428 research outputs found
Planetesimal collisions in binary systems
We study the collisional evolution of km-sized planetesimals in tight binary
star systems to investigate whether accretion towards protoplanets can proceed
despite the strong gravitational perturbations from the secondary star. The
orbits of planetesimals are numerically integrated in two dimensions under the
influence of the two stars and gas drag. The masses and orbits of the
planetesimals are allowed to evolve due to collisions with other planetesimals
and accretion of collisional debris. In addition, the mass in debris can evolve
due to planetesimal-planetesimal collisions and the creation of new
planetesimals. We show that it is possible in principle for km-sized
planetesimals to grow by two orders of magnitude in size if the efficiency of
planetesimal formation is relatively low. We discuss the limitations of our
two-dimensional approach.Comment: 5 pages, 5 figures, accepted for publication in MNRA
Against all odds? Forming the planet of the HD196885 binary
HD196885Ab is the most "extreme" planet-in-a-binary discovered to date, whose
orbit places it at the limit for orbital stability. The presence of a planet in
such a highly perturbed region poses a clear challenge to planet-formation
scenarios. We investigate this issue by focusing on the planet-formation stage
that is arguably the most sensitive to binary perturbations: the mutual
accretion of kilometre-sized planetesimals. To this effect we numerically
estimate the impact velocities amongst a population of circumprimary
planetesimals. We find that most of the circumprimary disc is strongly hostile
to planetesimal accretion, especially the region around 2.6AU (the planet's
location) where binary perturbations induce planetesimal-shattering of
more than 1km/s. Possible solutions to the paradox of having a planet in such
accretion-hostile regions are 1) that initial planetesimals were very big, at
least 250km, 2) that the binary had an initial orbit at least twice the present
one, and was later compacted due to early stellar encounters, 3) that
planetesimals did not grow by mutual impacts but by sweeping of dust (the
"snowball" growth mode identified by Xie et al., 2010b), or 4) that HD196885Ab
was formed not by core-accretion but by the concurent disc instability
mechanism. All of these 4 scenarios remain however highly conjectural.Comment: accepted for publication by Celestial Mechanics and Dynamical
Astronomy (Special issue on EXOPLANETS
Dynamics of Planetesimals due to Gas Drag from an Eccentric Precessing Disk
We analyze the dynamics of individual kilometer-size planetesimals in
circumstellar orbits of a tight binary system. We include both the
gravitational perturbations of the secondary star and a non-linear gas drag
stemming from an eccentric gas disk with a finite precession rate. We consider
several precession rates and eccentricities for the gas, and compare the
results with a static disk in circular orbit.
The disk precession introduces three main differences with respect to the
classical static case: (i) The equilibrium secular solutions generated by the
gas drag are no longer fixed points in the averaged system, but limit cycles
with frequency equal to the precession rate of the gas. The amplitude of the
cycle is inversely dependent on the body size, reaching negligible values for
km size planetesimals. (ii) The maximum final eccentricity attainable
by small bodies is restricted to the interval between the gas eccentricity and
the forced eccentricity, and apsidal alignment is no longer guaranteed for
planetesimals strongly coupled with the gas. (iii) The characteristic
timescales of orbital decay and secular evolution decrease significantly with
increasing precession rates, with values up to two orders of magnitude smaller
than for static disks.
Finally, we apply this analysis to the -Cephei system and estimate
impact velocities for different size bodies and values of the gas eccentricity.
For high disk eccentricities, we find that the disk precession decreases the
velocity dispersion between different size planetesimals, thus contributing to
accretional collisions in the outer parts of the disk. The opposite occurs for
almost circular gas disks, where precession generates an increase in the
relative velocities.Comment: 11 pages, 9 figures. Accepted in MNRA
The Color Distribution in the Edgeworth-Kuiper Belt
We have started since 1997 the Meudon Multicolor Survey of Outer Solar System
Objects with the aim of collecting a large and homogeneous set of color data
for Trans-Neptunian and Centaurs objects [...] We have a combined sample of 52
B-R color measurements for 8 Centaurs, 22 Classicals, 13 Plutinos, 8 Scattered
objects and 1 object with unidentified dynamical class. This dataset is the
largest single and homogeneous published dataset to date [...]. A strong
(color) correlation with mean excitation velocity points toward a space
weathering/impact origin for the color diversity. However, thorough modeling of
the collisional/dynamical environment in the Edgeworth-Kuiper belt needs to be
done in order to confirm this scenario. We found also that the Classical TNOs
consist in the superposition of two distinct populations: the dynamically Cold
Classical TNOs (red colors, low i, small sizes) and the dynamically Hot
Classical TNOs (diverse colors, moderate and high i, larger sizes). [...] Our
specific observation strategy [...] permitted us to highlight a few objects
suspected to have true compositional and/or texture variation on their
surfaces. These are 1998 HK151, 1999 DF9, 1999 OY3, 2000 GP183, 2000 OK67, and
2001 KA77 and should be prime targets for further observations [...]. Our
survey has also highlighted 1998 SN165 whose colors and dynamical properties
puts it in a new dynamical class distinct from the Classicals, its previously
assigned dynamical class.Comment: Accepted for publication in Astronomical Journal (38 pages, inc. 11
figures
Secular dynamics of planetesimals in tight binary systems: Application to Gamma-Cephei
The secular dynamics of small planetesimals in tight binary systems play a
fundamental role in establishing the possibility of accretional collisions in
such extreme cases. The most important secular parameters are the forced
eccentricity and secular frequency, which depend on the initial conditions of
the particles, as well as on the mass and orbital parameters of the secondary
star. We construct a second-order theory (with respect to the masses) for the
planar secular motion of small planetasimals and deduce new expressions for the
forced eccentricity and secular frequency. We also reanalyze the radial
velocity data available for Gamma-Cephei and present a series of orbital
solutions leading to residuals compatible with the best fits. Finally, we
discuss how different orbital configurations for Gamma-Cephei may affect the
dynamics of small bodies in circunmstellar motion. For Gamma-Cephei, we find
that the classical first-order expressions for the secular frequency and forced
eccentricity lead to large inaccuracies around 50 % for semimajor axes larger
than one tenth the orbital separation between the stellar components. Low
eccentricities and/or masses reduce the importance of the second-order terms.
The dynamics of small planetesimals only show a weak dependence with the
orbital fits of the stellar components, and the same result is found including
the effects of a nonlinear gas drag. Thus, the possibility of planetary
formation in this binary system largely appears insensitive to the orbital fits
adopted for the stellar components, and any future alterations in the system
parameters (due to new observations) should not change this picture. Finally,
we show that planetesimals migrating because of gas drag may be trapped in
mean-motion resonances with the binary, even though the migration is divergent.Comment: 11 pages, 9 figure
High Contrast Imaging of the Close Environment of HD 142527 -
Context. It has long been suggested that circumstellar disks surrounding
young stars may be the signposts of planets, and still more since the recent
discoveries of embedded substellar companions. The planet-disk interaction may
create, according to models, large structures, gaps, rings or spirals, in the
disk. In that sense, the Herbig star HD 142527 is particularly compelling as,
its massive disk displays intriguing asymmetries that suggest the existence of
a dynamical peturber of unknown nature. Aims. Our goal was to obtain deep
thermal images of the close circumstellar environment of HD 142527 to re-image
the reported close-in structures (cavity, spiral arms) of the disk and to
search for stellar and substellar companions that could be connected to their
presence. Results. The circumstellar environment of HD 142527 is revealed at an
unprecedented spatial resolution down to the sub arcsecond level for the first
time at 3.8 microns. Our images reveal important radial and azimuthal
asymmetries which invalidate an elliptical shape for the disk as previously
proposed. It rather suggests a bright inhomogeneous spiral arm plus various
fainter spiral arms. We also confirm an inner cavity down to 30 AU and two
important dips at position angles of 0 and 135 deg. The detection performance
in angular differential imaging enables the exploration of the planetary mass
regime for projected physical separations as close as 40 AU. The use of our
detection map together with Monte Carlo simulations sets stringent constraints
on the presence of planetary mass, brown dwarf or stellar companions as a
function of the semi-major axis. They severely constrain the presence of
massive giant planets with semi-major axis beyond 50AU, i.e. probably within
the large disk's cavity that radially extends up to 145 AU or even further
outside.Comment: 8 pages, 7 figures, accepted in A&
Neptune Trojans and Plutinos: colors, sizes, dynamics, and their possible collisions
Neptune Trojans and Plutinos are two subpopulations of trans-Neptunian
objects located in the 1:1 and the 3:2 mean motion resonances with Neptune,
respectively, and therefore protected from close encounters with the planet.
However, the orbits of these two kinds of objects may cross very often,
allowing a higher collisional rate between them than with other kinds of
trans-Neptunian objects, and a consequent size distribution modification of the
two subpopulations.
Observational colors and absolute magnitudes of Neptune Trojans and Plutinos
show that i) there are no intrinsically bright (large) Plutinos at small
inclinations, ii) there is an apparent excess of blue and intrinsically faint
(small) Plutinos, and iii) Neptune Trojans possess the same blue colors as
Plutinos within the same (estimated) size range do.
For the present subpopulations we analyzed the most favorable conditions for
close encounters/collisions and address any link there could be between those
encounters and the sizes and/or colors of Plutinos and Neptune Trojans. We also
performed a simultaneous numerical simulation of the outer Solar System over 1
Gyr for all these bodies in order to estimate their collisional rate.
We conclude that orbital overlap between Neptune Trojans and Plutinos is
favored for Plutinos with large libration amplitudes, high eccentricities, and
small inclinations. Additionally, with the assumption that the collisions can
be disruptive creating smaller objects not necessarily with similar colors, the
present high concentration of small Plutinos with small inclinations can thus
be a consequence of a collisional interaction with Neptune Trojans and such
hypothesis should be further analyzed.Comment: 15 pages, 9 figures, 6 tables, accepted for publication in A&
Manual dexterity, but not cerebral palsy, predicts cognitive functioning after neonatal stroke
AIM: To disentangle the respective impacts of manual dexterity and cerebral palsy (CP) in cognitive functioning after neonatal arterial ischaemic stroke.
METHOD: The population included 60 children (21 females, 39 males) with neonatal arterial ischaemic stroke but not epilepsy. The presence of CP was assessed clinically at the age of 7 years and 2 months (range 6y 11mo-7y 8mo) using the definition of the Surveillance of CP in Europe network. Standardized tests (Nine-Hole Peg Test and Box and Blocks Test) were used to quantify manual (finger and hand respectively) dexterity. General cognitive functioning was evaluated with the Wechsler Intelligence Scale for Children, Fourth Edition. Simple and multiple linear regression models were performed while controlling for socio-economic status, lesion side, and sex.
RESULTS: Fifteen children were diagnosed with CP. In simple regression models, both manual dexterity and CP were associated with cognitive functioning (β=0.41 [p=0.002] and β=0.31 [p=0.019] respectively). However, in multiple regression models, manual dexterity was the only associated variable of cognitive functioning, whether or not a child had CP (β=0.35; p=0.007). This result was reproduced in models with other covariables (β=0.31; p=0.017).
INTERPRETATION: As observed in typically developing children, manual dexterity is related to cognitive functioning in children having suffered a focal brain insult during the neonatal period.
WHAT THIS PAPER ADDS: Manual dexterity predicts cognitive functioning after neonatal arterial ischaemic stroke. Correlations between manual dexterity and cognitive functioning occur irrespective of sex, lesion side, presence of cerebral palsy, and socio-economic status. Residual motor ability may support cognitive functioning
Specification of data collection on animal diseases to increase the preparedness of the AHAW panel to answer future mandates – CFP/EFSA/AHAW/2010/01
Morphology of the very inclined debris disk around HD 32297
Direct imaging of circumstellar disks at high angular resolution is mandatory
to provide morphological information that bring constraints on their
properties, in particular the spatial distribution of dust. New techniques
combining observing strategy and data processing now allow very high contrast
imaging with 8-m class ground-based telescopes (10^-4 to 10^-5 at ~1") and
complement space telescopes while improving angular resolution at near infrared
wavelengths. We carried out a program at the VLT with NACO to image known
debris disks with higher angular resolution in the near IR than ever before in
order to study morphological properties and ultimately to detect signpost of
planets. The observing method makes use of advanced techniques: Adaptive
Optics, Coronagraphy and Differential Imaging, a combination designed to
directly image exoplanets with the upcoming generation of "planet finders" like
GPI (Gemini Planet Imager) and SPHERE (Spectro-Polarimetric High contrast
Exoplanet REsearch). Applied to extended objects like circumstellar disks, the
method is still successful but produces significant biases in terms of
photometry and morphology. We developed a new model-matching procedure to
correct for these biases and hence to bring constraints on the morphology of
debris disks. From our program, we present new images of the disk around the
star HD 32297 obtained in the H (1.6mic) and Ks (2.2mic) bands with an
unprecedented angular resolution (~65 mas). The images show an inclined thin
disk detected at separations larger than 0.5-0.6". The modeling stage confirms
a very high inclination (i=88{\deg}) and the presence of an inner cavity inside
r_0~110AU. We also found that the spine (line of maximum intensity along the
midplane) of the disk is curved and we attributed this feature to a large
anisotropic scattering factor (g~0.5, valid for an non-edge on disk). Abridged
...Comment: 12 pages, 10 figures, accepted for publication in Astronomy and
Astrophysic
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