3,841 research outputs found
Global MHD simulations of stratified and turbulent protoplanetary discs. I. Model properties
We present the results of global 3-D MHD simulations of stratified and
turbulent protoplanetary disc models. The aim of this work is to develop thin
disc models capable of sustaining turbulence for long run times, which can be
used for on-going studies of planet formation in turbulent discs. The results
are obtained using two codes written in spherical coordinates: GLOBAL and
NIRVANA. Both are time--explicit and use finite differences along with the
Constrained Transport algorithm to evolve the equations of MHD. In the presence
of a weak toroidal magnetic field, a thin protoplanetary disc in hydrostatic
equilibrium is destabilised by the magnetorotational instability (MRI). When
the resolution is large enough (25 vertical grid cells per scale height), the
entire disc settles into a turbulent quasi steady-state after about 300 orbits.
Angular momentum is transported outward such that the standard alpha parameter
is roughly 4-6*10^{-3}. We find that the initial toroidal flux is expelled from
the disc midplane and that the disc behaves essentially as a quasi-zero net
flux disc for the remainder of the simulation. As in previous studies, the disc
develops a dual structure composed of an MRI--driven turbulent core around its
midplane, and a magnetised corona stable to the MRI near its surface. By
varying disc parameters and boundary conditions, we show that these basic
properties of the models are robust. The high resolution disc models we present
in this paper achieve a quasi--steady state and sustain turbulence for hundreds
of orbits. As such, they are ideally suited to the study of outstanding
problems in planet formation such as disc--planet interactions and dust
dynamics.Comment: 19 pages, 29 figures, accepted in Astronomy & Astrophysic
Test results for composite specimens and elements containing joints and cutouts
A program was conducted to develop the technology for joints and cutouts in a composite fuselage that meets all design requirements of a large transport aircraft for the 1990s. An advanced trijet derivative of the DC-10 was selected as the baseline aircraft. Design and analysis of a 30-foot-long composite fuselage barrel provided a realistic basis for the test effort. The primary composite material was Hexcel F584 resin on 12 K IM6 fiber, in tape and broadgoods form. Fiberglass broadgoods were used in E-glass and S-glass fiber form in the cutout region of some panels. Additionally, injection-molded chopped graphite fiber/PEEK was used for longeron-to-frame shear clips. The test effort included four groups of test specimens, beginning with coupon specimens of mono-layer and cross-piled laminates, progressing through increasingly larger and more complex specimens, and ending with two 4- by 5-foot curved fuselage side panels. One of the side panels incorporated a transverse skin splice, while the second included two cabin window cutouts
Magnetically Driven Jets in the Kerr Metric
We compute a series of three-dimensional general relativistic
magnetohydrodynamic simulations of accretion flows in the Kerr metric to
investigate the properties of the unbound outflows that result. The overall
strength of these outflows increases sharply with increasing black hole
rotation rate, but a number of generic features are found in all cases. The
mass in the outflow is concentrated in a hollow cone whose opening angle is
largely determined by the effective potential for matter orbiting with angular
momentum comparable to that of the innermost stable circular orbit. The
dominant force accelerating the matter outward comes from the pressure of the
accretion disk's corona. The principal element that shapes the outflow is
therefore the centrifugal barrier preventing accreting matter from coming close
to the rotation axis. Inside the centrifugal barrier, the cone contains very
little matter and is dominated by electromagnetic fields that rotate at a rate
tied closely to the rotation of the black hole. These fields carry an
outward-going Poynting flux whose immediate energy source is the rotating
spacetime of the Kerr black hole. When the spin parameter a/M of the black hole
exceeds ~0.9, the energy carried to infinity by these outflows can be
comparable to the nominal radiative efficiency predicted in the Novikov-Thorne
model. Similarly, the expelled angular momentum can be comparable to that
accreted by the black hole. Both the inner electromagnetic part and the outer
matter part can contribute in significant fashion to the energy and angular
momentum of the outflow.Comment: 43 pages 12 figures To Appear in the Astrophysical Journal replaced
figure 3c with correct imag
A Meiotic Tapas Menu
this past fall (September 13-18), the 7th European Meiosis Meeting held in San Lorenzo de El Escorial brought us'Meiosis in Madrid.''This is a sister conference to the Gordon Meiosis series held every other year in the United States, and is an international small-format forum for sharing hot, new results, with an emphasis on unpublished work. At the conference, 21 countries were represented by 167 participants, with a nearly equal split between male and female scientists. The aim of the meeting was to present and promote in-depth discussions about all aspects of meiotic chromosome dynamics, recombination, and segregation. Rather than providing a comprehensive description of the meeting abstracts, this report will briefly describe meeting highlights, guided in large part by feedback from session chairs
Temperature-driven transition from the Wigner Crystal to the Bond-Charge-Density Wave in the Quasi-One-Dimensional Quarter-Filled band
It is known that within the interacting electron model Hamiltonian for the
one-dimensional 1/4-filled band, the singlet ground state is a Wigner crystal
only if the nearest neighbor electron-electron repulsion is larger than a
critical value. We show that this critical nearest neighbor Coulomb interaction
is different for each spin subspace, with the critical value decreasing with
increasing spin. As a consequence, with the lowering of temperature, there can
occur a transition from a Wigner crystal charge-ordered state to a spin-Peierls
state that is a Bond-Charge-Density Wave with charge occupancies different from
the Wigner crystal. This transition is possible because spin excitations from
the spin-Peierls state in the 1/4-filled band are necessarily accompanied by
changes in site charge densities. We apply our theory to the 1/4-filled band
quasi-one-dimensional organic charge-transfer solids in general and to 2:1
tetramethyltetrathiafulvalene (TMTTF) and tetramethyltetraselenafulvalene
(TMTSF) cationic salts in particular. We believe that many recent experiments
strongly indicate the Wigner crystal to Bond-Charge-Density Wave transition in
several members of the TMTTF family. We explain the occurrence of two different
antiferromagnetic phases but a single spin-Peierls state in the generic phase
diagram for the 2:1 cationic solids. The antiferromagnetic phases can have
either the Wigner crystal or the Bond-Charge-Spin-Density Wave charge
occupancies. The spin-Peierls state is always a Bond-Charge-Density Wave.Comment: 12 pages, 8 EPS figures. Longer version of previous manuscript.
Contains new numerical data as well as greatly expanded discussio
On the Ionisation Fraction in Protoplanetary Disks II: The Effect of Turbulent Mixing on Gas--phase Chemistry
We calculate the ionisation fraction in protostellar disk models using two
different gas-phase chemical networks, and examine the effect of turbulent
mixing by modelling the diffusion of chemical species vertically through the
disk. The aim is to determine in which regions of the disk gas can couple to a
magnetic field and sustain MHD turbulence. We find that the effect of diffusion
depends crucially on the elemental abundance of heavy metals (magnesium)
included in the chemical model. In the absence of heavy metals, diffusion has
essentially no effect on the ionisation structure of the disks, as the
recombination time scale is much shorter than the turbulent diffusion time
scale. When metals are included with an elemental abundance above a threshold
value, the diffusion can dramatically reduce the size of the magnetically
decoupled region, or even remove it altogther. For a complex chemistry the
elemental abundance of magnesium required to remove the dead zone is 10(-10) -
10(-8). We also find that diffusion can modify the reaction pathways, giving
rise to dominant species when diffusion is switched on that are minor species
when diffusion is absent. This suggests that there may be chemical signatures
of diffusive mixing that could be used to indirectly detect turbulent activity
in protoplanetary disks. We find examples of models in which the dead zone in
the outer disk region is rendered deeper when diffusion is switched on. Overall
these results suggest that global MHD turbulence in protoplanetary disks may be
self-sustaining under favourable circumstances, as turbulent mixing can help
maintain the ionisation fraction above that necessary to ensure good coupling
between the gas and magnetic field.Comment: 11 pages, 7 figures; accepted for publication in A &
Numerical evidence for `multi-scalar stars'
We present a class of general relativistic soliton-like solutions composed of
multiple minimally coupled, massive, real scalar fields which interact only
through the gravitational field. We describe a two-parameter family of
solutions we call ``phase-shifted boson stars'' (parameterized by central
density rho_0 and phase delta), which are obtained by solving the ordinary
differential equations associated with boson stars and then altering the phase
between the real and imaginary parts of the field. These solutions are similar
to boson stars as well as the oscillating soliton stars found by Seidel and
Suen [E. Seidel and W.M. Suen, Phys. Rev. Lett. 66, 1659 (1991)]; in
particular, long-time numerical evolutions suggest that phase-shifted boson
stars are stable. Our results indicate that scalar soliton-like solutions are
perhaps more generic than has been previously thought.Comment: Revtex. 4 pages with 4 figures. Submitted to Phys. Rev.
BOSS Ultracool Dwarfs I: Colors and Magnetic Activity of M and L dwarfs
We present the colors and activity of ultracool (M7-L8) dwarfs from the Tenth
Data Release of the Sloan Digital Sky Survey (SDSS). We combine previous
samples of SDSS M and L dwarfs with new data obtained from the Baryon
Oscillation Sky Survey (BOSS) to produce the BOSS Ultracool Dwarf (BUD) sample
of 11820 M7-L8 dwarfs. By combining SDSS data with photometry from the Two
Micron All Sky Survey and the Wide-Field Infrared Sky Explorer mission, we
present ultracool dwarf colors from to as a function of spectral
type, and extend the SDSS-2MASS-WISE color locus to include ultracool dwarfs.
The , , and colors provide the best indication of spectral type
for M7-L3 dwarfs. We also examine ultracool dwarf chromospheric activity
through the presence and strength of H emission. The fraction of active
dwarfs rises through the M spectral sequence until it reaches 90% at
spectral type L0. The fraction of active dwarfs then declines to 50% at
spectral type L5; no H emission is observed in the late-L dwarfs in the
BUD sample. The fraction of active L0-L5 dwarfs is much higher than previously
observed. The strength of activity declines with spectral type from M7 through
L3, after which the data do not show a clear trend. Using one-dimensional
chromosphere models, we explore the range of filling factors and chromospheric
temperature structures that are consistent with H observations of M0-L7
dwarfs. M dwarf chromospheres have a similar, smoothly varying range of
temperature and surface coverage while L dwarf chromospheres are cooler and
have smaller filling factors.Comment: 24 pages and 13 figures, submitted to AJ. A short video describing
these results can be found at https://www.youtube.com/watch?v=wwX5WkuJCU
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