2,077 research outputs found
Planet formation in highly inclined binaries
We explore planet formation in binary systems around the central star where
the protoplanetary disk plane is highly inclined with respect to the companion
star orbit. This might be the most frequent scenario for binary separations
larger than 40 AU, according to Hale (1994). We focus on planetesimal accretion
and compute average impact velocities in the habitable region and up to 6 AU
from the primary.Comment: Accepted for publication on A&
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
A new code to study structures in collisionally active, perturbed debris discs. Application to binaries
Debris discs are traditionally studied using two distinct types of numerical
models: statistical particle-in-a-box codes to study their collisional and size
distribution evolution, and dynamical N-body models to study their spatial
structure. The absence of collisions from N-body codes is in particular a major
shortcoming, as collisional processes are expected to significantly alter the
results obtained from pure N-body runs. We present a new numerical model, to
study the spatial structure of perturbed debris discs at dynamical and
collisional steady-state. We focus on the competing effects between
gravitational perturbations by a massive body (planet or star), collisional
production of small grains, and radiation pressure placing these grains in
possibly dynamically unstable regions. We consider a disc of parent bodies at
dynamical steady-state, from which small radiation-pressure-affected grains are
released in a series of runs, each corresponding to a different orbital
position of the perturber, where particles are assigned a collisional
destruction probability. These collisional runs produce successive position
maps that are then recombined, following a complex procedure, to produce
surface density profiles for each orbital position of the perturbing body. We
apply our code to the case of a circumprimary disc in a binary. We find
pronounced structures inside and outside the dynamical stability regions. For
low , the disc's structure is time varying, with spiral arms in the
dynamically "forbidden" region precessing with the companion star. For high
, the disc is strongly asymmetric but time invariant, with a pronounced
density drop in the binary's periastron direction. (better resolution version
of the paper at http://lesia.obspm.fr/perso/philippe-thebault/theb2011.pdf)Comment: accepted for publication in Astronomy and Astrophysics /// There is a
problem with the way Fig.1 looks on the astro-ph file. You can retrieve a
correct version of the full paper at
http://lesia.obspm.fr/perso/philippe-thebault/theb2011.pd
On the eccentricity of self-gravitating circumstellar disks in eccentric binary systems
We study the evolution of circumstellar massive disks around the primary star
of a binary system focusing on the computation of disk eccentricity. In
particular, we concentrate on its dependence on the binary eccentricity.
Self-gravity is included in our numerical simulations. Our standard model
assumes a semimajor axis for the binary of 30 AU, the most probable value
according to the present binary statistics.Comment: Accepted for publication on A&
Vertical structure of debris discs
The vertical thickness of debris discs is often used as a measure of these
systems' dynamical excitation and as clues to the presence of hidden massive
perturbers such as planetary embryos. However, this argument could be flawed
because the observed dust should be naturally placed on inclined orbits by the
combined effect of radiation pressure and mutual collisions. We critically
reinvestigate this issue and numerically estimate what the "natural" vertical
thickness of a collisionally evolving disc is, in the absence of any additional
perturbing body. We use a deterministic collisional code, following the
dynamical evolution of a population of indestructible test grains suffering
mutual inelastic impacts. Grain differential sizes as well as the effect of
radiation pressure are taken into account. We find that, under the coupled
effect of radiation pressure and collisions, grains naturally acquire
inclinations of a few degrees. The disc is stratified with respect to grain
sizes, with the smallest grains having the largest vertical dispersion and the
bigger ones clustered closer to the midplane. Debris discs should have a
minimum "natural" observed aspect ratio at visible to
mid-IR wavelengths where the flux is dominated by the smallest bound grains.
These values are comparable to the estimated thicknesses of many vertically
resolved debris discs, as is illustrated with the specific example of AU Mic.
For all systems with , the presence (or absence) of embedded
perturbing bodies cannot be inferred from the vertical dispersion of the discComment: accepted for publication in Astronomy and Astrophysics (full abstract
in the pdf file
Debris discs in binaries: a numerical study
Debris disc analysis and modelling provide crucial information about the
structure and the processes at play in extrasolar planetary systems. In binary
systems, this issue is more complex because the disc should in addition respond
to the companion star's perturbations. We explore the dynamical evolution of a
collisionally active debris disc for different initial parent body populations,
diverse binary configurations and optical depths. We focus on the radial extent
and size distribution of the disc at a stationary state. We numerically follow
the evolution of massless small grains, initially produced from a
circumprimary disc of parent bodies following a size distribution in ds . Grains are submitted to both stars' gravity as well as
radiation pressure. In addition, particles are assigned an empirically derived
collisional lifetime. For all the binary configurations the disc extends far
beyond the critical semimajor axis for orbital stability. This is due
to the steady production of small grains, placed on eccentric orbits reaching
beyond by radiation pressure. The amount of matter beyond acrit
depends on the balance between collisional production and dynamical removal
rates: it increases for more massive discs as well as for eccentric binaries.
Another important effect is that, in the dynamically stable region, the disc is
depleted from its smallest grains. Both results could lead to observable
signatures. We have shown that a companion star can never fully truncate a
collisionally active disc. For eccentric companions, grains in the unstable
regions can significantly contribute to the thermal emission in the mid-IR.
Discs with sharp outer edges, especially bright ones such as HR4796A, are
probably shaped by other mechanisms.Comment: accepted for publication in A&
LIDT-DD: A new self-consistent debris disc model including radiation pressure and coupling collisional and dynamical evolution
In most current debris disc models, the dynamical and the collisional
evolutions are studied separately, with N-body and statistical codes,
respectively, because of stringent computational constraints. We present here
LIDT-DD, the first code able to mix both approaches in a fully self-consistent
way. Our aim is for it to be generic enough so as to be applied to any
astrophysical cases where we expect dynamics and collisions to be deeply
interlocked with one another: planets in discs, violent massive breakups,
destabilized planetesimal belts, exozodiacal discs, etc. The code takes its
basic architecture from the LIDT3D algorithm developed by Charnoz et al.(2012)
for protoplanetary discs, but has been strongly modified and updated in order
to handle the very constraining specificities of debris discs physics:
high-velocity fragmenting collisions, radiation-pressure affected orbits,
absence of gas, etc. In LIDT-DD, grains of a given size at a given location in
a disc are grouped into "super-particles", whose orbits are evolved with an
N-body code and whose mutual collisions are individually tracked and treated
using a particle-in-a-box prescription. To cope with the wide range of possible
dynamics, tracers are sorted and regrouped into dynamical families depending on
their orbits. The code retrieves the classical features known for debris discs,
such as the particle size distributions in unperturbed discs, the outer radial
density profiles (slope in -1.5) outside narrow collisionally active rings, and
the depletion of small grains in "dynamically cold" discs. The potential of the
new code is illustrated with the test case of the violent breakup of a massive
planetesimal within a debris disc. The main potential future applications of
the code are planet/disc interactions, and more generally any configurations
where dynamics and collisions are expected to be intricately connected.Comment: Accepted for publication in A&A. 20 pages, 17 figures. Abstract
shortened for astro-p
Superpositions of the cosmological constant allow for singularity resolution and unitary evolution in quantum cosmology
A novel approach to quantization is shown to allow for superpositions of the
cosmological constant in isotropic and homogeneous mini-superspace models.
Generic solutions featuring such superpositions display unitary evolution and
resolution of the classical singularity. Physically well-motivated cosmological
solutions are constructed. These particular solutions exhibit characteristic
features of a cosmic bounce including universal phenomenology that can be
rendered insensitive to Planck-scale physics in a natural manner.Comment: Version accepted to Physics Letters B. Minor revisions,
clarifications added. 7 pages, 3 figure
Relative velocities among accreting planetesimals in binary systems: the circumbinary case
We numerically investigate the possibility of planetesimal accretion in
circumbinary disks, under the coupled influence of both stars' secular
perturbations and friction due to the gaseous component of the protoplanetary
disk. We focus on one crucial parameter: the distribution of encounter
velocities between planetesimals in the 0.5 to 100km size range. An extended
range of binary systems with differing orbital parameters is explored. The
resulting encounter velocities are compared to the threshold velocities below
which the net outcome of a collision is accumulation into a larger body instead
of mass erosion. For each binary configuration, we derive the critical radial
distance from the binary barycenter beyond which planetesimal accretion is
possible. This critical radial distance is smallest for equal-mass binaries on
almost circular orbits. It shifts to larger values for increasing
eccentricities and decreasing mass ratio. The importance of the planetesimals'
orbital alignments of planetesimals due to gas drag effects is discussed.Comment: accepted for publication in MNRA
Critical Temperature Range in Standard and Ni-bearing Spheroidal Graphite Cast Irons
Describing the conditions for reaustenitization of spheroidal graphite cast irons is of interest for their heat-treatment after casting, e.g. for manufacturing austempered ductile irons. Differential thermal analysis has been used to characterize the direct eutectoid transformation and the reverse transformation, i.e. the reaustenitization. This has been applied to a standard and a Ni-bearing alloy, with a ferritic matrix for the former, both a ferritic and a pearlitic matrix for the latter. The results are discussed in relation with the stable and metastable three phase fields. While earlier description of the direct eutectoid transformation is confirmed, the one for reverse eutectoid has been found more complex and is amended
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