4,067 research outputs found
Rapid planetesimal formation in turbulent circumstellar discs
The initial stages of planet formation in circumstellar gas discs proceed via
dust grains that collide and build up larger and larger bodies (Safronov 1969).
How this process continues from metre-sized boulders to kilometre-scale
planetesimals is a major unsolved problem (Dominik et al. 2007): boulders stick
together poorly (Benz 2000), and spiral into the protostar in a few hundred
orbits due to a head wind from the slower rotating gas (Weidenschilling 1977).
Gravitational collapse of the solid component has been suggested to overcome
this barrier (Safronov 1969, Goldreich & Ward 1973, Youdin & Shu 2002). Even
low levels of turbulence, however, inhibit sedimentation of solids to a
sufficiently dense midplane layer (Weidenschilling & Cuzzi 1993, Dominik et al.
2007), but turbulence must be present to explain observed gas accretion in
protostellar discs (Hartmann 1998). Here we report the discovery of efficient
gravitational collapse of boulders in locally overdense regions in the
midplane. The boulders concentrate initially in transient high pressures in the
turbulent gas (Johansen, Klahr, & Henning 2006), and these concentrations are
augmented a further order of magnitude by a streaming instability (Youdin &
Goodman 2005, Johansen, Henning, & Klahr 2006, Johansen & Youdin 2007) driven
by the relative flow of gas and solids. We find that gravitationally bound
clusters form with masses comparable to dwarf planets and containing a
distribution of boulder sizes. Gravitational collapse happens much faster than
radial drift, offering a possible path to planetesimal formation in accreting
circumstellar discs.Comment: To appear in Nature (30 August 2007 issue). 18 pages (in referee
mode), 3 figures. Supplementary Information can be found at 0708.389
Staircase Quantum Dots Configuration in Nanowires for Optimized Thermoelectric Power
The performance of thermoelectric energy harvesters can be improved by nanostructures that exploit inelastic transport processes. One prototype is the three-terminal hopping thermoelectric device where electron hopping between quantum-dots are driven by hot phonons. Such three-terminal hopping thermoelectric devices have potential in achieving high efficiency or power via inelastic transport and without relying on heavy-elements or toxic compounds. We show in this work how output power of the device can be optimized via tuning the number and energy configuration of the quantum-dots embedded in parallel nanowires. We find that the staircase energy configuration with constant energy-step can improve the power factor over a serial connection of a single pair of quantum-dots. Moreover, for a fixed energy-step, there is an optimal length for the nanowire. Similarly for a fixed number of quantum-dots there is an optimal energy-step for the output power. Our results are important for future developments of high-performance nanostructured thermoelectric devices
Complete intersections: Moduli, Torelli, and good reduction
We study the arithmetic of complete intersections in projective space over
number fields. Our main results include arithmetic Torelli theorems and
versions of the Shafarevich conjecture, as proved for curves and abelian
varieties by Faltings. For example, we prove an analogue of the Shafarevich
conjecture for cubic and quartic threefolds and intersections of two quadrics.Comment: 37 pages. Typo's fixed. Expanded Section 2.
The low level of debris disk activity at the time of the Late Heavy Bombardment: a Spitzer study of Praesepe
We present 24 micron photometry of the intermediate-age open cluster
Praesepe. We assemble a catalog of 193 probable cluster members that are
detected in optical databases, the Two Micron All Sky Survey (2MASS), and at 24
micron, within an area of ~ 2.47 square degrees. Mid-IR excesses indicating
debris disks are found for one early-type and for three solar-type stars.
Corrections for sampling statistics yield a 24 micron excess fraction (debris
disk fraction) of 6.5 +- 4.1% for luminous and 1.9 +- 1.2% for solar-type
stars. The incidence of excesses is in agreement with the decay trend of debris
disks as a function of age observed for other cluster and field stars. The
values also agree with those for older stars, indicating that debris generation
in the zones that emit at 24 micron falls to the older 1-10 Gyr field star
sample value by roughly 750 Myr.
We discuss our results in the context of previous observations of excess
fractions for early- and solar-type stars. We show that solar-type stars lose
their debris disk 24 micron excesses on a shorter timescale than early-type
stars. Simplistic Monte Carlo models suggest that, during the first Gyr of
their evolution, up to 15-30% of solar-type stars might undergo an orbital
realignment of giant planets such as the one thought to have led to the Late
Heavy Bombardment, if the length of the bombardment episode is similar to the
one thought to have happened in our Solar System.
In the Appendix, we determine the cluster's parameters via boostrap Monte
Carlo isochrone fitting, yielding an age of 757 Myr (+- 36 Myr at 1 sigma
confidence) and a distance of 179 pc (+- 2 pc at 1 sigma confidence), not
allowing for systematic errors.Comment: 22 pages, 14 figures, 9 tables, emulateapj format; Accepted for
publication in The Astrophysical Journa
Region graph partition function expansion and approximate free energy landscapes: Theory and some numerical results
Graphical models for finite-dimensional spin glasses and real-world
combinatorial optimization and satisfaction problems usually have an abundant
number of short loops. The cluster variation method and its extension, the
region graph method, are theoretical approaches for treating the complicated
short-loop-induced local correlations. For graphical models represented by
non-redundant or redundant region graphs, approximate free energy landscapes
are constructed in this paper through the mathematical framework of region
graph partition function expansion. Several free energy functionals are
obtained, each of which use a set of probability distribution functions or
functionals as order parameters. These probability distribution
function/functionals are required to satisfy the region graph
belief-propagation equation or the region graph survey-propagation equation to
ensure vanishing correction contributions of region subgraphs with dangling
edges. As a simple application of the general theory, we perform region graph
belief-propagation simulations on the square-lattice ferromagnetic Ising model
and the Edwards-Anderson model. Considerable improvements over the conventional
Bethe-Peierls approximation are achieved. Collective domains of different sizes
in the disordered and frustrated square lattice are identified by the
message-passing procedure. Such collective domains and the frustrations among
them are responsible for the low-temperature glass-like dynamical behaviors of
the system.Comment: 30 pages, 11 figures. More discussion on redundant region graphs. To
be published by Journal of Statistical Physic
Turbulence and galactic structure
Interstellar turbulence is driven over a wide range of scales by processes
including spiral arm instabilities and supernovae, and it affects the rate and
morphology of star formation, energy dissipation, and angular momentum transfer
in galaxy disks. Star formation is initiated on large scales by gravitational
instabilities which control the overall rate through the long dynamical time
corresponding to the average ISM density. Stars form at much higher densities
than average, however, and at much faster rates locally, so the slow average
rate arises because the fraction of the gas mass that forms stars at any one
time is low, ~10^{-4}. This low fraction is determined by turbulence
compression, and is apparently independent of specific cloud formation
processes which all operate at lower densities. Turbulence compression also
accounts for the formation of most stars in clusters, along with the cluster
mass spectrum, and it gives a hierarchical distribution to the positions of
these clusters and to star-forming regions in general. Turbulent motions appear
to be very fast in irregular galaxies at high redshift, possibly having speeds
equal to several tenths of the rotation speed in view of the morphology of
chain galaxies and their face-on counterparts. The origin of this turbulence is
not evident, but some of it could come from accretion onto the disk. Such high
turbulence could help drive an early epoch of gas inflow through viscous
torques in galaxies where spiral arms and bars are weak. Such evolution may
lead to bulge or bar formation, or to bar re-formation if a previous bar
dissolved. We show evidence that the bar fraction is about constant with
redshift out to z~1, and model the formation and destruction rates of bars
required to achieve this constancy.Comment: in: Penetrating Bars through Masks of Cosmic Dust: The Hubble Tuning
Fork strikes a New Note, Eds., K. Freeman, D. Block, I. Puerari, R. Groess,
Dordrecht: Kluwer, in press (presented at a conference in South Africa, June
7-12, 2004). 19 pgs, 5 figure
X-ray monitoring of classical novae in the central region of M 31. II. Autumn and winter 2007/2008 and 2008/2009
[Abridged] Classical novae (CNe) represent the major class of supersoft X-ray
sources (SSSs) in the central region of our neighbouring galaxy M 31. We
performed a dedicated monitoring of the M 31 central region with XMM-Newton and
Chandra between Nov 2007 and Feb 2008 and between Nov 2008 and Feb 2009
respectively, in order to find SSS counterparts of CNe, determine the duration
of their SSS phase and derive physical outburst parameters. We systematically
searched our data for X-ray counterparts of CNe and determined their X-ray
light curves and spectral properties. We detected in total 17 X-ray
counterparts of CNe in M 31, only four of which were known previously. These
latter sources are still active 12.5, 11.0, 7.4 and 4.8 years after the optical
outburst. From the 17 X-ray counterparts 13 were classified as SSSs. Four novae
displayed short SSS phases (< 100 d). Based on these results and previous
studies we compiled a catalogue of all novae with SSS counterparts in M 31
known so far. We used this catalogue to derive correlations between the
following X-ray and optical nova parameters: turn-on time, turn-off time,
effective temperature (X-ray), t2 decay time and expansion velocity of the
ejected envelope (optical). Furthermore, we found a first hint for the
existence of a difference between SSS parameters of novae associated with the
stellar populations of the M 31 bulge and disk. Additionally, we conducted a
Monte Carlo Markov Chain simulation on the intrinsic fraction of novae with SSS
phase. This simulation showed that the relatively high fraction of novae
without detected SSS emission might be explained by the inevitably incomplete
coverage with X-ray observations in combination with a large fraction of novae
with short SSS states, as expected from the WD mass distribution. In order to
verify our results with an increased sample further monitoring observations are
needed.Comment: 31 pages, 23 figures, 10 tables; submitted to A&
Atmospheric Heating and Wind Acceleration: Results for Cool Evolved Stars based on Proposed Processes
A chromosphere is a universal attribute of stars of spectral type later than
~F5. Evolved (K and M) giants and supergiants (including the zeta Aurigae
binaries) show extended and highly turbulent chromospheres, which develop into
slow massive winds. The associated continuous mass loss has a significant
impact on stellar evolution, and thence on the chemical evolution of galaxies.
Yet despite the fundamental importance of those winds in astrophysics, the
question of their origin(s) remains unsolved. What sources heat a chromosphere?
What is the role of the chromosphere in the formation of stellar winds? This
chapter provides a review of the observational requirements and theoretical
approaches for modeling chromospheric heating and the acceleration of winds in
single cool, evolved stars and in eclipsing binary stars, including physical
models that have recently been proposed. It describes the successes that have
been achieved so far by invoking acoustic and MHD waves to provide a physical
description of plasma heating and wind acceleration, and discusses the
challenges that still remain.Comment: 46 pages, 9 figures, 1 table; modified and unedited manuscript;
accepted version to appear in: Giants of Eclipse, eds. E. Griffin and T. Ake
(Berlin: Springer
The next detectors for gravitational wave astronomy
This paper focuses on the next detectors for gravitational wave astronomy
which will be required after the current ground based detectors have completed
their initial observations, and probably achieved the first direct detection of
gravitational waves. The next detectors will need to have greater sensitivity,
while also enabling the world array of detectors to have improved angular
resolution to allow localisation of signal sources. Sect. 1 of this paper
begins by reviewing proposals for the next ground based detectors, and presents
an analysis of the sensitivity of an 8 km armlength detector, which is proposed
as a safe and cost-effective means to attain a 4-fold improvement in
sensitivity. The scientific benefits of creating a pair of such detectors in
China and Australia is emphasised. Sect. 2 of this paper discusses the high
performance suspension systems for test masses that will be an essential
component for future detectors, while sect. 3 discusses solutions to the
problem of Newtonian noise which arise from fluctuations in gravity gradient
forces acting on test masses. Such gravitational perturbations cannot be
shielded, and set limits to low frequency sensitivity unless measured and
suppressed. Sects. 4 and 5 address critical operational technologies that will
be ongoing issues in future detectors. Sect. 4 addresses the design of thermal
compensation systems needed in all high optical power interferometers operating
at room temperature. Parametric instability control is addressed in sect. 5.
Only recently proven to occur in Advanced LIGO, parametric instability
phenomenon brings both risks and opportunities for future detectors. The path
to future enhancements of detectors will come from quantum measurement
technologies. Sect. 6 focuses on the use of optomechanical devices for
obtaining enhanced sensitivity, while sect. 7 reviews a range of quantum
measurement options
Natural history, phenotypic spectrum, and discriminative features of multisystemic RFC1 disease
Objective To delineate the full phenotypic spectrum, discriminative features, piloting longitudinal progression data, and sample size calculations of replication factor complex subunit 1 (RFC1) repeat expansions, recently identified as causing cerebellar ataxia, neuropathy, vestibular areflexia syndrome (CANVAS).
Methods Multimodal RFC1 repeat screening (PCR, Southern blot, whole-exome/genome sequencing?based approaches) combined with cross-sectional and longitudinal deep phenotyping in (1) cross-European cohort A (70 families) with ?2 features of CANVAS or ataxia with chronic cough (ACC) and (2) Turkish cohort B (105 families) with unselected late-onset ataxia.
Results Prevalence of RFC1 disease was 67% in cohort A, 14% in unselected cohort B, 68% in clinical CANVAS, and 100% in ACC. RFC1 disease was also identified in Western and Eastern Asian individuals and even by whole-exome sequencing. Visual compensation, sensory symptoms, and cough were strong positive discriminative predictors (>90%) against RFC1-negative patients. The phenotype across 70 RFC1-positive patients was mostly multisystemic (69%), including dysautonomia (62%) and bradykinesia (28%) (overlap with cerebellar-type multiple system atrophy [MSA-C]), postural instability (49%), slow vertical saccades (17%), and chorea or dystonia (11%). Ataxia progression was ?1.3 Scale for the Assessment and Rating of Ataxia points per year (32 cross-sectional, 17 longitudinal assessments, follow-up ?9 years [mean 3.1 years]) but also included early falls, variable nonlinear phases of MSA-C?like progression (SARA points 2.5?5.5 per year), and premature death. Treatment trials require 330 (1-year trial) and 132 (2-year trial) patients in total to detect 50% reduced progression.
Conclusions RFC1 disease is frequent and occurs across continents, with CANVAS and ACC as highly diagnostic phenotypes yet as variable, overlapping clusters along a continuous multisystemic disease spectrum, including MSA-C-overlap. Our natural history data help to inform future RFC1 treatment trials.
Classification of Evidence This study provides Class II evidence that RFC1 repeat expansions are associated with CANVAS and ACC.FUNDING: Study Funding This work was supported via the European Union’s Horizon 2020 research and innovation program by the BMBF under the frame of the E-Rare-3 network PREPARE (01GM1607; to M. Synofzik,M.A., H.P., B.P.v.d.W.), by the DFG under the frame of EJP-RD network PROSPAX (No. 441409627; M. Synofzik, B.P.v.d.W., A.N.B.), and grant 779257 “Solve-RD” (toM. Synofzik, B.P.v.d.W.). B.P.v.d.W. receives additional research support from ZonMW, Hersenstichting, Gossweiler Foundation, uniQure, and Radboud University Medical Centre. T.B.H. was supported by the DFG (No 418081722). A.T. receives funding from the University of T¨ubingen, medical faculty, for the Clinician Scientist Program grant 439-0-0. A.C. thanks Medical Research Council, MR/T001712/1) and Fondazione CARIPLO (2019-1836) for grant support. L.S., T.K., B.P.v.d.W., and M. Synofzik are members of the European Reference Network for Rare Neurological Diseases, project 739510. A.N.B. is supported by the Suna and Inan Kirac Foundation and Koç University School of Medicine
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