155 research outputs found
Grain Size segregation in debris discs
In most debris discs, dust grain dynamics is strongly affected by stellar
radiation pressure. As this mechanism is size-dependent, we expect dust grains
to be spatially segregated according to their sizes. However, because of the
complex interplay between radiation pressure, collisions and dynamical
perturbations, this spatial segregation of the particle size distribution (PSD)
has proven difficult to investigate with numerical models. We propose to
explore this issue using a new-generation code that can handle some of the
coupling between dynamical and collisional effects. We investigate how PSDs
behave in both unperturbed discs "at rest" and in discs pertubed by planetary
objects. We use the DyCoSS code of Thebault(2012) to investigate the coupled
effect of collisions, radiation pressure and dynamical perturbations in systems
having reached a steady state. We consider 2 setups: a narrow ring perturbed by
an exterior planet, and an extended disc into which a planet is embedded. For
both setups we consider an additional unperturbed case with no planet. We also
investigate how possible spatial size segregation affect disc images at
different wavelengths. We find that PSDs are always strongly spatially
segregated. The only case for which they follow a standard dn/dr = C.r**(-3.5)
law is for an unperturbed narrow ring, but only within the parent body ring
itself. For all other configurations, the PSD can strongly depart from such
power laws and have strong spatial gradients. As an example, the geometrical
cross section of the disc is rarely dominated by the smallest grains on bound
orbits, as it is expected to be in standard PSDs in s**q with q<-3. Although
the exact profiles and spatial variations of PSDs are a complex function of the
considered set-up, we are however able to derive some robust results that
should be useful for image-or-SED-fitting models of observed discs.Comment: Accepted in A&A // Figure quality has been downgraded. A high-res
version of the paper can be found at
http://lesia.obspm.fr/perso/philippe-thebault/sizepap_rev.pdf /V2: typos
correcte
Extrasolar comets : the origin of dust in exozodiacal disks?
Comets have been invoked in numerous studies as a potentially important
source of dust and gas around stars, but none has studied the thermo-physical
evolution, out-gassing rate, and dust ejection of these objects in such stellar
systems. We investigate the thermo-physical evolution of comets in
exo-planetary systems in order to provide valuable theoretical data required to
interpret observations of gas and dust. We use a quasi 3D model of cometary
nucleus to study the thermo-physical evolution of comets evolving around a
single star from 0.1 to 50 AU, whose homogeneous luminosity varies from 0.1 to
70 solar luminosities. This paper provides mass ejection, lifetimes, and the
rate of dust and water gas mass productions for comets as a function of the
distance to the star and stellar luminosity. Results show significant physical
changes to comets at high stellar luminosities. The models are presented in
such a manner that they can be readily applied to any planetary system. By
considering the examples of the Solar System, Vega and HD 69830, we show that
dust grains released from sublimating comets have the potential to create the
observed (exo)zodiacal emission. We show that observations can be reproduced by
1 to 2 massive comets or by a large number of comets whose orbits approach
close to the star. Our conclusions depend on the stellar luminosity and the
uncertain lifetime of the dust grains. We find, as in previous studies, that
exozodiacal dust disks can only survive if replenished by a population of
typically sized comets renewed from a large and cold reservoir of cometary
bodies beyond the water ice line. These comets could reach the inner regions of
the planetary system following scattering by a (giant) planet.Comment: 21 pages, 10 figure
Scattering of small bodies by planets: a potential origin for exozodiacal dust ?
High levels of exozodiacal dust are observed around a growing number of main
sequence stars. The origin of such dust is not clear, given that it has a short
lifetime against both collisions and radiative forces. Even a collisional
cascade with km-sized parent bodies, as suggested to explain outer debris
discs, cannot survive sufficiently long. In this work we investigate whether
the observed exozodiacal dust could originate from an outer planetesimal belt.
We investigate the scattering processes in stable planetary systems in order to
determine whether sufficient material could be scattered inwards in order to
retain the exozodiacal dust at its currently observed levels. We use N-body
simulations to investigate the efficiency of this scattering and its dependence
on the architecture of the planetary system. The results of these simulations
can be used to assess the ability of hypothetical chains of planets to produce
exozodi in observed systems. We find that for older (>100Myr) stars with
exozodiacal dust, a massive, large radii (>20AU) outer belt and a chain of
tightly packed, low-mass planets would be required in order to retain the dust
at its currently observed levels. This brings into question how many, if any,
real systems possess such a contrived architecture and are therefore capable of
scattering at sufficiently high rates to retain exozodi dust on long
timescales
Investigating the flyby scenario for the HD 141569 system
HD 141569, a triple star system, has been intensively observed and studied
for its massive debris disk. It was rather regarded as a gravitationally bound
triple system but recent measurements of the HD 141569A radial velocity seem to
invalidate this hypothesis. The flyby scenario has therefore to be investigated
to test its compatibility with the observations. We present a study of the
flyby scenario for the HD141569 system, by considering 3 variants: a sole
flyby, a flyby associated with one planet and a flyby with two planets. We use
analytical calculations and perform N-body numerical simulations of the flyby
encounter. The binary orbit is found to be almost fixed by the observational
constraint on a edge-on plane with respect to the observers. If the binary has
had an influence on the disk structure, it should have a passing time at the
periapsis between 5000 and 8000 years ago and a distance at periapsis between
600 and 900 AU. The best scenario for reproducing the disk morphology is a
flyby with only 1 planet. For a 2 Mj (resp. 8 Mj) planet, its eccentricity must
be around 0.2 (resp. below 0.1). In the two cases, its apoapsis is about 130
AU. Although the global disk shape is reasonably well reproduced, some features
cannot be explain by the present model and the likehood of the flyby event
remains an issue. Dynamically speaking, HD 141569 is still a puzzling system
Signatures of massive collisions in debris discs
Violent stochastic collisional events have been invoked as a possible
explanation for some debris discs displaying pronounced asymmetries or having a
great luminosity excess. So far, no thorough modelling of the consequences of
such events has been carried out, mainly because of the extreme numerical
challenge of coupling the dynamical and collisional evolution of dust.
We perform the first fully self-consistent modelling of the aftermath of
massive breakups in debris discs. We follow the collisional and dynamical
evolution of dust released after the breakup of a Ceres-sized body at 6 AU from
its central star. We investigate the duration, magnitude and spatial structure
of the signature left by such a violent event, as well as its observational
detectability.
We use the recently developed LIDT-DD code (Kral et al., 2013), which handles
the coupled collisional and dynamical evolution of debris discs. The main focus
is placed on the complex interplay between destructive collisions, Keplerian
dynamics and radiation pressure forces. We use the GRaTer package to estimate
the system's luminosity at different wavelengths.
The breakup of a Ceres-sized body at 6 AU creates an asymmetric dust disc
that is homogenized, by the coupled action of collisions and dynamics, on a
timescale of a few years. The luminosity excess in the breakup's
aftermath should be detectable by mid-IR photometry, from a 30 pc distance,
over a period of years that exceeds the duration of the asymmetric
phase of the disc (a few years). As for the asymmetric structures, we
derive synthetic images for the SPHERE/VLT and MIRI/JWST instruments, showing
that they should be clearly visible and resolved from a 10 pc distance. Images
at 1.6m (marginally), 11.4 and 15.5m would show the inner disc
structures while 23m images would display the outer disc asymmetries.Comment: 16 pages, 14 figures, abstract shortened, accepted for publication in
A&
Polarization of stars with debris disks: comparing observations with models
The Space telescope carried out an unprecedented survey of nearby
stars for debris disks. The dust present in these debris disks scatters and
polarizes stellar light in the visible part of the spectrum. We explore what
can be learned with aperture polarimetry and detailed radiative transfer
modelling about stellar systems with debris disks. We present a polarimetric
survey, with measurements from the literature, of candidate stars observed by
DEBRIS and DUNES surveys. We perform a statistical analysis of the
polarimetric data with the detection of far-infrared excess by and
with a sample of 223 stars. Monte Carlo simulations were performed to
determine the effects of various model parameters on the polarization level and
find the mass required for detection with current instruments. Eighteen stars
were detected with a polarization per cent and
, but only two of them have a debris disk. No statistically
significant difference is found between the different groups of stars, with,
without, and unknown status for far-infrared excess, and presence of
polarization. The simulations show that the integrated polarization is rather
small, usually per cent for typical masses detected by their
far-infrared excess for hot and most warm disks. Masses observed in cold disks
can produce polarization levels above per cent since there is usually
more dust in them than in closer disks. We list five factors which can explain
the observed low-polarization detection rate. Observations with high-precision
polarimeters should lead to additional constraints on models of unresolved
debris disks.Comment: Corrected some quotations and typos and deleted superfluous
references. 20 pages, 5 figure
Insights on the dynamical history of the Fomalhaut system - Investigating the Fom c hypothesis
The eccentric shape of the debris disk observed around Fomalhaut was first
attributed to Fom b, a companion detected near the belt inner-edge, but new
constraints on its orbit revealed that it is belt-crossing, highly eccentric
, and can hardly account for the shape of the belt. The best
scenario to explain this paradox is that there is another massive body in this
system, Fom c, which drives the debris disk shape. The resulting planetary
system is highly unstable, which hints at a dynamical scenario involving a
recent scattering of Fom b on its current orbit, potentially with the putative
Fom c.
Our goal is to give insights on the probability for Fom b to have been set on
its highly eccentric orbit by a close-encounter with the putative Fom c. We aim
to study in particular the part played by mean-motion resonances with Fom c,
which could have brought Fom b sufficiently close to Fom c for it to be
scattered on its current orbit, but also delay this scattering event.
Using N-body simulations, we found that the generation of orbits similar to
that of Fom b, either in term of dimensions or orientation, is a robust process
involving a scattering event and a further secular evolution of inner material
with an eccentric massive body such as the putative Fom c. We found in
particular that mean-motion resonances can delay scattering events, and thus
the production of Fom b-like orbits, on timescales comparable to the age of the
system, thus explaining the witnessing of an unstable configuration.
We conclude that Fom b probably originated from an inner resonance with Fom
c, which is at least Neptune-Saturn size, and was set on its current orbit by a
scattering event with Fom c. Since Fom b could not have formed from material in
resonance, our scenario also hints at former migration processes in this
planetary system
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