7,440 research outputs found
Automated visual tracking for studying the ontogeny of zebrafish swimming
The zebrafish Danio rerio is a widely used model organism in studies of genetics, developmental biology, and recently, biomechanics. In order to quantify changes in swimming during all stages of development, we have developed a visual tracking system that estimates the posture of fish. Our current approach assumes planar motion of the fish, given image sequences taken from a top view. An accurate geometric fish model is automatically designed and fit to the images at each time frame. Our approach works across a range of fish shapes and sizes and is therefore well suited for studying the ontogeny of fish swimming, while also being robust to common environmental occlusions. Our current analysis focuses on measuring the influence of vertebra development on the swimming capabilities of zebrafish. We examine wild-type zebrafish and mutants with stiff vertebrae (stocksteif) and quantify their body kinematics as a function of their development from larvae to adult (mutants made available by the Hubrecht laboratory, The Netherlands). By tracking the fish, we are able to measure the curvature and net acceleration along the body that result from the fish's body wave. Here, we demonstrate the capabilities of the tracking system for the escape response of wild-type zebrafish and stocksteif mutant zebrafish. The response was filmed with a digital high-speed camera at 1500 frames s–1. Our approach enables biomechanists and ethologists to process much larger datasets than possible at present. Our automated tracking scheme can therefore accelerate insight in the swimming behavior of many species of (developing) fish
Hot DQ White Dwarfs: Something Different
We present a detailed analysis of all the known Hot DQ white dwarfs in the
Fourth Data Release of the Sloan Digital Sky Survey (SDSS) recently found to
have carbon dominated atmospheres. Our spectroscopic and photometric analysis
reveals that these objects all have effective temperatures between ~18,000 and
24,000 K. The surface composition is found to be completely dominated by
carbon, as revealed by the absence of Hbeta and HeI 4471 lines (or
determination of trace amount in a few cases). We find that the surface gravity
of all objects but one seems to be ''normal'' and around log g = 8.0 while one
is likely near log g = 9.0. The presence of a weak magnetic field is directly
detected by spectropolarimetry in one object and is suspected in two others. We
propose that these strange stars could be cooled down versions of the weird
PG1159 star H1504+65 and form a new family of hydrogen and helium deficient
objects following the post-AGB phase. Finally, we present the results of full
nonadiabatic calculations dedicated specifically to each of the Hot DQ that
show that only SDSS J142625.70+575218.4 is expected to exhibit luminosity
variations. This result is in excellent agreement with recent observations by
Montgomery et al. who find that J142625.70+575218.4 is the only pulsator among
6 Hot DQ white dwarfs surveyed in February 2008.Comment: 33 pages, 7 figures, accepted for publication in Ap
An asteroseismic test of diffusion theory in white dwarfs
The helium-atmosphere (DB) white dwarfs are commonly thought to be the
descendants of the hotter PG1159 stars, which initially have uniform He/C/O
atmospheres. In this evolutionary scenario, diffusion builds a pure He surface
layer which gradually thickens as the star cools. In the temperature range of
the pulsating DB white dwarfs (T_eff ~ 25,000 K) this transformation is still
taking place, allowing asteroseismic tests of the theory. We have obtained
dual-site observations of the pulsating DB star CBS114, to complement existing
observations of the slightly cooler star GD358. We recover the 7 independent
pulsation modes that were previously known, and we discover 4 new ones to
provide additional constraints on the models. We perform objective global
fitting of our updated double-layered envelope models to both sets of
observations, leading to determinations of the envelope masses and pure He
surface layers that qualitatively agree with the expectations of diffusion
theory. These results provide new asteroseismic evidence supporting one of the
central assumptions of spectral evolution theory, linking the DB white dwarfs
to PG1159 stars.Comment: 7 pages, 3 figures, 3 tables, accepted for publication in A&
On a Conjecture of Rapoport and Zink
In their book Rapoport and Zink constructed rigid analytic period spaces
for Fontaine's filtered isocrystals, and period morphisms from PEL
moduli spaces of -divisible groups to some of these period spaces. They
conjectured the existence of an \'etale bijective morphism of
rigid analytic spaces and of a universal local system of -vector spaces on
. For Hodge-Tate weights and we construct in this article an
intrinsic Berkovich open subspace of and the universal local
system on . We conjecture that the rigid-analytic space associated with
is the maximal possible , and that is connected. We give
evidence for these conjectures and we show that for those period spaces
possessing PEL period morphisms, equals the image of the period morphism.
Then our local system is the rational Tate module of the universal
-divisible group and enjoys additional functoriality properties. We show
that only in exceptional cases equals all of and when the
Shimura group is we determine all these cases.Comment: v2: 48 pages; many new results added, v3: final version that will
appear in Inventiones Mathematica
Static displacements and chemical correlations in alloys
Recent experiments in metallic solid solutions have revealed interesting
correlations between static pair-displacements and the ordering behavior of
these alloys. This paper discusses a simple theoretical model which
successfully explains these observations and which provides a natural framework
for analyzing experimental measurements of pair-displacements and chemical
correlations in solid solutions. The utility and scope of this model is
demonstrated by analyzing results of experiments on and alloys
and results of simulations of and alloys.Comment: 12 page
Interplay between magnetic anisotropy and interlayer coupling in nanosecond magnetization reversal of spin-valve trilayers
The influence of magnetic anisotropy on nanosecond magnetization reversal in
coupled FeNi/Cu/Co trilayers was studied using a photoelectron emission
microscope combined with x-ray magnetic circular dicroism. In quasi-isotropic
samples the reversal of the soft FeNi layer is determined by domain wall
pinning that leads to the formation of small and irregular domains. In samples
with uniaxial magnetic anisotropy, the domains are larger and the influence of
local interlayer coupling dominates the domain structure and the reversal of
the FeNi layer
The Origin of Mercury's Surface Composition, an Experimental Investigation
Introduction: Results from MESSENGER spacecraft have confirmed the reduced nature of Mercury, based on its high core/mantle ratio and its FeO-poor and S-rich surface. Moreover, high resolution images revealed large volcanic plains and abundant pyroclastic deposits, suggesting major melting stages of the Mercurian mantle. In addition, MESSENGER has provided the most precise data to date on major elemental compositions of Mercury's surface. These results revealed considerable chemical heterogeneities that suggested several stages of differentiation and re-melting processes. This interpretation was challenged by our experimental previous study, which showed a similar compositional variation in the melting products of enstatite chondrites, which are a possible Mercury analogue. However, these experimental melts were obtained over a limited range of pressure (1 bar to 1 gigapascal) and were not compared to the most recent elemental maps. Therefore, here we extend the experimental dataset to higher pressures and perform a more quantitative comparison with Mercury's surface compositions measured by MESSENGER. In particular, we test whether these chemical heterogeneities result from mixing between polybaric melts. Our experiments and models show that the majority of chemical diversity of Mercury's surface can result from melting of a primitive mantle compositionally similar to enstatite chondrites in composition at various depths and degrees of melting. The high-Mg region's composition is reproduced by melting at high pressure (3 gigapascals) (Tab. 1), which is consistent with previous interpretation as being a large degraded impact basin based on its low elevation and thin average crust. While low-Mg NVP (North Volcanic Plains) are the result of melting at low pressure (1 bar), intermediate-Mg NVP, Caloris Basin and Rachmaninoff result from mixing of a high-pressure (3 gigapascals) and low-pressure components (1 bar for Rachmaninoff and 1 gigapascal for the other regions) (Tab. 1). Moreover, all compositions suggest mixing between low and high degree melts that indicate important differentiation processes
Melting of the Primitive Mercurian Mantle, Insights into the Origin of Its Surface Composition
Recent findings of the MESSENGER mission on Mercury have brought new evidence for its reducing nature, widespread volcanism and surface compositional heteregeneity. MESSENGER also provided major elemental ratios of its surface that can be used to infer large-scale differentiation processes and the thermal history of the planet. Mercury is known as being very reduced, with very low Fe-content and high S and alkali contents on its surface. Its bulk composition is therefore likely close to EH enstatite chondrites. In order to elucidate the origin of the chemical diversity of Mercury's surface, we determined the melting properties of EH enstatite chondrites, at pressures between 1 bar and 3 GPa and oxygen fugacity of IW-3 to IW-5, using piston-cylinder experiments, combined with a previous study on EH4 melting at 1 bar. We found that the presence of Ca-rich sulfide melts induces significant decrease of Ca-content in silicate melts at low pressure and low degree of melting (F). Also at pressures lower than 3 GPa, the SiO2-content decreases with F, while it increases at 3 GPa. This is likely due to the chemical composition of the bulk silicate which has a (Mg+Fe+Ca)/Si ratio very close to 1 and to the change from incongruent to congruent melting of enstatite. We then tested whether the various chemical compositions of Mercury's surface can result from mixing between two melting products of EH chondrites. We found that the majority of the geochemical provinces of Mercury's surface can be explained by mixing of two melts, with the exception of the High-Al plains that require an Al-rich source. Our findings indicate that Mercury's surface could have been produced by polybaric melting of a relatively primitive mantle
The pulsating DA white dwarf star EC 14012-1446: results from four epochs of time-resolved photometry
The pulsating DA white dwarfs are the coolest degenerate stars that undergo
self-driven oscillations. Understanding their interior structure will help to
understand the previous evolution of the star. To this end, we report the
analysis of more than 200 h of time-resolved CCD photometry of the pulsating DA
white dwarf star EC 14012-1446 acquired during four observing epochs in three
different years, including a coordinated three-site campaign. A total of 19
independent frequencies in the star's light variations together with 148
combination signals up to fifth order could be detected. We are unable to
obtain the period spacing of the normal modes and therefore a mass estimate of
the star, but we infer a fairly short rotation period of 0.61 +/- 0.03 d,
assuming the rotationally split modes are l=1. The pulsation modes of the star
undergo amplitude and frequency variations, in the sense that modes with higher
radial overtone show more pronounced variability and that amplitude changes are
always accompanied by frequency variations. Most of the second-order
combination frequencies detected have amplitudes that are a function of their
parent mode amplitudes, but we found a few cases of possible resonantly excited
modes. We point out the complications in the analysis and interpretation of
data sets of pulsating white dwarfs that are affected by combination
frequencies of the form f_A+f_B-f_C intruding into the frequency range of the
independent modes.Comment: 14 pages, 6 figures, 6 tables. MNRAS, in pres
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