaporation and condensation
coefficients are equal when systems are not far from equilibrium and smaller
than one, and decrease with increasing temperature. For the same reduced
temperature , where is the critical temperature, these two
coefficients are higher for LJ dimers and trimers than for monomers, in
contrast to the traditional viewpoint that they are close to unity for
monatomic molecules and decrease for polyatomic molecules. Furthermore, data
for the two coefficients collapse onto a master curve when plotted against a
translational length ratio between the liquid and vapor phase.Comment: revised version, 15 pages, 15 figures, to appear in J. Chem. Phy
Signatures of QCD Phase Transition in a Newborn Compact Star
We study the scenario that a new born strange quark star cools to the Quantum
ChromoDynamics (QCD) phase transition temperature and converts to a neutron
star, and we calculate the evolution of temperature and luminosity of the
compact star. We argue that the conversion energy released can be of the order
erg. We also propose that a second neutrino burst will be emitted at
the completion of this phase transition.Comment: 6 pages, 1 figure, minor modified and subtitles added, typos
corrected. Accepted for publication in Monthly Notice Lette
Iron K-alpha Fluorescent Line Profiles from Spiral Accretion Flows in AGNs
We present 6.4 keV iron K-alpha fluorescent line profiles predicted for a
relativistic black hole accretion disk in the presence of a spiral motion in
Kerr geometry, the work extended from an earlier literature motivated by recent
magnetohydrodynamic (MHD) simulations. The velocity field of the spiral motion,
superposed on the background Keplerian flow, results in a complicated redshift
distribution in the accretion disk. An X-ray source attributed to a localized
flaring region on the black hole symmetry axis illuminates the iron in the
disk. The emissivity form becomes very steep because of the light bending
effect from the primary X-ray source to the disk. The predicted line profile is
calculated for various spiral waves, and we found, regardless of the source
height, that: (i) a multiple-peak along with a classical double-peak structure
generally appears, (ii) such a multiple-peak can be categorized into two types,
sharp sub-peaks and periodic spiky peaks, (iii) a tightly-packed spiral wave
tends to produce more spiky multiple peaks, whereas (iv) a spiral wave with a
larger amplitude seems to generate more sharp sub-peaks, (v) the effect seems
to be less significant when the spiral wave is centrally concentrated, (vi) the
line shape may show a drastic change (forming a double-peak, triple-peak or
multiple-peak feature) as the spiral wave rotates with the disk. Our results
emphasize that around a rapidly-rotating black hole an extremely redshifted
iron line profile with a noticeable spike-like feature can be realized in the
presence of the spiral wave. Future X-ray observations, from {\it Astro-E2} for
example, will have sufficient spectral resolution for testing our spiral wave
model which exhibits unique spike-like features.Comment: 30 pages, 10 figures, submitted to ApJ, will be presented at 204th
Meeting of AAS in Denve
Constraining the Black Hole Mass Spectrum with LISA Observations II: Direct comparison of detailed models
A number of scenarios have been proposed for the origin of the supermassive
black holes (SMBHs) that are found in the centres of most galaxies. Many such
scenarios predict a high-redshift population of massive black holes (MBHs),
with masses in the range 100 to 100000 times that of the Sun. When the Laser
Interferometer Space Antenna (LISA) is finally operational, it is likely that
it will detect on the order of 100 of these MBH binaries as they merge. The
differences between proposed population models produce appreciable effects in
the portion of the population which is detectable by LISA, so it is likely that
the LISA observations will allow us to place constraints on them. However,
gravitational wave detectors such as LISA will not be able to detect all such
mergers nor assign precise black hole parameters to the merger, due to weak
gravitational wave signal strengths. This paper explores LISA's ability to
distinguish between several MBH population models. In this way, we go beyond
predicting a LISA observed population and consider the extent to which LISA
observations could inform astrophysical modellers. The errors in LISA parameter
estimation are applied with a direct method which generates random sample
parameters for each source in a population realisation. We consider how the
distinguishability varies depending on the choice of source parameters (1 or 2
parameters chosen from masses, redshift or spins) used to characterise the
model distributions, with confidence levels determined by 1 and 2-dimensional
tests based on the Kolmogorov-Smirnov test.Comment: 32 pages, 21 Figures, 6 Tables; Submitted to MNRA
Anisotropic multi-gap superfluid states in nuclear matter
It is shown that under changing density or temperature a nucleon Fermi
superfluid can undergo a phase transition to an anisotropic superfluid state,
characterized by nonvanishing gaps in pairing channels with singlet-singlet
(SS) and triplet-singlet (TS) pairing of nucleons (in spin and isospin spaces).
In the SS pairing channel nucleons are paired with nonzero orbital angular
momentum. Such two-gap states can arise as a result of branching from the
one-gap solution of the self-consistent equations, describing SS or TS pairing
of nucleons, that depends on the relationship between SS and TS coupling
constants at the branching point. The density/temperature dependence of the
order parameters and the critical temperature for transition to the anisotropic
two-gap state are determined in a model with the SkP effective interaction. It
is shown that the anisotropic SS-TS superfluid phase corresponds to a
metastable state in nuclear matter.Comment: Prepared with RevTeX4, 7p., 5 fi
Constraining properties of the black hole population using LISA
LISA should detect gravitational waves from tens to hundreds of systems
containing black holes with mass in the range from 10 thousand to 10 million
solar masses. Black holes in this mass range are not well constrained by
current electromagnetic observations, so LISA could significantly enhance our
understanding of the astrophysics of such systems. In this paper, we describe a
framework for combining LISA observations to make statements about massive
black hole populations. We summarise the constraints that LISA observations of
extreme-mass-ratio inspirals might be able to place on the mass function of
black holes in the LISA range. We also describe how LISA observations can be
used to choose between different models for the hierarchical growth of
structure in the early Universe. We consider four models that differ in their
prescription for the initial mass distribution of black hole seeds, and in the
efficiency of accretion onto the black holes. We show that with as little as 3
months of LISA data we can clearly distinguish between these models, even under
relatively pessimistic assumptions about the performance of the detector and
our knowledge of the gravitational waveforms.Comment: 12 pages, 3 figures, submitted to Class. Quantum Grav. for
proceedings of 8th LISA Symposium; v2 minor changes for consistency with
accepted versio
Neutrino Emission from Cooper Pairs and Minimal Cooling of Neutron Stars
The minimal cooling paradigm for neutron star cooling assumes that enhanced
cooling due to neutrino emission from any direct Urca process, due either to
nucleons or to exotica such as hyperons, Bose condensates, or deconfined
quarks, does not occur. This scenario was developed to replace and extend the
so-called standard cooling scenario to include neutrino emission from the
Cooper pair breaking and formation processes that occur near the critical
temperature for superfluid/superconductor pairing.
Recently, it has been found that Cooper-pair neutrino emission from the
vector channel is suppressed by a large factor compared to the original
estimates that violated vector current conservation. We show that Cooper-pair
neutrino emission remains, nevertheless, an efficient cooling mechanism through
the axial channel. As a result, the elimination of neutrino emission from
Cooper-paired nucleons through the vector channel has only minor effects on the
long-term cooling of neutron stars within the minimal cooling paradigm. We
further quantify precisely the effect of the size of the neutron 3P2 gap and
demonstrate that consistency between observations and the minimal cooling
paradigm requires that the critical temperature T_c for this gap covers a range
of values between T_c^min 0.5 \times 10^9 K in
the core of the star. In addition, it is required that young neutron stars have
heterogenous envelope compositions: some must have light-element compositions
and others must have heavy-element compositions. Unless these two conditions
are fulfilled, about half of the observed young cooling neutron stars are
inconsistent with the minimal cooling paradigm and provide evidence for the
existence of enhanced cooling.Comment: 13 pages, 9 figures, submitted to Ap
Effects of Uniaxial Stress on Antiferromagnetic Moment in the Heavy Electron Compound URu_2Si_2
We have performed the elastic neutron scattering experiments under uniaxial
stress \sigma along the tetragonal [100], [110] and [001] directions for
URu2Si2. For \sigma // [100] and [110], the antiferromagnetic moment \mu_o is
strongly enhanced from 0.02 \mu_B (\sigma=0) to 0.22 \mu_B (\sigma=2.5 kbar) at
1.5 K. The rate of increase d\mu_o/d\sigma is roughly estimated to be ~ 0.1
\mu_B/kbar, which is much larger than that for the hydrostatic pressure (~
0.025 \mu_B/kbar). Above 2.5 kbar, \mu_o shows a tendency to saturate similar
to the behavior in the hydrostatic pressure. For \sigma // [001], on the other
hand, \mu_o shows only a slight increase to 0.028 \mu_B (\sigma = 4.6 kbar)
with a rate of ~ 0.002 \mu_B/kbar. The observed anisotropy suggests that the
competition between the hidden order and the antiferromagnetic state in URu2Si2
is strongly coupled with the tetragonal four-fold symmetry and the c/a ratio,
or both.Comment: 3 pages, 3 eps figures, Proceedings of Int. Conf. on Strongly
Correlated Electrons with Orbital Degrees of Freedom (Sendai, Japan,
September 11-14, 2001
Chandra Observations of G11.2-0.3: Implications for Pulsar Ages
We present Chandra X-ray Observatory imaging observations of the young
Galactic supernova remnant G11.2-0.3. The image shows that the previously known
young 65-ms X-ray pulsar is at position (J2000) RA 18h 11m 29.22s, DEC -19o 25'
27.''6, with 1 sigma error radius 0.''6. This is within 8'' of the geometric
center of the shell. This provides strong confirming evidence that the system
is younger, by a factor of ~12, than the characteristic age of the pulsar. The
age discrepancy suggests that pulsar characteristic ages can be poor age
estimators for young pulsars. Assuming conventional spin down with constant
magnetic field and braking index, the most likely explanation for the age
discrepancy in G11.2-0.3 is that the pulsar was born with a spin period of ~62
ms. The Chandra image also reveals, for the first time, the morphology of the
pulsar wind nebula. The elongated hard-X-ray structure can be interpreted as
either a jet or a Crab-like torus seen edge on. This adds to the growing list
of highly aspherical pulsar wind nebulae and argues that such structures are
common around young pulsars.Comment: 16 pages, 3 figures, Accepted for publication in ApJ. For a full
resolution version of Fig 1, see
http://www.physics.mcgill.ca/~vkaspi/G11.2-0.3/f1.ep
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