1,808 research outputs found
The dynamics of spiral arms in pure stellar disks
It has been believed that spirals in pure stellar disks, especially the ones
spontaneously formed, decay in several galactic rotations due to the increase
of stellar velocity dispersions. Therefore, some cooling mechanism, for example
dissipational effects of the interstellar medium, was assumed to be necessary
to keep the spiral arms. Here we show that stellar disks can maintain spiral
features for several tens of rotations without the help of cooling, using a
series of high-resolution three-dimensional -body simulations of pure
stellar disks. We found that if the number of particles is sufficiently large,
e.g., , multi-arm spirals developed in an isolated disk can
survive for more than 10 Gyrs. We confirmed that there is a self-regulating
mechanism that maintains the amplitude of the spiral arms. Spiral arms increase
Toomre's of the disk, and the heating rate correlates with the squared
amplitude of the spirals. Since the amplitude itself is limited by the value of
, this makes the dynamical heating less effective in the later phase of
evolution. A simple analytical argument suggests that the heating is caused by
gravitational scattering of stars by spiral arms, and that the self-regulating
mechanism in pure-stellar disks can effectively maintain spiral arms on a
cosmological timescale. In the case of a smaller number of particles, e.g.,
, spiral arms grow faster in the beginning of the simulation
(while is small) and they cause a rapid increase of . As a result, the
spiral arms become faint in several Gyrs.Comment: 18 pages, 19 figures, accepted for Ap
The formation of Kuiper-belt Binaries through Exchange Reactions
Recent observations have revealed an unexpectedly high binary fraction among
the Trans-Neptunian Objects (TNOs) that populate the Kuiper-belt. The
discovered binaries have four characteristics they comprise a few percent of
the TNOs, the mass ratio of their components is close to unity, their internal
orbits are highly eccentric, and the orbits are more than 100 times wider than
the primary's radius. In contrast, theories of binary asteroid formation tend
to produce close, circular binaries. Therefore, a new approach is required to
explain the unique characteristics of the TNO binaries. Two models have been
proposed. Both, however, require extreme assumptions on the size distribution
of TNOs. Here we show a mechanism which is guaranteed to produces binaries of
the required type during the early TNO growth phase, based on only one
plausible assumption, namely that initially TNOs were formed through
gravitational instabilities of the protoplanetary dust layer.Comment: 12pages, 4 figure
Dust accretion onto high-mass planets
We study the accretion of dust particles of various sizes onto embedded
massive gas giant planets, where we take into account the structure of the gas
disk due to the presence of the planet. The accretion rate of solids is
important for the structure of giant planets: it determines the growth rate of
the solid core that may be present as well as their final enrichment in solids.
We use the RODEO hydrodynamics solver to solve the flow equations for the gas,
together with a particle approach for the dust. The solver for the particles'
equations of motion is implicit with respect to the drag force, which allows us
to treat the whole dust size spectrum. We find that dust accretion is limited
to the smallest particle sizes. The largest particles get trapped in outer
mean-motion resonances with the planet, while particles of intermediate size
are pushed away from the orbit of the planet by the density structure in the
gas disk. Only particles smaller than approximately s_max =10 micron may
accrete on a planet with the mass of Jupiter. For a ten times less massive
planet s_max=100 micron. The strongly reduced accretion of dust makes it very
hard to enrich a newly formed giant planet in solids.Comment: 15 pages, 18 figures, accepted for publication in A&
Fluoride-containing bioactive glasses: Effect of glass design and structure on degradation, pH and apatite formation in simulated body fluid
NOTICE: this is the author’s version of a work that was accepted for publication in Acta Biomaterialia. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Acta Biomaterialia, [VOL 6, ISSUE 8, (2010)] DOI: 10.1016/j.actbio.2010.01.04
Functionalization of different polymers with sulfonic groups as a way to coat them with a biomimetic apatite layer
Covalent coupling of sulfonic group (–SO3H)
was attempted on different polymers to evaluate efficacy of
this functional group in inducing nucleation of apatite in
body environment, and thereupon to design a simple biomimetic
process for preparing bonelike apatite-polymer
composites. Substrates of polyethylene terephthalate
(PET), polycaprolactam (Nylon 6), high molecular weight
polyethylene (HMWPE) and ethylene-vinyl alcohol copolymer
(EVOH) were subjected to sulfonation by being
soaked in sulfuric acid (H2SO4) or chlorosulfonic acid
(ClSO3H) with different concentrations. In order to incorporate
calcium ions, the sulfonated substrates were soaked
in saturated solution of calcium hydroxide (Ca(OH)2). The
treated substrates were soaked in a simulated body fluid
(SBF). Fourier transformed infrared spectroscopy, thin-film
X-ray diffraction, and scanning electron microscopy
showed that the sulfonation and subsequent Ca(OH)2
treatments allowed formation of –SO3H groups binding
Ca2+ ions on the surface of HMWPE and EVOH, but not on
PET and Nylon 6. The HMWPE and EVOH could thus
form bonelike apatite layer on their surfaces in SBF within
7 d. These results indicate that the –SO3H groups are
effective for inducing apatite nucleation, and thereby that
surface sulfonation of polymers are effective pre-treatment
method for preparing biomimetic apatite on their surfaces
Migration then assembly: Formation of Neptune mass planets inside 1 AU
We demonstrate that the observed distribution of `Hot Neptune'/`Super-Earth'
systems is well reproduced by a model in which planet assembly occurs in situ,
with no significant migration post-assembly. This is achieved only if the
amount of mass in rocky material is -- interior to 1
AU. Such a reservoir of material implies that significant radial migration of
solid material takes place, and that it occur before the stage of final planet
assembly.
The model not only reproduces the general distribution of mass versus period,
but also the detailed statistics of multiple planet systems in the sample.
We furthermore demonstrate that cores of this size are also likely to meet
the criterion to gravitationally capture gas from the nebula, although
accretion is rapidly limited by the opening of gaps in the gas disk. If the
mass growth is limited by this tidal truncation, then the scenario sketched
here naturally produces Neptune-mass objects with substantial components of
both rock and gas, as is observed.
The quantitative expectations of this scenario are that most planets in the
`Hot Neptune/Super-Earth' class inhabit multiple-planet systems, with
characteristic orbital spacings. The model also provides a natural division
into gas-rich (Hot Neptune) and gas-poor (Super-Earth) classes at fixed period.
The dividing mass ranges from at 10 day orbital periods to
at 100 day orbital periods. For orbital periods
days, the division is less clear because a gas atmosphere may be significantly
eroded by stellar radiation.Comment: 41 pages in preprint style, 15 figures, final version accepted to Ap
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