4,978 research outputs found
Evolution of reconnection along an arcade of magnetic loops
RHESSI observations of a solar flare showing continuous motions of double
hard X-ray sources interpreted as footpoints of magnetic loops are presented.
The temporal evolution shows many distinct emission peaks of duration of some
tens of seconds ('elementary flare bursts'). Elementary flare bursts have been
interpreted as instabilities or oscillations of the reconnection process
leading to an unsteady release of magnetic energy. These interpretations based
on two-dimensional concepts cannot explain these observations, showing that the
flare elements are displaced in a third dimension along the arcade. Therefore,
the observed flare elements are not a modulation of the reconnection process,
but originate as this process progresses along an arcade of magnetic loops.
Contrary to previous reports, we find no correlation between footpoint motion
and hard X-ray flux. This flare apparently contradicts the predictions of the
standard translation invariant 2.5D reconnection models.Comment: 4 pages, 3 figures, to be published in Astrophysical Journal Letter
Migration and giant planet formation
We extend the core-accretion model of giant gaseous planets by Pollack et al.
(\cite{P96}) to include migration, disc evolution and gap formation. Starting
with a core of a fraction of an Earth's mass located at 8 AU, we end our
simulation with the onset of runaway gas accretion when the planet is at 5.5 AU
1 Myr later. This timescale is about a factor ten shorter than the one found by
Pollack et al. (\cite{P96}) even though the disc was less massive initially and
viscously evolving. Other initial conditions can lead to even shorter
timescales. The reason for this speed-up is found to result from the fact that
a moving planet does not deplete its feeding zone to the extend of a static
planet. Thus, the uncomfortably long formation timescale associated with the
core-accretion scenario can be considerably reduced and brought in much better
agreement with the typical disc lifetimes inferred from observations of young
circumstellar discs.Comment: 9 pages, 2 figures, published in A&A Letter
Theoretical and material studies on thin-film electroluminescent devices
The effect of surface nucleation processes on the quality of ZnS layers grown on (001) GaAs substrates by molecular beam epitaxy is reported. Reflection high energy electron diffraction indicated that nucleation at high temperatures produced more planar surfaces than nucleation at low temperatures, but the crystalline quality as accessed by x ray double crystal diffractometry is relatively independent of nucleation temperature. A critical factor in layer quality was the initial roughness of the GaAs surfaces
Spectral Hardening of Large Solar Flares
RHESSI observations are used to quantitatively study the hard X-ray evolution
in 5 large solar flares selected for spectral hardening in the course of the
event. The X-ray bremsstrahlung emission from non-thermal electrons is
characterized by two spectroscopically distinct phases: impulsive and gradual.
The impulsive phase usually consists of several emission spikes following a
soft-hard-soft spectral pattern, whereas the gradual stage manifests itself as
spectral hardening while the flux slowly decreases. Both the soft-hard-soft
(impulsive) phase and the hardening (gradual) phase are well described by
piecewise linear dependence of the photon spectral index on the logarithm of
the hard X-ray flux. The different linear parts of this relation correspond to
different rise and decay phases of emission spikes. The temporal evolution of
the spectra is compared with the configuration and motion of the hard X-ray
sources in RHESSI images. These observations reveal that the two stages of
electron acceleration causing these two different behaviors are closely related
in space and time. The transition between the impulsive and gradual phase is
found to be smooth and progressive rather than abrupt. This suggests that they
arise because of a slow change in a common accelerator rather than being caused
by two independent and distinct acceleration processes. We propose that the
hardening during the decay phase is caused by continuing particle acceleration
with longer trapping in the accelerator before escape.Comment: accepted by Ap
Theory of planet formation and comparison with observation: Formation of the planetary mass-radius relationship
The planetary mass-radius diagram is an observational result of central
importance to understand planet formation. We present an updated version of our
planet formation model based on the core accretion paradigm which allows to
calculate planetary radii and luminosities during the entire formation and
evolution of the planets. We first study with it the formation of Jupiter, and
compare with previous works. Then we conduct planetary population synthesis
calculations to obtain a synthetic mass-radius diagram which we compare with
the observed one. Except for bloated Hot Jupiters which can be explained only
with additional mechanisms related to their proximity to the star, we find a
good agreement of the general shape of the observed and the synthetic
mass-radius diagram. This shape can be understood with basic concepts of the
core accretion model.Comment: Proceedings Haute Provence Observatory Colloquium: Detection and
Dynamics of Transiting Exoplanets (23-27 August 2010). Edited by F. Bouchy,
R. F. Diaz & C. Moutou. Extended version: 17 pages, 8 figure
Planet Population Synthesis
With the increasing number of exoplanets discovered, statistical properties
of the population as a whole become unique constraints on planet formation
models provided a link between the description of the detailed processes
playing a role in this formation and the observed population can be
established. Planet population synthesis provides such a link. The approach
allows to study how different physical models of individual processes (e.g.,
proto-planetary disc structure and evolution, planetesimal formation, gas
accretion, migration, etc.) affect the overall properties of the population of
emerging planets. By necessity, planet population synthesis relies on
simplified descriptions of complex processes. These descriptions can be
obtained from more detailed specialised simulations of these processes. The
objective of this chapter is twofold: 1) provide an overview of the physics
entering in the two main approaches to planet population synthesis and 2)
present some of the results achieved as well as illustrate how it can be used
to extract constraints on the models and to help interpret observations.Comment: 23 pages, 8 figures, accepted for publication as a chapter in
Protostars and Planets VI, University of Arizona Press (2014), eds. H.
Beuther, R. Klessen, C. Dullemond, Th. Henning. Updated references relative
to v
Genomic aberrations in normal tissue adjacent to HER2-amplified breast cancers: field cancerization or contaminating tumor cells?
Field cancerization effects as well as isolated tumor cell foci extending well beyond the invasive tumor margin have been described previously to account for local recurrence rates following breast conserving surgery despite adequate surgical margins and breast radiotherapy. To look for evidence of possible tumor cell contamination or field cancerization by genetic effects, a pilot study (Study 1: 12 sample pairs) followed by a verification study (Study 2: 20 sample pairs) were performed on DNA extracted from HER2-positive breast tumors and matching normal adjacent mammary tissue samples excised 1-3 cm beyond the invasive tumor margin. High-resolution molecular inversion probe (MIP) arrays were used to compare genomic copy number variations, including increased HER2 gene copies, between the paired samples; as well, a detailed histologic and immunohistochemical (IHC) re-evaluation of all Study 2 samples was performed blinded to the genomic results to characterize the adjacent normal tissue composition bracketing the DNA-extracted samples. Overall, 14/32 (44 %) sample pairs from both studies produced genome-wide evidence of genetic aberrations including HER2 copy number gains within the adjacent normal tissue samples. The observed single-parental origin of monoallelic HER2 amplicon haplotypes shared by informative tumor-normal pairs, as well as commonly gained loci elsewhere on 17q, suggested the presence of contaminating tumor cells in the genomically aberrant normal samples. Histologic and IHC analyses identified occult 25-200 μm tumor cell clusters overexpressing HER2 scattered in more than half, but not all, of the genomically aberrant normal samples re-evaluated, but in none of the genomically normal samples. These genomic and microscopic findings support the conclusion that tumor cell contamination rather than genetic field cancerization represents the likeliest cause of local clinical recurrence rates following breast conserving surgery, and mandate caution in assuming the genomic normalcy of histologically benign appearing peritumor breast tissue
Device and method for frictionally testing materials for ignitability
Test apparatus for determining ignition characteristics of various metal in oxidizer environments simulating operating conditions for materials is invented. The test apparatus has a chamber through which the oxidizing agent flows, and means for mounting a stationary test sample therein, a powered, rotating shaft in the chamber rigidly mounts a second test sample. The shaft is axially movable to bring the samples into frictional engagement and heated to the ignition point. Instrumentation connected to the apparatus provides for observation of temperatures, pressures, loads on and speeds of the rotating shaft, and torques whereby components of stressed oxygen systems can be selected which will avoid accidental fires under working conditions
The spectral evolution of impulsive solar X-ray flares. II.Comparison of observations with models
We study the evolution of the spectral index and the normalization (flux) of
the non-thermal component of the electron spectra observed by RHESSI during 24
solar hard X-ray flares. The quantitative evolution is confronted with the
predictions of simple electron acceleration models featuring the soft-hard-soft
behaviour. The comparison is general in scope and can be applied to different
acceleration models, provided that they make predictions for the behavior of
the spectral index as a function of the normalization. A simple stochastic
acceleration model yields plausible best-fit model parameters for about 77% of
the 141 events consisting of rise and decay phases of individual hard X-ray
peaks. However, it implies unphysically high electron acceleration rates and
total energies for the others. Other simple acceleration models such as
constant rate of accelerated electrons or constant input power have a similar
failure rate. The peaks inconsistent with the simple acceleration models have
smaller variations in the spectral index. The cases compatible with a simple
stochastic model require typically a few times 10^36 electrons accelerated per
second at a threshold energy of 18 keV in the rise phases and 24 keV in the
decay phases of the flare peaks.Comment: 9 pages, 4 figures, accepted for publication by A&
A multipotent precursor in the thymus maps to the branching point of the T versus B lineage decision
Hematopoietic precursors continuously colonize the thymus where they give rise mainly to T cells, but also to B and dendritic cells. The lineage relationship between these three cell types is unclear, and it remains to be determined if precursors in the thymus are multipotent, oligopotent, or lineage restricted. Resolution of this question necessitates the determination of the clonal differentiation potential of the most immature precursors in the thymus. Using a CC chemokine receptor 9–enhanced green fluorescent protein knock-in allele like a surface marker of unknown function, we identify a multipotent precursor present in bone marrow, blood, and thymus. Single cells of this precursor give rise to T, B, and dendritic cells. A more differentiated stage of this multipotent precursor in the thymus has lost the capacity to generate B but not T, dendritic, and myeloid cells. Thus, the newly identified precursor maps to the branching point of the T versus B lineage decision in the hematopoietic lineage hierarchy
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