5,767 research outputs found
Distinguishing an ejected blob from alternative flare models at the Galactic centre with GRAVITY
The black hole at the Galactic centre exhibits regularly flares of radiation,
the origin of which is still not understood. In this article, we study the
ability of the near-future GRAVITY infrared instrument to constrain the nature
of these events. We develop realistic simulations of GRAVITY astrometric data
sets for various flare models. We show that the instrument will be able to
distinguish an ejected blob from alternative flare models, provided the blob
inclination is >= 45deg, the flare brightest magnitude is 14 <= mK <= 15 and
the flare duration is >= 1h30.Comment: 11 pages, 9 figures, accepted by MNRA
Travelling waves in a nonlinear degenerate diffusion model for bacterial pattern formation
We study a reaction diffusion model recently proposed in [5] to describe the spatiotemporal evolution of the bacterium Bacillus subtilis on agar plates containing nutrient. An interesting mathematical feature of the model, which is a coupled pair of partial differential equations, is that the bacterial density satisfies a degenerate nonlinear diffusion equation. It was shown numerically that this model can exhibit quasi-one-dimensional constant speed travelling wave solutions. We present an analytic study of the existence and uniqueness problem for constant speed travelling wave solutions. We find that such solutions exist only for speeds greater than some threshold speed giving minimum speed waves which have a sharp profile. For speeds greater than this minimum speed the waves are smooth. We also characterise the dependence of the wave profile on the decay of the front of the initial perturbation in bacterial density. An investigation of the partial differential equation problem establishes,via a global existence and uniqueness argument, that these waves are the only long time solutions supported by the problem. Numerical solutions of the partial differential equation problem are presented and they confirm the results of the analysis
Dynamical Friction of Double Perturbers in a Gaseous Medium
In many astrophysical situations, as in the coalescence of supermassive black
hole pairs at gas rich galactic nuclei, the dynamical friction experienced by
an object is a combination of its own wake as well as the wakes of its
companions. Using a semi-analytic approach, we investigate the composite wake
due to, and the resulting drag forces on, double perturbers that are placed at
the opposite sides of the orbital center and move on a circular orbit in a
uniform gaseous medium. The circular orbit makes the wake of each perturber
asymmetric, creating an overdense tail at the trailing side. The tail not only
drags the perturber backward but it also exerts a positive torque on the
companion. For equal-mass perturbers, the positive torque created by the
companion wake is, on average, a fraction ~40-50% of the negative torque
created by its own wake, but this fraction may be even larger for perturbers
moving subsonically. This suggests that the orbital decay of a perturber in a
double system, especially in the subsonic regime, can take considerably longer
than in isolation. We provide the fitting formulae for the forces due to the
companion wake and discuss our results in light of recent numerical simulations
for mergers of binary black holes.Comment: 4 pages, 3 figures, accepted for publication in ApJ
Demography of Yellow-Bellied Marmont Populations
This is the publisher's version, also available electronically from http://www.jstor.org/stable/info/1935452Marmot (Marmota flaviventris) populations are colonial or satellite. The number of adults of colonial populations is relatively stable; fluctuations occur primarily because of changes in numbers of young and yearlings. Population trends among five colonial populations are dissimilar. Satellite populations are unstable and reproduce at a lower rate than do colonial populations. Satellite marmots are shorter resident than colonial marmots. Both colonial and satellite females usually are longer resident than males. All adult colonial males are 41% of adult colonial females are recruited from other places; all satellite adults are recruited from other places. Losses of colonial marmots are attributed primarily to mortality during hibernation and emigration. Predation appears to be a minor source of mortality of colonial marmots, but may be of greater significance to satellite populations. Demographic relationships of individual colonies appear to be density-independent. Dispersal of colonial animals occurs primarily among yearlings, which have a higher expectation of reaching sexual maturity than young have. The major cause of dispersal is social pressure, but social stress is not simply density-dependent. The colonial social organization is more adaptive than the more nearly solitary (=satellite)
A Disk Census for Young Brown Dwarfs
Recent surveys have identified sub-stellar objects down to planetary masses
in nearby star-forming regions. Reliable determination of the disk frequency in
young brown dwarfs is of paramount importance to understanding their origin.
Here we report the results of a systematic study of infrared L'-band
(3.8-micron) disk excess in ~50 spectroscopically confirmed objects near and
below the sub-stellar boundary in several young clusters. Our observations,
using the ESO Very Large Telescope, Keck I and the NASA Infrared Telescope
Facility, reveal that a significant fraction of brown dwarfs harbor disks at a
very young age. Their inner disk lifetimes do not appear to be vastly different
from those of disks around T Tauri stars. Our findings are consistent with the
hypothesis that sub-stellar objects form via a mechanism similar to solar-mass
stars.Comment: accepted for publication in The Astronomical Journa
Enhanced Eddy Activity in the Beaufort Gyre in Response to Sea Ice Loss
The Beaufort Gyre freshwater content has increased since the 1990s, potentially stabilizing in recent years. The mechanisms proposed to explain the stabilization involve either mesoscale eddy activity that opposes Ekman pumping or the reduction of Ekman pumping due to reduced sea ice?ocean surface stress. However, the relative importance of these mechanisms is unclear. Here, we present observational estimates of the Beaufort Gyre mechanical energy budget and show that energy dissipation and freshwater content stabilization by eddies increased in the late-2000s. The loss of sea ice and acceleration of ocean currents after 2007 resulted in enhanced mechanical energy input but without corresponding increases in potential energy storage. To balance the energy surplus, eddy dissipation and its role in gyre stabilization must have increased after 2007. Our results imply that declining Arctic sea ice will lead to an increasingly energetic Beaufort Gyre with eddies playing a greater role in its stabilization
Saturation of Magnetorotational Instability through Magnetic Field Generation
The saturation mechanism of Magneto-Rotational Instability (MRI) is examined
through analytical quasilinear theory and through nonlinear computation of a
single mode in a rotating disk. We find that large-scale magnetic field is
generated through the alpha effect (the correlated product of velocity and
magnetic field fluctuations) and causes the MRI mode to saturate. If the
large-scale plasma flow is allowed to evolve, the mode can also saturate
through its flow relaxation. In astrophysical plasmas, for which the flow
cannot relax because of gravitational constraints, the mode saturates through
field generation only.Comment: 9 pages, 10 figures to appear in ApJ, Jun 2009, 10 v69
Anomalous high energy dispersion in photoemission spectra from insulating cuprates
Angle resolved photoelectron spectroscopic measurements have been performed
on an insulating cuprate Ca_2CuO_2Cl_2. High resolution data taken along the
\Gamma to (pi,pi) cut show an additional dispersive feature that merges with
the known dispersion of the lowest binding energy feature, which follows the
usual strongly renormalized dispersion of ~0.35 eV. This higher energy part
reveals a dispersion that is very close to the unrenormalized band predicted by
band theory. A transfer of spectral weight from the low energy feature to the
high energy feature is observed as the \Gamma point is approached. By comparing
with theoretical calculations the high energy feature observed here
demonstrates that the incoherent portion of the spectral function has
significant structure in momentum space due to the presence of various energy
scales.Comment: 5 pages, 3 figure
Dispersion in the lifetime and accretion rate of T Tauri discs
We compare evolutionary models for protoplanetary discs that include disc
winds with observational determinations of the disc lifetime and accretion rate
in Taurus. Using updated estimates for stellar ages in Taurus, together with
published classifications, we show that the evolution of the disc fraction with
stellar age is similar to that derived for ensembles of stars within young
clusters. Around 30 percent of stars lose their discs within 1 Myr, while the
remainder have disc lifetimes that are typically in the 1-10 Myr range. We show
that the latter range of ages is consistent with theoretical models for disc
evolution, provided that there is a dispersion of around 0.5 in the log of the
initial disc mass. The same range of initial conditions brackets the observed
variation in the accretion rate of Classical T Tauri stars at a given age. We
discuss the expected lifetime of discs in close binary systems, and show that
our models predict that the disc lifetime is almost constant for separations
exceeding 10 au. This implies a low predicted fraction of binaries that pair a
Classical T Tauri star with a Weak-lined T Tauri star, and is in better
agreement with observations of the disc lifetime in binaries than disc models
that do not include disc mass loss in a wind.Comment: MNRAS, in pres
Possible manifestation of spin fluctuations in the temperature behavior of resistivity in Sm_{1.85}Ce_{0.15}CuO_4 thin films
A pronounced step-like (kink) behavior in the temperature dependence of
resistivity is observed in the optimally-doped
thin films around and attributed to
manifestation of strong spin fluctuations induced by moments with the
energy . In addition to fluctuation
induced contribution due to thermal broadening effects (of the
width ), the experimental data are found to be well fitted
accounting for residual (zero-temperature) , electron-phonon and electron-electron contributions. The
best fits produced , ,
, and for estimates of the plasmon frequency, the
impurity scattering rate, electron-phonon coupling constant, and the Fermi
energy, respectively.Comment: 6 pages (REVTEX4), 2 EPS figures; accepted for publication in JETP
Letter
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