450 research outputs found
Wind-shearing in gaseous protoplanetary disks and the evolution of binary planetesimals
One of the first stages of planet formation is the growth of small
planetesimals. This early stage occurs much before the dispersal of most of the
gas from the protoplanetary disk. Due to their different aerodynamic
properties, planetesimals of different sizes and shapes experience different
drag forces from the gas during this time. Such differential forces produce a
wind-shearing (WISH) effect between close by, different size planetesimals. For
any two planetesimals, a WISH radius can be considered, at which the
differential acceleration due to the wind becomes greater than the mutual
gravitational pull between the planetesimals. We find that the WISH radius
could be much smaller than the Hill radius, i.e. WISH could play a more
important role than tidal perturbations by the star. Here we study the WISH
radii for planetesimal pairs of different sizes and compare the effects of wind
and gravitational shearing (drag force vs. gravitational tidal force). We then
discuss the role of WISH for the stability and survival of binary
planetesimals. Binaries are sheared apart by the wind if they are wider than
their WISH radius. WISH-stable binaries can inspiral and possibly coalesce due
to gas drag. Here, we calculate the WISH radius and the gas drag-induced merger
timescale, providing stability and survival criteria for gas-embedded binary
planetesimals. Our results suggest that even WISH-stable binaries may merge in
times shorter than the lifetime of the gaseous disk. This may constrain
currently observed binary planetesimals to have formed far from the star or at
a late stage after the dispersal of most of the disk gas. We note that the WISH
radius may also be important for other processes such as planetesimal erosion
and planetesimal encounters and collisions in a gaseous environment.Comment: ApJ, in pres
The rate of WD-WD head-on collisions in isolated triples is too low to explain standard type Ia supernovae
Type Ia supernovae (Ia-SNe) are thought to arise from the thermonuclear
explosions of white dwarfs (WDs). The progenitors of such explosions are still
highly debated; in particular the conditions leading to detonations in WDs are
not well understood in most of the suggested progenitor models. Nevertheless,
direct head-on collisions of two WDs were shown to give rise to detonations and
produce Ia-SNe - like explosions, and were suggested as possible progenitors.
The rates of such collisions in dense globular clusters are far below the
observed rates of type Ia SNe, but it was suggested that quasi-secular
evolution of hierarchical triples could produce a high rate of such collisions.
Here we used detailed triple stellar evolution populations synthesis models
coupled with dynamical secular evolution to calculate the rates of WD-WD
collisions in triples and their properties. We explored a range of models with
different realistic initial conditions and derived the expected SNe total mass,
mass-ratio and delay time distributions for each of the models. We find that
the SNe rate from WD-WD collisions is of the order of 0.1% of the observed
Ia-SNe rate across all our models, and the delay-time distribution is almost
uniform in time, and is inconsistent with observations. We conclude that SNe
from WD-WD collisions in isolated triples can at most provide for a small
fraction of Ia-SNe, and can not serve as the main progenitors of such
explosions.Comment: 13 pages, 4 figures, submitted to A&
Luminous Supernovae
Supernovae (SNe), the luminous explosions of stars, were observed since
antiquity, with typical peak luminosity not exceeding 1.2x10^{43} erg/s
(absolute magnitude >-19.5 mag). It is only in the last dozen years that
numerous examples of SNe that are substantially super-luminous (>7x10^{43}
erg/s; <-21 mag absolute) were well-documented. Reviewing the accumulated
evidence, we define three broad classes of super-luminous SN events (SLSNe).
Hydrogen-rich events (SLSN-II) radiate photons diffusing out from thick
hydrogen layers where they have been deposited by strong shocks, and often show
signs of interaction with circumstellar material. SLSN-R, a rare class of
hydrogen-poor events, are powered by very large amounts of radioactive 56Ni and
arguably result from explosions of very massive stars due to the pair
instability. A third, distinct group of hydrogen-poor events emits photons from
rapidly-expanding hydrogen-poor material distributed over large radii, and are
not powered by radioactivity (SLSN-I). These may be the hydrogen-poor analogs
of SLSN-II.Comment: This manuscript has been accepted for publication in Science (to
appear August 24). This version has not undergone final editing. Please refer
to the complete version of record at http://www.sciencemag.org/. The
manuscript may not be reproduced or used in any manner that does not fall
within the fair use provisions of the Copyright Act without the prior,
written permission of AAA
Classical Diffusion of a quantum particle in a noisy environment
We study the spreading of a quantum-mechanical wavepacket in a
one-dimensional tight-binding model with a noisy potential, and analyze the
emergence of classical diffusion from the quantum dynamics due to decoherence.
We consider a finite correlation time of the noisy environment, and treat the
system by utilizing the separation of fast (dephasing) and slow (diffusion)
processes. We show that classical diffusive behavior emerges at long times, and
we calculate analytically the dependence of the classical diffusion coefficient
on the noise magnitude and correlation time. This method provides a general
solution to this problem for arbitrary conditions of the noisy environment. The
results are relevant to a large variety of physical systems, from electronic
transport in solid state physics, to light transmission in optical devices,
diffusion of excitons, and quantum computation
Universal computation by multi-particle quantum walk
A quantum walk is a time-homogeneous quantum-mechanical process on a graph
defined by analogy to classical random walk. The quantum walker is a particle
that moves from a given vertex to adjacent vertices in quantum superposition.
Here we consider a generalization of quantum walk to systems with more than one
walker. A continuous-time multi-particle quantum walk is generated by a
time-independent Hamiltonian with a term corresponding to a single-particle
quantum walk for each particle, along with an interaction term. Multi-particle
quantum walk includes a broad class of interacting many-body systems such as
the Bose-Hubbard model and systems of fermions or distinguishable particles
with nearest-neighbor interactions. We show that multi-particle quantum walk is
capable of universal quantum computation. Since it is also possible to
efficiently simulate a multi-particle quantum walk of the type we consider
using a universal quantum computer, this model exactly captures the power of
quantum computation. In principle our construction could be used as an
architecture for building a scalable quantum computer with no need for
time-dependent control
The Mass-Loss Induced Eccentric Kozai Mechanism: A New Channel for the Production of Close Compact Object-Stellar Binaries
Over a broad range of initial inclinations and eccentricities an appreciable
fraction of hierarchical triple star systems with similar masses are
essentially unaffected by the Kozai-Lidov mechanism (KM) until the primary in
the central binary evolves into a compact object. Once it does, it may be much
less massive than the other components in the ternary, enabling the "eccentric
Kozai mechanism (EKM):" the mutual inclination between the inner and outer
binary can flip signs driving the inner binary to very high eccentricity,
leading to a close binary or collision. We demonstrate this "Mass-loss Induced
Eccentric Kozai" (MIEK) mechanism by considering an example system and defining
an ad-hoc minimal separation between the inner two members at which tidal
affects become important. For fixed initial masses and semi-major axes, but
uniform distributions of eccentricity and cosine of the mutual inclination,
~10% of systems interact tidally or collide while the primary is on the MS due
to the KM or EKM. Those affected by the EKM are not captured by earlier
quadrupole-order secular calculations. We show that fully ~30% of systems
interact tidally or collide for the first time as the primary swells to AU
scales, mostly as a result of the KM. Finally, ~2% of systems interact tidally
or collide for the first time after the primary sheds most of its mass and
becomes a WD, mostly as a result of the MIEK mechanism. These findings motivate
a more detailed study of mass-loss in triple systems and the formation of close
NS/WD-MS and NS/WD-NS/WD binaries without an initial common envelope phase.Comment: 12 pages, 6 figures, 1 table. Accepted for publication in ApJ. For a
brief video explaining this paper, see http://youtu.be/4CdTOF17q5
Realization of quantum walks with negligible decoherence in waveguide lattices
Quantum random walks are the quantum counterpart of classical random walks, and were recently studied in the context of quantum computation. Physical implementations of quantum walks have only been made in very small scale systems severely limited by decoherence. Here we show that the propagation of photons in waveguide lattices, which have been studied extensively in recent years, are essentially an implementation of quantum walks. Since waveguide lattices are easily constructed at large scales and display negligible decoherence, they can serve as an ideal and versatile experimental playground for the study of quantum walks and quantum algorithms. We experimentally observe quantum walks in large systems (similar to 100 sites) and confirm quantum walks effects which were studied theoretically, including ballistic propagation, disorder, and boundary related effects
Pharmacological targeting of AKAP-directed compartmentalized cAMP signalling
The second messenger cyclic adenosine monophosphate (cAMP) can bind and activate protein kinase A (PKA). The cAMP/PKA system is ubiquitous and involved in a wide array of biological processes and therefore requires tight spatial and temporal regulation. Important components of the safeguard system are the A-kinase anchoring proteins (AKAPs), a heterogeneous family of scaffolding proteins defined by its ability to directly bind PKA. AKAPs tether PKA to specific subcellular compartments, and they bind further interaction partners to create local signalling hubs. The recent discovery of new AKAPs and advances in the field that shed light on the relevance of these hubs for human disease highlight unique opportunities for pharmacological modulation. This review exemplifies how interference with signalling, particularly cAMP signalling, at such hubs can reshape signalling responses and discusses how this could lead to novel pharmacological concepts for the treatment of disease with an unmet medical need such as cardiovascular disease and cancer
Hypervelocity Planets and Transits Around Hypervelocity Stars
The disruption of a binary star system by the massive black hole at the
Galactic Centre, SgrA*, can lead to the capture of one star around SgrA* and
the ejection of its companion as a hypervelocity star (HVS). We consider the
possibility that these stars may have planets and study the dynamics of these
planets. Using a direct -body integration code, we simulated a large number
of different binary orbits around SgrA*. For some orbital parameters, a planet
is ejected at a high speed. In other instances, a HVS is ejected with one or
more planets orbiting around it. In these cases, it may be possible to observe
the planet as it transits the face of the star. A planet may also collide with
its host star. In such cases the atmosphere of the star will be enriched with
metals. In other cases, a planet is tidally disrupted by SgrA*, leading to a
bright flare.Comment: 8 pages, 5 figures, 2 tables, accepted for publication in MNRA
Quantum Walk in Position Space with Single Optically Trapped Atoms
The quantum walk is the quantum analogue of the well-known random walk, which
forms the basis for models and applications in many realms of science. Its
properties are markedly different from the classical counterpart and might lead
to extensive applications in quantum information science. In our experiment, we
implemented a quantum walk on the line with single neutral atoms by
deterministically delocalizing them over the sites of a one-dimensional
spin-dependent optical lattice. With the use of site-resolved fluorescence
imaging, the final wave function is characterized by local quantum state
tomography, and its spatial coherence is demonstrated. Our system allows the
observation of the quantum-to-classical transition and paves the way for
applications, such as quantum cellular automata.Comment: 7 pages, 4 figure
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