263 research outputs found
A Dust Cloud of Ganymede Maintained by Hypervelocity Impacts of Interplanetary Micrometeoroids
A dust cloud of Ganymede has been detected by in-situ measurements with the
dust detector onboard the Galileo spacecraft. The dust grains have been sensed
at altitudes below five Ganymede radii (Ganymede radius = ). Our
analysis identifies the particles in the dust cloud surrounding Ganymede by
their impact direction, impact velocity, and mass distribution and implies that
they have been kicked up by hypervelocity impacts of micrometeoroids onto the
satellite's surface. We calculate the radial density profile of the particles
ejected from the satellite by interplanetary dust grains. We assume the yields,
mass and velocity distributions of the ejecta obtained from laboratory impact
experiments onto icy targets and consider the dynamics of the ejected grains in
ballistic and escaping trajectories near Ganymede. The spatial dust density
profile calculated with interplanetary particles as impactors is consistent
with the profile derived from the Galileo measurements. The contribution of
interstellar grains as projectiles is negligible. Dust measurements in the
vicinities of satellites by spacecraft detectors are suggested as a beneficial
tool to obtain more knowledge about the satellite surfaces, as well as dusty
planetary rings maintained by satellites through the impact ejecta mechanism.Comment: 31 pages, 10 figures, accepted for Planetary and Space Scienc
Mars Phobos and Deimos Survey (M-PADS) – A Martian Moons Orbiter and Phobos Lander
We describe a Mars 'Micro Mission' for detailed study of the martian satellites Phobos and Deimos. The mission involves two ~330 kg spacecraft equipped with solar electric propulsion to reach Mars orbit. The two spacecraft are stacked for launch: an orbiter for remote investigation of the moons and in situ studies of their environment in Mars orbit, and another carrying a lander for in situ measurements on the surface of Phobos (or alternatively Deimos). Phobos and Deimos remain only partially studied, and Deimos less well than Phobos. Mars has almost always been the primary mission objective, while the more dedicated Phobos project (1988-89) failed to realise its full potential. Many questions remain concerning the moons' origins, evolution, physical nature and composition. Current missions, such as Mars Express, are extending our knowledge of Phobos in some areas but largely neglect Deimos. The objectives of M-PADS focus on: origins and evolution, interactions with Mars, volatiles and interiors, surface features, and differences. The consequent measurement requirements imply both landed and remote sensing payloads. M-PADS is expected to accommodate a 60 kg orbital payload and a 16 kg lander payload. M-PADS resulted from a BNSC-funded study carried out in 2003 to define candidate Mars Micro Mission concepts for ESA's Aurora programme
Galileo dust data from the jovian system: 2000 to 2003
The Galileo spacecraft was orbiting Jupiter between Dec 1995 and Sep 2003.
The Galileo dust detector monitored the jovian dust environment between about 2
and 370 R_J (jovian radius R_J = 71492 km). We present data from the Galileo
dust instrument for the period January 2000 to September 2003. We report on the
data of 5389 particles measured between 2000 and the end of the mission in
2003. The majority of the 21250 particles for which the full set of measured
impact parameters (impact time, impact direction, charge rise times, charge
amplitudes, etc.) was transmitted to Earth were tiny grains (about 10 nm in
radius), most of them originating from Jupiter's innermost Galilean moon Io.
Their impact rates frequently exceeded 10 min^-1. Surprisingly large impact
rates up to 100 min^-1 occurred in Aug/Sep 2000 when Galileo was at about 280
R_J from Jupiter. This peak in dust emission appears to coincide with strong
changes in the release of neutral gas from the Io torus. Strong variability in
the Io dust flux was measured on timescales of days to weeks, indicating large
variations in the dust release from Io or the Io torus or both on such short
timescales. Galileo has detected a large number of bigger micron-sized
particles mostly in the region between the Galilean moons. A surprisingly large
number of such bigger grains was measured in March 2003 within a 4-day interval
when Galileo was outside Jupiter's magnetosphere at approximately 350 R_J
jovicentric distance. Two passages of Jupiter's gossamer rings in 2002 and 2003
provided the first actual comparison of in-situ dust data from a planetary ring
with the results inferred from inverting optical images.Comment: 59 pages, 13 figures, 6 tables, submitted to Planetary and Space
Scienc
Modulation of a protein free-energy landscape by circular permutation
Circular permutations usually retain the native structure and function of a protein while inevitably perturb its folding dynamics. By using simulations with a structure-based model and a rigorous methodology to determine free-energy surfaces from trajectories we evaluate the effect of a circular permutation on the free-energy landscape of the protein T4 lysozyme. We observe changes which, while subtle, largely affect the cooperativity between the two subdomains. Such a change in cooperativity has been previously experimentally observed and recently also characterized using single molecule optical tweezers and the Crooks relation. The free-energy landscapes show that both the wild type and circular permutant have an on-pathway intermediate, previously experimentally characterized, where one of the subdomains is completely formed. The landscapes, however, differ in the position of the rate-limiting step for folding, which occurs before the intermediate in the wild-type and after in the circular permutant. This shift of transition state explains the observed change in the cooperativity. The underlying free-energy landscape thus provides a microscopic description of the folding dynamics and the connection between circular permutation and the loss of cooperativity experimentally observed
Incidence of debris discs around FGK stars in the solar neighbourhood
Debris discs are a consequence of the planet formation process and constitute
the fingerprints of planetesimal systems. Their solar system's counterparts are
the asteroid and Edgeworth-Kuiper belts. The aim of this paper is to provide
robust numbers for the incidence of debris discs around FGK stars in the solar
neighbourhood. The full sample of 177 FGK stars with d<20 pc proposed for the
DUNES survey is presented. Herschel/PACS observations at 100 and 160 micron
complemented with data at 70 micron, and at 250, 350 and 500 micron SPIRE
photometry, were obtained. The 123 objects observed by the DUNES collaboration
were presented in a previous paper. The remaining 54 stars, shared with the
DEBRIS consortium and observed by them, and the combined full sample are
studied in this paper. The incidence of debris discs per spectral type is
analysed and put into context together with other parameters of the sample,
like metallicity, rotation and activity, and age.
The subsample of 105 stars with d<15 pc containing 23 F, 33 G and 49 K stars,
is complete for F stars, almost complete for G stars and contains a substantial
number of K stars to draw solid conclusions on objects of this spectral type.
The incidence rates of debris discs per spectral type 0.26 (6 objects with
excesses out of 23 F stars), 0.21 (7 out of 33 G stars) and 0.20 (10 out of 49
K stars), the fraction for all three spectral types together being 0.22 (23 out
of 105 stars). Uncertainties corresponding to a 95% confidence level are given
in the text for all these numbers. The medians of the upper limits of
L_dust/L_* for each spectral type are 7.8E-7 (F), 1.4E-6 (G) and 2.2E-6 (K);
the lowest values being around 4.0E-7. The incidence of debris discs is similar
for active (young) and inactive (old) stars. The fractional luminosity tends to
drop with increasing age, as expected from collisional erosion of the debris
belts.Comment: 31 pages, 15 figures, 10 tables, 2 appendice
Deep Learning for Brain Tumor Segmentation in Radiosurgery: Prospective Clinical Evaluation
Stereotactic radiosurgery is a minimally-invasive treatment option for a
large number of patients with intracranial tumors. As part of the therapy
treatment, accurate delineation of brain tumors is of great importance.
However, slice-by-slice manual segmentation on T1c MRI could be time-consuming
(especially for multiple metastases) and subjective. In our work, we compared
several deep convolutional networks architectures and training procedures and
evaluated the best model in a radiation therapy department for three types of
brain tumors: meningiomas, schwannomas and multiple brain metastases. The
developed semiautomatic segmentation system accelerates the contouring process
by 2.2 times on average and increases inter-rater agreement from 92.0% to
96.5%
Exploring the Free Energy Landscape: From Dynamics to Networks and Back
The knowledge of the Free Energy Landscape topology is the essential key to
understand many biochemical processes. The determination of the conformers of a
protein and their basins of attraction takes a central role for studying
molecular isomerization reactions. In this work, we present a novel framework
to unveil the features of a Free Energy Landscape answering questions such as
how many meta-stable conformers are, how the hierarchical relationship among
them is, or what the structure and kinetics of the transition paths are.
Exploring the landscape by molecular dynamics simulations, the microscopic data
of the trajectory are encoded into a Conformational Markov Network. The
structure of this graph reveals the regions of the conformational space
corresponding to the basins of attraction. In addition, handling the
Conformational Markov Network, relevant kinetic magnitudes as dwell times or
rate constants, and the hierarchical relationship among basins, complete the
global picture of the landscape. We show the power of the analysis studying a
toy model of a funnel-like potential and computing efficiently the conformers
of a short peptide, the dialanine, paving the way to a systematic study of the
Free Energy Landscape in large peptides.Comment: PLoS Computational Biology (in press
Dust in the inner regions of debris disks around A stars
We present infrared interferometric observations of the inner regions of two
A-star debris disks, beta Leo and zeta Lep, using the FLUOR instrument at the
CHARA interferometer on both short (30 m) and long (>200 m) baselines. For the
target stars, the short baseline visibilities are lower than expected for the
stellar photosphere alone, while those of a check star, delta Leo, are not. We
interpret this visibility offset of a few percent as a near-infrared excess
arising from dust grains which, due to the instrumental field of view, must be
located within several AU of the central star. For beta Leo, the near-infrared
excess producing grains are spatially distinct from the dust which produces the
previously known mid-infrared excess. For zeta Lep, the near-infrared excess
may be spatially associated with the mid-infrared excess producing material. We
present simple geometric models which are consistent with the near and
mid-infrared excess and show that for both objects, the near-infrared producing
material is most consistent with a thin ring of dust near the sublimation
radius with typical grain sizes smaller than the nominal radiation pressure
blowout radius. Finally, we discuss possible origins of the near-infrared
emitting dust in the context of debris disk evolution models.Comment: 20 pages, 2 figures, to appear in the Astrophysical Journa
Hot exozodiacal dust resolved around Vega with IOTA/IONIC
Although debris discs have been detected around a significant number of
main-sequence stars, only a few of them are known to harbour hot dust in their
inner part where terrestrial planets may have formed. Thanks to infrared
interferometric observations, it is possible to obtain a direct measurement of
these regions, which are of prime importance for preparing future exo-Earth
characterisation missions. In this context, we have resolved the exozodiacal
dust disc around Vega with the help of infrared stellar interferometry and
estimated the integrated H-band flux originating from the first few AUs of the
debris disc. Using precise H-band interferometric measurements obtained with
the 3-telescope IOTA/IONIC interferometer (Mount Hopkins, Arizona), thorough
modelling of both interferometric data (squared visibility and closure phase)
and spectral energy distribution was performed to constrain the nature of the
near-infrared excess emission. The most straightforward scenario consists in a
compact dust disc producing a thermal emission that is largely dominated by
small grains located between 0.1 and 0.3 AU from Vega and accounting for 1.23
+/- 0.45% of the near-infrared stellar flux for our best-fit model. This flux
ratio is shown to vary slightly with the geometry of the model used to fit our
interferometric data (variations within +/-0.19%). Initially revealed by K-band
CHARA/FLUOR observations, the presence of hot exozodiacal dust in the vicinity
of Vega is confirmed by our H-band IOTA/IONIC measurements at the 3-sigma
level. Whereas the origin of the dust is still uncertain, its presence and the
possible connection with the outer disc suggest that the Vega system is
currently undergoing major dynamical perturbations.Comment: 10 pages, 9 figures, accepted for publication in A&
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