3,325 research outputs found
Gas- and dust evolution in protoplanetary disks
Context. Current models of the size- and radial evolution of dust in
protoplanetary disks generally oversimplify either the radial evolution of the
disk (by focussing at one single radius or by using steady state disk models)
or they assume particle growth to proceed monodispersely or without
fragmentation. Further studies of protoplanetary disks - such as observations,
disk chemistry and structure calculations or planet population synthesis models
- depend on the distribution of dust as a function of grain size and radial
position in the disk.
Aims. We attempt to improve upon current models to be able to investigate how
the initial conditions, the build-up phase, and the evolution of the
protoplanetary disk influence growth and transport of dust.
Methods. We introduce a new version of the model of Brauer et al. (2008) in
which we now include the time-dependent viscous evolution of the gas disk, and
in which more advanced input physics and numerical integration methods are
implemented.
Results. We show that grain properties, the gas pressure gradient, and the
amount of turbulence are much more influencing the evolution of dust than the
initial conditions or the build-up phase of the protoplanetary disk. We
quantify which conditions or environments are favorable for growth beyond the
meter size barrier. High gas surface densities or zonal flows may help to
overcome the problem of radial drift, however already a small amount of
turbulence poses a much stronger obstacle for grain growth.Comment: accepted to A&
Guaranteed emergence of genuine entanglement in 3-qubit evolving systems
Multipartite entanglement has been shown to be of particular relevance for a
better understanding and exploitation of the dynamics and flow of entanglement
in multiparty systems. This calls for analysis aimed at identifying the
appropriate processes that guarantee the emergence of multipartite entanglement
in a wide range of scenarios. Here we carry on such analysis considering a
system of two initially entangled qubits, one of which is let to interact with
a third qubit according to an arbitrary unitary evolution. We establish
necessary and sufficient conditions on the corresponding Kraus operators, to
discern whether the evolved state pertains to either one of the classes of
3-qubit pure states that exhibit some kind of entanglement, namely biseparable,
W-, and GHZ- genuine entangled classes. Our results provide a classification of
the Kraus operators according to their capacity of producing 3-qubit
entanglement, and pave the way for extending the analysis to larger systems and
determining the particular interactions that must be implemented in order to
create, enhance and distribute entanglement in a specific manner.Comment: Two new subsections included. Accepted for publication in The
European Physical Journal
Dust retention in protoplanetary disks
Context: Protoplanetary disks are observed to remain dust-rich for up to
several million years. Theoretical modeling, on the other hand, raises several
questions. Firstly, dust coagulation occurs so rapidly, that if the small dust
grains are not replenished by collisional fragmentation of dust aggregates,
most disks should be observed to be dust poor, which is not the case. Secondly,
if dust aggregates grow to sizes of the order of centimeters to meters, they
drift so fast inwards, that they are quickly lost.
Aims: We attempt to verify if collisional fragmentation of dust aggregates is
effective enough to keep disks 'dusty' by replenishing the population of small
grains and by preventing excessive radial drift.
Methods: With a new and sophisticated implicitly integrated coagulation and
fragmentation modeling code, we solve the combined problem of coagulation,
fragmentation, turbulent mixing and radial drift and at the same time solve for
the 1-D viscous gas disk evolution.
Results: We find that for a critical collision velocity of 1 m/s, as
suggested by laboratory experiments, the fragmentation is so effective, that at
all times the dust is in the form of relatively small particles. This means
that radial drift is small and that large amounts of small dust particles
remain present for a few million years, as observed. For a critical velocity of
10 m/s, we find that particles grow about two orders of magnitude larger, which
leads again to significant dust loss since larger particles are more strongly
affected by radial drift.Comment: Letter accepted 3 July 2009, included comments of language edito
Twisted Conjugacy Classes in Abelian Extensions of Certain Linear Groups
Given an automorphism , one has an action of
on itself by -twisted conjugacy, namely, . The orbits
of this action are called -twisted conjugacy classes. One says that
has the -property if there are infinitely many
-twisted conjugacy classes for every automorphism of . In
this paper we show that SL and its congruence subgroups have
the -property. Further we show that any (countable) abelian extension
of has the -property where is a torsion free
non-elementary hyperbolic group, or SL, Sp or
a principal congruence subgroup of SL or the fundamental group
of a complete Riemannian manifold of constant negative curvature
Dust size distributions in coagulation/fragmentation equilibrium: Numerical solutions and analytical fits
Context. Grains in circumstellar disks are believed to grow by mutual
collisions and subsequent sticking due to surface forces. Results of many
fields of research involving circumstellar disks, such as radiative transfer
calculations, disk chemistry, magneto-hydrodynamic simulations largely depend
on the unknown grain size distribution.
Aims. As detailed calculations of grain growth and fragmentation are both
numerically challenging and computationally expensive, we aim to find simple
recipes and analytical solutions for the grain size distribution in
circumstellar disks for a scenario in which grain growth is limited by
fragmentation and radial drift can be neglected.
Methods. We generalize previous analytical work on self-similar steady-state
grain distributions. Numerical simulations are carried out to identify under
which conditions the grain size distributions can be understood in terms of a
combination of power-law distributions. A physically motivated fitting formula
for grain size distributions is derived using our analytical predictions and
numerical simulations.
Results. We find good agreement between analytical results and numerical
solutions of the Smoluchowski equation for simple shapes of the kernel
function. The results for more complicated and realistic cases can be fitted
with a physically motivated "black box" recipe presented in this paper. Our
results show that the shape of the dust distribution is mostly dominated by the
gas surface density (not the dust-to-gas ratio), the turbulence strength and
the temperature and does not obey an MRN type distribution.Comment: 16 pages, 9 figures, accepted for publication in A&
First-principles calculation of positron lifetimes and affinities in perfect and imperfect transition-metal carbides and nitrides
First-principles electronic structure and positron-state calculations for transition-metal carbides and nitrides are performed. Perfect NaCl structures as well as structures with metal or carbon/nitrogen vacancies are considered. The positron affinities and lifetimes are determined. The trends are discussed and the results are compared with recent positron lifetime measurements for group-IV and -V refractory metal carbides. The present analysis suggests, contradictory to an earlier interpretation, that positrons are trapped and annihilated at both carbon and metal vacancies. The concentration of metal vacancies detected by positron annihilation methods is probably very low, below the sensitivity limit of other experimental methods.Peer reviewe
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Thesis Statement For M.F.A.
Several months ago, problems I had committed myself to began tieing down my work. The solution seemed out of reach. But through a paring-down process, a means toward resolution emerged. By elimination of multiple motifs, story lines, staging and cartoon characterizations, a greater interaction between image, theme and materials occurred. I am pleased with the directness and the isolated intensity of the images.
To say less in this case is to say more. Now the work is a synthesis of styles and attitudes I exhibited previously. This has also settled the conflict between my drawing and painting styles which has bound me for so long.
In light of past accusations of overver-balization, I have decided to structure this thesis in a different manner than the norm. Hopefully, this brevity of approach will provide more clarity to the viewer's apprehension of the work. I am convinced that this is the best form in which to snap the fetters of over-conceptualization and gratuitous verbiage.
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Electric and acoustic stimulation during movement preparation can facilitate movement execution in healthy participants and stroke survivors
There has been increasing interest in the use of loud acoustic stimulation (LAS) to gain insight into the preparation and initiation of motor actions. Typically, LAS presented during movement preparation in healthy participants culminates in the earlier than normal initiation of the prepared movement and an increase in the magnitude of the response. Recent reports have shown LAS can also facilitate movement in chronic stroke survivors. This suggests that current therapies for motor recovery after stroke might benefit from employing such alternate methods of triggering movement. In this study we sought to test a new way to facilitate motor actions that could be of relevance in clinical settings. Five individuals with chronic motor impairments due to stroke and eight healthy young adults performed a functional reaching task in response to a visual go-signal. On 30% of the trials, LAS or electric stimuli (collectively, sensory stimuli) were unexpectedly presented in synchrony with the go-signal. Both healthy and stroke participants reacted with shorter latencies and executed faster responses when sensory stimulation was synchronized with the go-signal. We have replicated previous findings showing acoustic stimuli can aid movement execution in chronic stroke survivors and demonstrated the same type of effect can be achieved using electric stimulation. Thus, these two types of sensory stimuli can be easily integrated with current devices available to assist people with stroke to engage in rehabilitation efforts
A simple model for the evolution of the dust population in protoplanetary disks
Context: The global size and spatial distribution of dust is an important
ingredient in the structure and evolution of protoplanetary disks and in the
formation of larger bodies, such as planetesimals. Aims: We aim to derive
simple equations that explain the global evolution of the dust surface density
profile and the upper limit of the grain size distribution and which can
readily be used for further modeling or for interpreting of observational data.
Methods: We have developed a simple model that follows the upper end of the
dust size distribution and the evolution of the dust surface density profile.
This model is calibrated with state-of-the-art simulations of dust evolution,
which treat dust growth, fragmentation, and transport in viscously evolving gas
disks. Results: We find very good agreement between the full dust-evolution
code and the toy model presented in this paper. We derive analytical profiles
that describe the dust-to-gas ratios and the dust surface density profiles well
in protoplanetary disks, as well as the radial flux by solid material "rain
out", which is crucial for triggering any gravity assisted formation of
planetesimals. We show that fragmentation is the dominating effect in the inner
regions of the disk leading to a dust surface density exponent of -1.5, while
the outer regions at later times can become drift-dominated, yielding a dust
surface density exponent of -0.75. Our results show that radial drift is not
efficient in fragmenting dust grains. This supports the theory that small dust
grains are resupplied by fragmentation due to the turbulent state of the disk.Comment: 12 pages, 10 figures, accepted to A&
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