9,225 research outputs found
Dust Size Growth and Settling in a Protoplanetary Disk
We have studied dust evolution in a quiescent or turbulent protoplanetary
disk by numerically solving coagulation equation for settling dust particles,
using the minimum mass solar nebular model. As a result, if we assume an
ideally quiescent disk, the dust particles settle toward the disk midplane to
form a gravitationally unstable layer within 2x10^3 - 4x10^4 yr at 1 - 30 AU,
which is in good agreement with an analytic calculation by Nakagawa, Sekiya, &
Hayashi (1986) although they did not take into account the particle size
distribution explicitly. In an opposite extreme case of a globally turbulent
disk, on the other hand, the dust particles fluctuate owing to turbulent motion
of the gas and most particles become large enough to move inward very rapidly
within 70 - 3x10^4 yr at 1 - 30 AU, depending on the strength of turbulence.
Our result suggests that global turbulent motion should cease for the
planetesimal formation in protoplanetary disks.Comment: 27 pages, 8 figures, accepted for publication in the Ap
Roundabout relaxation: collective excitation requires a detour to equilibrium
Relaxation to equilibrium after strong and collective excitation is studied,
by using a Hamiltonian dynamical system of one dimensional XY model. After an
excitation of a domain of elements, the excitation is concentrated to fewer
elements, which are made farther away from equilibrium, and the excitation
intensity increases logarithmically with . Equilibrium is reached only after
taking this ``roundabout'' route, with the time for relaxation diverging
asymptotically as with .Comment: 4 pages, 5 figure
Mutations in shaking-B prevent electrical synapse formation in the Drosophila giant fiber system
The giant fiber system (GFS) is a simple network of neurons that mediates visually elicited escape behavior in Drosophila. The giant fiber (GF), the major component of the system, is a large, descending interneuron that relays visual stimuli to the motoneurons that innervate the tergotrochanteral jump muscle (TTM) and dorsal longitudinal flight muscles (DLMs). Mutations in the neural transcript from the shaking-B locus abolish the behavioral response by disrupting transmission at some electrical synapses in the GFS. This study focuses on the role of the gene in the development of the synaptic connections. Using an enhancer-trap line that expresses lacZ in the GFs, we show that the neurons develop during the first 30 hr of metamorphosis. Within the next 15 hr, they begin to form electrical synapses, as indicated by the transfer of intracellularly injected Lucifer yellow. The GFs dye-couple to the TTM motoneuron between 30 and 45 hr of metamorphosis, to the peripherally synapsing interneuron that drives the DLM motoneurons at approximately 48 hr, and to giant commissural interneurons in the brain at approximately 55 hr. Immunocytochemistry with shaking-B peptide antisera demonstrates that the expression of shaking-B protein in the region of GFS synapses coincides temporally with the onset of synaptogenesis; expression persists thereafter. The mutation shak-B2, which eliminates protein expression, prevents the establishment of dye coupling shaking-B, therefore, is essential for the assembly and/or maintenance of functional gap junctions at electrical synapses in the GFS
3D Distribution of Molecular Gas in the Barred Milky Way
We present a new model of the three-dimensional distribution of molecular gas
in the Milky Way Galaxy, based on CO line data. Our analysis is based on a
gas-flow simulation of the inner Galaxy using smoothed-particle hydrodynamics
(SPH) using a realistic barred gravitional potential derived from the observed
COBE/DIRBE near-IR light distribution. The gas model prescribes the gas orbits
much better than a simple circular rotation model and is highly constrained by
observations, but it cannot predict local details. In this study, we provide a
3D map of the observed molecular gas distribution using the velocity field from
the SPH model. A comparison with studies of the Galactic Center region suggests
that the main structures are reproduced but somewhat stretched along the
line-of-sight, probably on account of limited resolution of the underlying SPH
simulation. The gas model will be publicly available and may prove useful in a
number of applications, among them the analysis of diffuse gamma-ray emission
as measured with GLAST.Comment: ApJ in pres
Collective motions in globally coupled tent maps with stochastic updating
We study a generalization of globally coupled maps, where the elements are
updated with probability . When is below a threshold , the
collective motion vanishes and the system is the stationary state in the large
size limit. We present the linear stability analysis.Comment: 6 pages including 5 figure
Red giant bound on the axion-electron coupling reexamined
If axions or other low-mass pseudoscalars couple to electrons (``fine
structure constant'' ) they are emitted from red giant stars by the
Compton process and by bremsstrahlung .
We construct a simple analytic expression for the energy-loss rate for all
conditions relevant for a red giant and include axion losses in evolutionary
calculations from the main sequence to the helium flash. We find that
\alpha_a\lapprox0.5\mn(-26) or m_a\lapprox 9\,\meV/\cos^2\beta lest the red
giant core at helium ignition exceed its standard mass by more than
0.025\,\MM_\odot, in conflict with observational evidence. Our bound is the
most restrictive limit on , but it does not exclude the possibility
that axion emission contributes significantly to the cooling of ZZ~Ceti stars
such as G117--B15A for which the period decrease was recently measured.Comment: 11 pages, uuencoded and compressed postscript fil
Axions and the pulsation periods of variable white dwarfs revisited
Axions are the natural consequence of the introduction of the Peccei-Quinn
symmetry to solve the strong CP problem. All the efforts to detect such elusive
particles have failed up to now. Nevertheless, it has been recently shown that
the luminosity function of white dwarfs is best fitted if axions with a mass of
a few meV are included in the evolutionary calculations. Our aim is to show
that variable white dwarfs can provide additional and independent evidence
about the existence of axions. The evolution of a white dwarf is a slow cooling
process that translates into a secular increase of the pulsation periods of
some variable white dwarfs, the so-called DAV and DBV types. Since axions can
freely escape from such stars, their existence would increase the cooling rate
and, consequently, the rate of change of the periods as compared with the
standard ones. The present values of the rate of change of the pulsation period
of G117-B15A are compatible with the existence of axions with the masses
suggested by the luminosity function of white dwarfs, in contrast with previous
estimations. Furthermore, it is shown that if such axions indeed exist, the
drift of the periods of pulsation of DBV stars would be noticeably perturbed.Comment: Accepted for publication in Astronomy & Astrophysic
Continuous Avalanche Segregation of Granular Mixtures in Thin Rotating Drums
We study segregation of granular mixtures in the continuous avalanche regime
(for frequencies above ~ 1 rpm) in thin rotating drums using a continuum theory
for surface flows of grains. The theory predicts profiles in agreement with
experiments only when we consider a flux dependent velocity of flowing grains.
We find the segregation of species of different size and surface properties,
with the smallest and roughest grains being found preferentially at the center
of the drum. For a wide difference between the species we find a complete
segregation in agreement with experiments. In addition, we predict a transition
to a smooth segregation regime - with an power-law decay of the concentrations
as a function of radial coordinate - as the size ratio between the grains is
decreased towards one.Comment: 4 pages, 4 figures, http://polymer.bu.edu/~hmaks
The Flow Of Granular Matter Under Reduced-Gravity Conditions
To gain a better understanding of the surfaces of planets and small bodies in
the solar system, the flow behavior of granular material for various gravity
levels is of utmost interest. We performed a set of reduced-gravity
measurements to analyze the flow behavior of granular matter with a quasi-2D
hourglass under coarse-vacuum conditions and with a tilting avalanche box. We
used the Bremen drop tower and a small centrifuge to achieve residual-gravity
levels between 0.01 g and 0.3 g. Both experiments were carried out with basalt
and glass grains as well as with two kinds of ordinary sand. For the hourglass
experiments, the volume flow through the orifice, the repose and friction
angles, and the flow behavior of the particles close to the surface were
determined. In the avalanche-box experiment, we measured the duration of the
avalanche, the maximum slope angle as well as the width of the avalanche as a
function of the gravity level.Comment: Accepted by "Proc. Powders and Grains 2009", Publisher AI
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