8,401 research outputs found
Comment on `Equilibrium crystal shape of the Potts model at the first-order transition point'
We comment on the article by Fujimoto (1997 J. Phys. A: Math. Gen., Vol. 30,
3779), where the exact equilibrium crystal shape (ECS) in the critical Q-state
Potts model on the square lattice was calculated, and its equivalence with ECS
in the Ising model was established. We confirm these results, giving their
alternative derivation applying the transformation properties of the
one-particle dispersion relation in the six-vertex model. It is shown, that
this dispersion relation is identical with that in the Ising model on the
square lattice.Comment: 4 pages, 1 figure, LaTeX2
A fundamental test for stellar feedback recipes in galaxy simulations
Direct comparisons between galaxy simulations and observations that both
reach scales < 100 pc are strong tools to investigate the cloud-scale physics
of star formation and feedback in nearby galaxies. Here we carry out such a
comparison for hydrodynamical simulations of a Milky Way-like galaxy, including
stochastic star formation, HII region and supernova feedback, and chemical
post-processing at 8 pc resolution. Our simulation shows excellent agreement
with almost all kpc-scale and larger observables, including total star
formation rates, radial profiles of CO, HI, and star formation through the
galactic disc, mass ratios of the ISM components, both whole-galaxy and
resolved Kennicutt-Schmidt relations, and giant molecular cloud properties.
However, we find that our simulation does not reproduce the observed
de-correlation between tracers of gas and star formation on < 100 pc scales,
known as the star formation 'uncertainty principle', which indicates that
observed clouds undergo rapid evolutionary lifecycles. We conclude that the
discrepancy is driven by insufficiently-strong pre-supernova feedback in our
simulation, which does not disperse the surrounding gas completely, leaving
star formation tracer emission too strongly associated with molecular gas
tracer emission, inconsistent with observations. This result implies that the
cloud-scale de-correlation of gas and star formation is a fundamental test for
feedback prescriptions in galaxy simulations, one that can fail even in
simulations that reproduce all other macroscopic properties of star-forming
galaxies.Comment: 13 pages, 10 figures, accepted for publication in MNRA
Mechanisms for High-frequency QPOs in Neutron Star and Black Hole Binaries
We explain the millisecond variability detected by Rossi X-ray Timing
Explorer (RXTE) in the X-ray emission from a number of low mass X-ray binary
systems (Sco X-1, 4U1728-34, 4U1608-522, 4U1636-536, 4U0614+091, 4U1735-44,
4U1820-30, GX5-1 and etc) in terms of dynamics of the centrifugal barrier, a
hot boundary region surrounding a neutron star. We demonstrate that this region
may experience the relaxation oscillations, and that the displacements of a gas
element both in radial and vertical directions occur at the same main
frequency, of order of the local Keplerian frequency. We show the importance of
the effect of a splitting of the main frequency produced by the Coriolis force
in a rotating disk for the interpretation of a spacing between the QPO peaks.
We estimate a magnitude of the splitting effect and present a simple formula
for the whole spectrum of the split frequencies. It is interesting that the
first three lowest-order overtones fall in the range of 200-1200 Hz and match
the kHz-QPO frequencies observed by RXTE. Similar phenomena should also occur
in Black Hole (BH) systems, but, since the QPO frequency is inversely
proportional to the mass of a compact object, the frequency of the
centrifugal-barrier oscillations in the BH systems should be a factor of 5-10
lower than that for the NS systems. The X-ray spectrum formed in this region is
a result of upscattering of a soft radiation (from a disk and a NS surface) off
relatively hot electrons in the boundary layer. We also briefly discuss some
alternative QPO models, including a possibility of acoustic oscillations in the
boundary layer, the proper stellar rotation, and g-mode disk oscillations.Comment: The paper is coming out in the Astrophysical Journal in the 1st of
May issue of 199
Comparing simulated Al maps to gamma-ray measurements
© ESO 2019.Context. The diffuse gamma-ray emission of at 1.8 MeV reflects ongoing nucleosynthesis in the Milky Way, and traces massive-star feedback in the interstellar medium due to its 1 Myr radioactive lifetime. Interstellar-medium morphology and dynamics are investigated in astrophysics through 3D hydrodynamic simulations in fine detail, as only few suitable astronomical probes are available. Aims. We compare a galactic-scale hydrodynamic simulation of the Galaxy's interstellar medium, including feedback and nucleosynthesis, with gamma-ray data on emission in the Milky Way extracting constraints that are only weakly dependent on the particular realisation of the simulation or Galaxy structure. Methods. Due to constraints and biases in both the simulations and the gamma-ray observations, such comparisons are not straightforward. For a direct comparison, we perform maximum likelihood fits of simulated sky maps as well as observation-based maximum entropy maps to measurements with INTEGRAL/SPI. To study general morphological properties, we compare the scale heights of emission produced by the simulation to INTEGRAL/SPI measurements.} Results. The direct comparison shows that the simulation describes the observed inner Galaxy well, but differs significantly from the observed full-sky emission morphology. Comparing the scale height distribution, we see similarities for small scale height features and a mismatch at larger scale heights. We attribute this to the prominent foreground emission sites that are not captured by the simulation.Peer reviewedFinal Accepted Versio
Effects of disorder on two coupled Hubbard chains at half-filling
We investigate the effects of quenched disorder on two chain Hubbard models
at half-filling by using bosonization and renormalization group methods. It is
found that the sufficiently strong forward scattering due to impurities and the
random gauge field, which is generated by impurity backward scattering, destroy
the charge gaps as well as the spin gaps. Random backward scattering due to
impurities then drives the resulting massless phase to the Anderson
localization phase. For intermediate strength of random forward scattering,
however, the spin gaps still survive, and only one of the charge gaps is
collapsed. In this parameter region, one of the charge degrees of freedom is in
the Anderson localized state, while the other one is still in the massive
state.Comment: 10 pages, RevTex, 3 eps figure
Charge and Spin Transport in the One-dimensional Hubbard Model
In this paper we study the charge and spin currents transported by the
elementary excitations of the one-dimensional Hubbard model. The corresponding
current spectra are obtained by both analytic methods and numerical solution of
the Bethe-ansatz equations. For the case of half-filling, we find that the
spin-triplet excitations carry spin but no charge, while charge -spin
triplet excitations carry charge but no spin, and both spin-singlet and charge
-spin-singlet excitations carry neither spin nor charge currents.Comment: 24 pages, 14 figure
Determining Initial States for Time-Parallel Simulations
In this paper, we propose a time-parallel simulation method which uses a pre-simulation to identify recurrent states. Also, an approximation technique is suggested for approximate Markovian modeling for queueing networks to extend the class of simulation models which can be simulated efficiently using our time-parallel simulation. A central server system and a virtual circuit of a packet-switched data communication network modeled by closed queueing networks are experimented with the proposed time-parallel simulation. Experiment results suggest that the proposed approach can exploit massive parallelism while yielding accurate results
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