4,774 research outputs found
Freely-Decaying, Homogeneous Turbulence Generated by Multi-scale Grids
We investigate wind tunnel turbulence generated by both conventional and
multi-scale grids. Measurements were made in a tunnel which has a large
test-section, so that possible side wall effects are very small and the length
assures that the turbulence has time to settle down to a homogeneous shear-free
state. The conventional and multi-scale grids were all designed to produce
turbulence with the same integral scale, so that a direct comparison could be
made between the different flows. Our primary finding is that the behavior of
the turbulence behind our multi-scale grids is virtually identical to that
behind the equivalent conventional grid. In particular, all flows exhibit a
power-law decay of energy, , where is very close to the
classical Saffman exponent of . Moreover, all spectra exhibit
classical Kolmogorov scaling, with the spectra collapsing on the integral
scales at small , and on the Kolmogorov micro-scales at large . Our
results are at odds with some other experiments performed on similar
multi-scale grids, where significantly higher energy decay exponents and
turbulence levels have been reported.Comment: 19 pages, 18 figure
Does the bonding rule break down in AsSe glass?
The local coordination numbers of AsSe glass were determined by a
combination of anomalous x-ray scattering experiments, reverse Monte Carlo
calculations, and {\it ab initio} molecular dynamics simulations. The
well-known `8- bonding rule' proposed by Mott breaks down around the As
atoms, exceeding the rule by 7--26%. An experimental prediction based on
mean-field theory agrees with the present experimental and theoretical results.
The fourfold coordinated As atoms likely form As-As wrong bond chains rather
than ethan-like configurations, which is identified as the origin for the
breakdown of the `8- bonding rule'.Comment: 6 pages, 6figures, 1table, submitted to Europhysics Letter
Episodic excursions of low-mass protostars on the Hertzsprung-Russell diagram
Following our recent work devoted to the effect of accretion on the
pre-main-sequence evolution of low-mass stars, we perform a detailed analysis
of episodic excursions of low-mass protostars in the Hertzsprung-Russell (H-R)
diagram triggered by strong mass accretion bursts typical of FU Orionis-type
objects (FUors). These excursions reveal themselves as sharp increases in the
stellar total luminosity and/or effective temperature of the protostar and can
last from hundreds to a few thousands of years, depending on the burst strength
and characteristics of the protostar. During the excursions, low-mass
protostars occupy the same part of the H-R diagram as young intermediate-mass
protostars in the quiescent phase of accretion. Moreover, the time spent by
low-mass protostars in these regions is on average a factor of several longer
than that spent by the intermediate-mass stars in quiescence. During the
excursions, low-mass protostars pass close to the position of most known FUors
in the H-R diagram, but owing to intrinsic ambiguity the model stellar
evolutionary tracks are unreliable in determining the FUor properties. We find
that the photospheric luminosity in the outburst state may dominate the
accretion luminosity already after a few years after the onset of the outburst,
meaning that the mass accretion rates of known FUors inferred from the
bolometric luminosity may be systematically overestimated, especially in the
fading phase.Comment: 15 pages, 12 figure
The Rossiter-McLaughlin effect and analytic radial velocity curves for transiting extrasolar planetary systems
A transiting extrasolar planet sequentially blocks off the light coming from
the different parts of the disk of the host star in a time dependent manner.
Due to the spin of the star, this produces an asymmetric distortion in the line
profiles of the stellar spectrum, leading to an apparent anomaly of the radial
velocity curves, known as the Rossiter - McLaughlin effect. Here, we derive
approximate but accurate analytic formulae for the anomaly of radial velocity
curves taking account of the stellar limb darkening. The formulae are
particularly useful in extracting information of the projected angle between
the planetary orbit axis and the stellar spin axis, \lambda, and the projected
stellar spin velocity, V sin I_s. We create mock samples for the radial curves
for the transiting extrasolar system HD209458, and demonstrate that constraints
on the spin parameters (V sin I_s, \lambda) may be significantly improved by
combining our analytic template formulae and the precision velocity curves from
high-resolution spectroscopic observations with 8-10 m class telescopes. Thus
future observational exploration of transiting systems using the Rossiter -
McLaughlin effect is one of the most important probes to better understanding
of the origin of extrasolar planetary systems, especially the origin of their
angular momentum.Comment: 39 pages, 16 figures, Accepted to ApJ. To match the published version
(ApJ 623, April 10 issue
Cosmological Growth History of Supermassive Black Holes and Demographics in the High-z Universe : Do Lyman-Break Galaxies Have Supermassive Black Holes?
We study the demographics of supermassive black holes (SMBHs) in the local
and high-z universe with hard X-ray luminosity functions (HXLFs) of AGNs.
First, we compare the mass accretion history at z>0 with optical luminosity
functions (OLFs) and that with HXLFs. We consider the constraints on parameters
of mass accretion (energy conversion efficiency and Eddington ratio) and
conclude that the constraints based on HXLFs are more adequate rather than that
based on OLFs. The sub-Eddington case is allowed only when we use HXLFs. Next,
we estimate the upper limit of the cumulative mass density of SMBHs at any
redshifts. For an application, we examine if Lyman-Break galaxies (LBGs) at z=3
already have SMBHs in their centers which is suggested by recent observations.
If most of LBGs already has massive SMBHs at z=3, the resultant mass density of
SMBHs at z=0 should exceed the observational estimate because such SMBHs should
further grow by accretion. We show that the special conditions should be met in
order that a large part of LBGs have SMBHs (for example, large energy
conversion efficiency and frequent mergers and/or direct formations at z>3).
The possibility that nearly all LBGs have SMBHs with large mass ratio, such as
M_BH/M_stellar > 0.005, is reliably ruled out.(abridged)Comment: 30 pages including 5 figures, accepted for publication in Ap
Occurrence Distribution of Polar Cap Patches: Dependences on UT, Season and Hemisphere
Polar cap patches are islands of enhanced electron density in the polar cap F region ionosphere, which sometimes affect the propagation of trans-ionospheric radio waves. Considering the intake of daytime sunlit plasma by the high-latitude convection as the primary cause of patches, the spatial overlap between the convection and the daytime sunlit plasma should be one of the critical factors controlling the generation of patches. To confirm this hypothesis, we statistically investigated the UT and seasonal distributions of patch occurrence frequency in both the hemispheres by using in situ plasma density data from the Swarm satellite. As a result, it was found that the occurrence distribution of patches is a complex function of UT, season and hemisphere, but it can be mostly interpreted by the spatial overlap between the high-latitude convection and the solar terminator. This suggests that polar cap patches are not necessarily phenomena that occur only during winter months. That is, patches can often be observed even in periods away from the winter solstice if the location of solar terminator in the magnetic coordinate system is appropriate for the generation of patches. For example, in the southern hemisphere, where the offset between the geographic and magnetic poles is larger than that in the northern hemisphere, the highest patch occurrence rate is obtained around the equinoctial periods. These results indicate that it is needed to take these dependences into account when we discuss and predict the space weather impacts of patches on the trans-ionospheric radio propagation
Structural and dynamical properties of liquid Si. An orbital-free molecular dynamics study
Several static and dynamic properties of liquid silicon near melting have
been determined from an orbital free {\em ab-initio} molecular dynamics
simulation. The calculated static structure is in good agreement with the
available X-ray and neutron diffraction data. The dynamical structure shows
collective density excitations with an associated dispersion relation which
closely follows recent experimental data. It is found that liquid silicon can
not sustain the propagation of shear waves which can be related to the power
spectrum of the velocity autocorrelation function. Accurate estimates have also
been obtained for several transport coefficients. The overall picture is that
the dynamic properties have many characteristics of the simple liquid metals
although some conspicuous differences have been found.Comment: 12 pages, 11 figure
Statistical Description of Hydrodynamic Processes in Ionic Melts with taking into account Polarization Effects
Statistical description of hydrodynamic processes for ionic melts is proposed
with taking into account polarization effects caused by the deformation of
external ionic shells. This description is carried out by means of the Zubarev
nonequilibrium statistical operator method, appropriate for investigations of
both strong and weak nonequilibrium processes. The nonequilibrium statistical
operator and the generalized hydrodynamic equations that take into account
polarization processes are received for ionic-polarization model of ionic
molten salts when the nonequilibrium averaged values of densities of ions
number, their momentum, dipole momentum and total energy are chosen for the
reduced description parameters. A spectrum of collective excitations is
investigated within the viscoelastic approximation for ion-polarization model
of ionic melts.Comment: 24 pages, RevTex4.1-format, no figure
Dynamical Expansion of Ionization and Dissociation Front around a Massive Star. II. On the Generality of Triggered Star Formation
We analyze the dynamical expansion of the HII region, photodissociation
region, and the swept-up shell, solving the UV- and FUV-radiative transfer, the
thermal and chemical processes in the time-dependent hydrodynamics code.
Following our previous paper, we investigate the time evolutions with various
ambient number densities and central stars. Our calculations show that basic
evolution is qualitatively similar among our models with different parameters.
The molecular gas is finally accumulated in the shell, and the gravitational
fragmentation of the shell is generally expected. The quantitative differences
among models are well understood with analytic scaling relations. The detailed
physical and chemical structure of the shell is mainly determined by the
incident FUV flux and the column density of the shell, which also follow the
scaling relations. The time of shell-fragmentation, and the mass of the
gathered molecular gas are sensitive tothe ambient number density. In the case
of the lower number density, the shell-fragmentation occurs over a longer
timescale, and the accumulated molecular gas is more massive. The variations
with different central stars are more moderate. The time of the
shell-fragmentation differs by a factor of several with the various stars of
M_* = 12-101 M_sun. According to our numerical results, we conclude that the
expanding HII region should be an efficient trigger for star formation in
molecular clouds if the mass of the ambient molecular material is large enough.Comment: 49 pages, including 17 figures ; Accepted for publication in Ap
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