936 research outputs found
Shear-melting of a hexagonal columnar crystal by proliferation of dislocations
A hexagonal columnar crystal undergoes a shear-melting transition above a
critical shear rate or stress. We combine the analysis of the shear-thinning
regime below the melting with that of synchrotron X-ray scattering data under
shear and propose the melting to be due to a proliferation of dislocations,
whose density is determined by both techniques to vary as a power law of the
shear rate with a 2/3 exponent, as expected for a creep model of crystalline
solids. Moreover, our data suggest the existence under shear of a line hexatic
phase, between the columnar crystal and the liquid phase
Spacecraft charging and ion wake formation in the near-Sun environment
A three-dimensional (3-D), self-consistent code is employed to solve for the
static potential structure surrounding a spacecraft in a high photoelectron
environment. The numerical solutions show that, under certain conditions, a
spacecraft can take on a negative potential in spite of strong photoelectron
currents. The negative potential is due to an electrostatic barrier near the
surface of the spacecraft that can reflect a large fraction of the
photoelectron flux back to the spacecraft. This electrostatic barrier forms if
(1) the photoelectron density at the surface of the spacecraft greatly exceeds
the ambient plasma density, (2) the spacecraft size is significantly larger
than local Debye length of the photoelectrons, and (3) the thermal electron
energy is much larger than the characteristic energy of the escaping
photoelectrons. All of these conditions are present near the Sun. The numerical
solutions also show that the spacecraft's negative potential can be amplified
by an ion wake. The negative potential of the ion wake prevents secondary
electrons from escaping the part of spacecraft in contact with the wake. These
findings may be important for future spacecraft missions that go nearer to the
Sun, such as Solar Orbiter and Solar Probe Plus.Comment: 25 pages, 7 figures, accepted for publication in Physics of Plasma
The origin of short-lived radionuclides and the astrophysical environment of solar system formation
Based on early solar system abundances of short-lived radionuclides (SRs),
such as Al (T Myr) and Fe (T Myr),
it is often asserted that the Sun was born in a large stellar cluster, where a
massive star contaminated the protoplanetary disk with freshly
nucleosynthesized isotopes from its supernova (SN) explosion. To account for
the inferred initial solar system abundances of short-lived radionuclides, this
supernova had to be close ( 0.3 pc) to the young ( 1 Myr)
protoplanetary disk.
Here we show that massive star evolution timescales are too long, compared to
typical timescales of star formation in embedded clusters, for them to explode
as supernovae within the lifetimes of nearby disks. This is especially true in
an Orion Nebular Cluster (ONC)-type of setting, where the most massive star
will explode as a supernova 5 Myr after the onset of star formation,
when nearby disks will have already suffered substantial photoevaporation
and/or formed large planetesimals.
We quantify the probability for {\it any} protoplanetary disk to receive SRs
from a nearby supernova at the level observed in the early solar system. Key
constraints on our estimate are: (1) SRs have to be injected into a newly
formed ( 1 Myr) disk, (2) the disk has to survive UV
photoevaporation, and (3) the protoplanetary disk must be situated in an
enrichment zone permitting SR injection at the solar system level without disk
disruption. The probability of protoplanetary disk contamination by a supernova
ejecta is, in the most favorable case, 3 10
Is a level III dissection necessary for a positive sentinel lymph node in melanoma?
Background For melanoma patients with a positive axillary SLN, the extent of ALND remains controversial, with debate over whether a level III dissection is needed. Methods We queried our IRB approved prospective database for patients with a positive axillary SLN who had a level I/II dissection only, and compared recurrence and complication rates to the existing literature. Results Between 1998 and 2008, 270 patients had 285 level I/II ALNDs for a positive SLN. Median number of SLN removed was 2, while the median number of involved SLN was 1 (range 1–4). An average of 18.7 nodes/ALND were removed, with 13% having positive non‐SLN. Post‐operative complications occurred in 31 patients (11%), primarily cellulitis (8%). After a mean follow‐up of 44 months, 14 patients had a regional recurrence in the axillary basin (5%). Conclusions The complication rate and regional recurrence rate for patients undergoing a level I/II ALND for a positive SLN are either lower than or on par with reported series of ALND for level I, II, and III dissections, suggesting that in this setting, the level III dissection may be of minimal benefit. J. Surg. Oncol. 2012; 105:225–228. © 2011 Wiley Periodicals, Inc.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/90389/1/22076_ftp.pd
Giant molecular clouds in the non-grand design spiral galaxy NGC 6946
We present high spatial resolution observations of Giant Molecular Clouds
(GMCs) in the eastern part of the nearby spiral galaxy NGC 6946 obtained with
the Combined Array for Research in Millimeter-wave Astronomy (CARMA). We have
observed 12CO(1-0), 12CO(2-1) and 13CO(1-0), achieving spatial resolutions of
5.4" x 5.0", 2.5" x 2.0" and 5.6" x 5.4" respectively over a region of 6 x 6
kpc. This region extends from 1.5 kpc to 8 kpc galactocentric radius, thus
avoiding the intense star formation in the central kpc. We have recovered
short-spacing u-v components by using single dish observations from the
Nobeyama 45m and IRAM 30m telescopes. Using the automated CPROPS algorithm we
identified 44 CO cloud complexes in the 12CO(1-0) map and 64 GMCs in the
12CO(2-1) maps. The sizes, line widths, and luminosities of the GMCs are
similar to values found in other extragalactic studies. We have classified the
clouds into on-arm and inter-arm clouds based on the stellar mass density
traced by the 3.6 um map. On-arm clouds present in general higher star
formation rates than clouds located on inter-arm regions. Although the star
formation efficiency shows no systematic trend with galactocentric radius, some
on-arm clouds -- which are more luminous and more massive compared to inter-arm
GMCs -- are also forming stars more efficiently than the rest of the identified
GMCs. These structures appear to be located on two specific regions in the
spiral arms. One of them shows a strong gradient, suggesting that this region
of high star formation efficiency may be the result of gas flow convergence.Comment: 23 pages, 21 figures, 3 tables. To be published in The Astrophysical
Journa
Toy amphiphiles on the computer: What can we learn from generic models?
Generic coarse-grained models are designed such that they are (i) simple and
(ii) computationally efficient. They do not aim at representing particular
materials, but classes of materials, hence they can offer insight into
universal properties of these classes. Here we review generic models for
amphiphilic molecules and discuss applications in studies of self-assembling
nanostructures and the local structure of bilayer membranes, i.e. their phases
and their interactions with nanosized inclusions. Special attention is given to
the comparison of simulations with elastic continuum models, which are, in some
sense, generic models on a higher coarse-graining level. In many cases, it is
possible to bridge quantitatively between generic particle models and continuum
models, hence multiscale modeling works on principle. On the other side,
generic simulations can help to interpret experiments by providing information
that is not accessible otherwise.Comment: Invited feature article, to appear in Macromolecular Rapid
Communication
Single‐Column Model Simulations of Subtropical Marine Boundary‐Layer Cloud Transitions Under Weakening Inversions
Results are presented of the GASS/EUCLIPSE single‐column model intercomparison study on the subtropical marine low‐level cloud transition. A central goal is to establish the performance of state‐of‐the‐art boundary‐layer schemes for weather and climate models for this cloud regime, using large‐eddy simulations of the same scenes as a reference. A novelty is that the comparison covers four different cases instead of one, in order to broaden the covered parameter space. Three cases are situated in the North‐Eastern Pacific, while one reflects conditions in the North‐Eastern Atlantic. A set of variables is considered that reflects key aspects of the transition process, making use of simple metrics to establish the model performance. Using this method, some longstanding problems in low‐level cloud representation are identified. Considerable spread exists among models concerning the cloud amount, its vertical structure, and the associated impact on radiative transfer. The sign and amplitude of these biases differ somewhat per case, depending on how far the transition has progressed. After cloud breakup the ensemble median exhibits the well‐known “too few too bright” problem. The boundary‐layer deepening rate and its state of decoupling are both underestimated, while the representation of the thin capping cloud layer appears complicated by a lack of vertical resolution. Encouragingly, some models are successful in representing the full set of variables, in particular, the vertical structure and diurnal cycle of the cloud layer in transition. An intriguing result is that the median of the model ensemble performs best, inspiring a new approach in subgrid parameterization
How to ask sensitive questions in conservation: A review of specialized questioning techniques
Tools for social research are critical for developing an understanding of conservation problems and assessing the feasibility of conservation actions. Social surveys are an essential tool frequently applied in conservation to assess both people’s behaviour and to understand its drivers. However, little attention has been given to the weaknesses and strengths of different survey tools. When topics of conservation concern are illegal or otherwise sensitive, data collected using direct questions are likely to be affected by non-response and social desirability biases, reducing their validity. These sources of bias associated with using direct questions on sensitive topics have long been recognised in the social sciences but have been poorly considered in conservation and natural resource management.
We reviewed specialized questioning techniques developed in a number of disciplines specifically for investigating sensitive topics. These methods ensure respondent anonymity, increase willingness to answer, and critically, make it impossible to directly link incriminating data to an individual. We describe each method and report their main characteristics, such as data requirements, possible data outputs, availability of evidence that they can be adapted for use in illiterate communities, and summarize their main advantages and disadvantages. Recommendations for their application in conservation are given. We suggest that the conservation toolbox should be expanded by incorporating specialized questioning techniques, developed specifically to increase response accuracy. By considering the limitations of each survey technique, we will ultimately contribute to more effective evaluations of conservation interventions and more robust policy decisions
The Hong-Ou-Mandel effect with atoms
Controlling light at the level of individual photons has led to advances in
fields ranging from quantum information and precision sensing to fundamental
tests of quantum mechanics. A central development that followed the advent of
single photon sources was the observation of the Hong-Ou- Mandel (HOM) effect,
a novel two-photon path interference phenomenon experienced by
indistinguishable photons. The effect is now a central technique in the field
of quantum optics, harnessed for a variety of applications such as diagnosing
single photon sources and creating probabilistic entanglement in linear quantum
computing. Recently, several distinct experiments using atomic sources have
realized the requisite control to observe and exploit Hong-Ou-Mandel
interference of atoms. This article provides a summary of this phenomenon and
discusses some of its implications for atomic systems. Transitioning from the
domain of photons to atoms opens new perspectives on fundamental concepts, such
as the classification of entanglement of identical particles. It aids in the
design of novel probes of quantities such as entanglement entropy by combining
well established tools of AMO physics - unity single-atom detection, tunable
interactions, and scalability - with the Hong-Ou-Mandel interference.
Furthermore, it is now possible for established protocols in the photon
community, such as measurement-induced entanglement, to be employed in atomic
experiments that possess deterministic single-particle production and
detection. Hence, the realization of the HOM effect with atoms represents a
productive union of central ideas in quantum control of atoms and photons.Comment: 19 pages, 7 figure
Dwarf galaxy populations in present-day galaxy clusters: I. Abundances and red fractions
We compare the galaxy population in the Virgo, Fornax, Coma and Perseus
cluster to a state-of-the-art semi-analytic model, focusing on the regime of
dwarf galaxies with luminosities from approximately 10^8 L_sun to 10^9 L_sun.
We find that the number density profiles of dwarfs in observed clusters are
reproduced reasonably well, and that the red fractions of model clusters
provide a good match to Coma and Perseus. On the other hand, the red fraction
among dwarf galaxies in Virgo is clearly lower than in model clusters. We argue
that this is mainly caused by the treatment of environmental effects in the
model. This explanation is supported by our finding that the colours of central
("field") dwarf galaxies are reproduced well, in contrast to previous claims.
Finally, we find that the dwarf-to-giant ratio in model clusters is too high.
This may indicate that the current model prescription for tidal disruption of
faint galaxies is still not efficient enough.Comment: 20 pages, 10 figures. Accepted by MNRAS. Includes the modifications
after referee report. Main results unchanged, interpretation slightly change
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