78 research outputs found
Dust production in debris discs: constraints on the smallest grains
The surface energy constraint puts a limit on the smallest fragment
that can be produced after a collision. Based on analytical
considerations, this mechanism has been recently identified as been potentially
able to prevent the production of small dust grains in debris discs and cut off
their size distribution at sizes larger than the blow-out size. We numerically
investigate the importance of this effect to find under which conditions it can
leave a signature in the small-size end of a disc's particle size distribution
(PSD). An important part of this work is to map out, in a disc at steady-state,
what is the most likely collisional origin for micron-sized grains, in terms of
the sizes of their collisional progenitors. We implement, for the first time,
the surface energy constraint into a collisional evolution code. We consider a
debris disc extending from 50 to 100AU and 2 different stellar types. We also
consider two levels of stirring in the disc: dynamically "hot" (e=0.075) and
"cold" (e=0.01). For all cases, we derive maps as a function of
target and projectile sizes, and , and compare them to equivalent
maps for the dust-production rate. We then compute disc-integrated PSDs and
estimate the imprint of the surface energy constraint. We find that the
(,) regions of high values do not coincide with those of
high dust production rate. As a consequence, the surface energy constraint has
generally a weak effect on the system's PSD. The maximum -induced
depletion of m-sized grains is % and is obtained for a sun-like
star and a dynamically hot case. For the e=0.01 cases, the surface energy
effect is negligible compared to the massive small grain depletion induced by
another mechanism: the natural imbalance between dust production and
destruction rates in low-stirring discs identified by Thebault&Wu(2008).Comment: Accepted for Publication in A&A (abstract truncated for astroph) (v3
and v4 corrected from minor language errors and typos
Transient events in bright debris discs: Collisional avalanches revisited
A collisional avalanche is set off by the breakup of a large planetesimal,
releasing small unbound grains that enter a debris disc located further away
from the star, triggering there a collisional chain reaction that can
potentially create detectable transient structures. We explore this mechanism,
using for the first time a code coupling dynamical and collisional evolutions,
and investigate if avalanches could explain the short-term luminosity
variations observed in some extremely bright discs. We consider two set-ups: a
cold disc case, with a dust release at 10au and an outer disc extending from 50
to 120au, and a warm disc case with the release at 1au and a 5-12au outer disc.
We find that avalanches could leave detectable structures on resolved images,
for both cold and warm disc cases, in discs with optical depth of a few
, provided that large dust masses
(10-510g) are initially released. The integrated
photometric excess due to an avalanche is limited, less than 10% for these
released dust masses, peaking in the mid-IR and becoming insignificant beyond
40-50m. Contrary to earlier studies, we do not obtain stronger
avalanches when increasing to higher values. Likewise, we do not observe
a significant luminosity deficit, as compared to the pre-avalanche level, after
the passage of the avalanche. These two results concur to make avalanches an
unlikely explanation for the sharp luminosity drops observed in some extremely
bright debris discs. The ideal configuration for observing an avalanche would
be a two-belt structure, with an inner belt of fractional luminosity >10
where breakups of massive planetesimals occur, and a more massive outer belt,
with of a few , into which the avalanche chain reaction
develops and propagates.Comment: Accepted for publication in Astronomy & Astrophysics (abstract
drastically shortened to meet astro-ph requirements
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
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
Planets in Binaries: Formation and Dynamical Evolution
Binary systems are very common among field stars. While this relatively small
number of planets in binaries is probably partly due to strong observational
biases, there is, however, statistical evidence that planets are indeed less
frequent in binaries with separations smaller than 100 au, strongly suggesting
that the presence of a close in companion star has an adverse effect on planet
formation. It is indeed possible for the gravitational pull of the second star
to affect all the different stages of planet formation, from proto-planetary
disk formation to dust accumulation into planetesimals, to the accretion of
these planetesimals into large planetary embryos and, eventually, the final
growth of these embryos into planets. For the crucial planetesimal accretion
phase, the complex coupling between dynamical perturbations from the binary and
friction due to gas in the protoplanetary disk suggests that planetesimal
accretion might be hampered due to increased, accretion hostile impact
velocities. Likewise, the interplay between the binary secular perturbations
and mean motion resonances lead to unstable regions, where not only planet
formation is inhibited, but where a massive body would be ejected from the
system on a hyperbolic orbit. The amplitude of these two main effects is
different for S and P type planets, so that a comparison between the two
populations might outline the influence of the companion star on the planet
formation process. Unfortunately, at present the two populations (circumstellar
or circumbinary) are not known equally well and different biases and
uncertainties prevent a quantitative comparison. We also highlight the long
term dynamical evolution of both S and P type systems and focus on how these
different evolutions influence the final architecture of planetary systems in
binaries.Comment: Published on Galaxies, vol. 7, issue 4, p. 8
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&
Le facteur d'élongation Spt2/Sin1 est impliqué dans l'assemblage du nucléosome couplé à la transcription
La structure de base de la chromatine est le nucléosome contenant 146 pb de l'ADN, enroulé autour d'un octamÚre d'histone composé de deux dimÚres d'histone H2A-H2B et d'un tétramÚre d'histone H3-H4. De nombreuses protéines nonhistone sont impliquées dans la régulation de la structure chromatinienne. Notre laboratoire s'intéresse à l'étude d'une protéine structurale de type HMG retrouvée chez Saccharomyes cerevisiae : Spt2/Sin1. Des études précédentes ont mis en évidence le rÎle de cette protéine dans de nombreux mécanismes transcriptionnels tels l'initiation, l'élongation et la terminaison. Pour mieux comprendre la fonction de ce facteur dans la régulation transcriptionnelle, nous avons décidé d'utiliser un modÚle fortement régulé par Spt2. Il a été démontré que la mutation spt2A déréprimait le gÚne SER3. De façon intéressante, la répression du gÚne SER3 est due à la transcription active d'un ARN non codant (ARNnc), en amont du gÚne, appelé SRG1. Ce mécanisme complexe de régulation est appelé interférence à la transcription. Mes résultats montrent clairement que la répression du gÚne SER3 n'est pas uniquement liée au taux de production de l'ARNnc mais aussi à la formation d'une structure chromatinienne propre permettant la régulation du mécanisme d'interférence. De plus, je montre que, comme Spt6, Spt2 joue un rÎle central dans l'établissement de cette structure chromatinienne répressive. Pour finir, en utilisant un test d'échange nucleosomal, je montre que la protéine Spt2 contribue à la déposition des histones H3 dans le sillage de l'ARNP II au niveau de SRG1. Cette nouvelle fonction de Spt2 suggÚre un rÎle de la protéine dans l'assemblage nucleosomal lié à la transcription
From Dust To Planetesimal: The Snowball Phase ?
The standard model of planet formation considers an initial phase in which
planetesimals form from a dust disk, followed by a phase of mutual
planetesimal-planetesimal collisions, leading eventually to the formation of
planetary embryos. However, there is a potential transition phase (which we
call the "snowball phase"), between the formation of the first planetesimals
and the onset of mutual collisions amongst them, which has often been either
ignored or underestimated in previous studies. In this snowball phase, isolated
planetesimals move on Keplerian orbits and grow solely via the direct accretion
of sub-cm sized dust entrained with the gas in the protoplanetary disk. Using a
simplified model in which planetesimals are progressively produced from the
dust, we consider the expected sizes to which the planetesimals can grow before
mutual collisions commence and derive the dependence of this size on a number
of critical parameters, including the degree of disk turbulence, the
planetesimal size at birth and the rate of planetesimal creation. For systems
in which turbulence is weak and the planetesimals are created at a low rate and
with relatively small birth size, we show that the snowball growth phase can be
very important, allowing planetesimals to grow by a factor of 10^6 in mass
before mutual collisions take over. In such cases, the snowball growth phase
can be the dominant mode to transfer mass from the dust to planetesimals.
Moreover, such growth can take place within the typical lifetime of a
protoplanetary gas disk. A noteworthy result is that ... ...(see the paper).
For the specific case of close binaries such as Alpha Centauri ... ... (see the
paper). From a more general perspective, these preliminary results suggest that
an efficient snowball growth phase provides a large amount of "room at the
bottom" for theories of planet formation.Comment: Accepted for publication in the Astrophysical Journal. 15 pages, 4
figures, 1 tabl
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