1,535 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
Schrodinger Evolution for the Universe: Reparametrization
Starting from a generalized Hamilton-Jacobi formalism, we develop a new
framework for constructing observables and their evolution in theories
invariant under global time reparametrizations. Our proposal relaxes the usual
Dirac prescription for the observables of a totally constrained system
(`perennials') and allows one to recover the influential partial and complete
observables approach in a particular limit. Difficulties such as the
non-unitary evolution of the complete observables in terms of certain partial
observables are explained as a breakdown of this limit. Identification of our
observables (`mutables') relies upon a physical distinction between gauge
symmetries that exist at the level of histories and states (`Type 1'), and
those that exist at the level of histories and not states (`Type 2'). This
distinction resolves a tension in the literature concerning the physical
interpretation of the partial observables and allows for a richer class of
observables in the quantum theory. There is the potential for the application
of our proposal to the quantization of gravity when understood in terms of the
Shape Dynamics formalism.Comment: 25 pages (including title page and references), 1 figur
Symplectic reduction and the problem of time in nonrelativistic mechanics
The deep connection between the interpretation of theories invariant under local symmetry transformations (i.e. gauge theories) and the philosophy of space and time can be illustrated nonrelativistically via the investigation of reparameterisation invariant reformulations of Newtonian mechanics, such as Jacobi's theory. Like general relativity, the canonical formulation of such theories feature Hamiltonian constraints; and like general relativity, the interpretation of these constraints along conventional Dirac lines is highly problematic in that it leads to a nonrelativistic variant of the infamous problem of time. I argue that, nonrelativistically at least, the source of the problem can be found precisely within the symplectic reduction that goes along with strict adherence to the Dirac view. Avoiding reduction, two viable alternative strategies for dealing with Hamiltonian constraints are available. Each is found to lead us to a novel and interesting re-conception of time and change within nonrelativistic mechanics. Both these strategies and the failure of reduction have important implications for the debate concerning the relational or absolute status of time within physical theory
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
Planet Signatures in Collisionally Active Debris Discs: scattered light images
Planet perturbations are often invoked as a potential explanation for many
spatial structures that have been imaged in debris discs. So far this issue has
been mostly investigated with collisionless N-body numerical models. We
numerically investigate how the coupled effect of collisions and radiation
pressure can affect the formation and survival of radial and azimutal
structures in a disc perturbed by a planet. We consider two set-ups: a planet
embedded within an extended disc and a planet exterior to an inner debris ring.
We use the DyCoSS code of Thebault(2012) and derive synthetic images of the
system in scattered light. The planet's mass and orbit, as well as the disc's
collisional activity are explored as free parameters.
We find that collisions always significantly damp planet-induced structures.
For the case of an embedded planet, the planet's signature, mostly a density
gap around its radial position, should remain detectable in head-on images if
M_planet > M_Saturn. If the system is seen edge-on, however, inferring the
presence of the planet is much more difficult, although some planet-induced
signatures might be observable under favourable conditions.
For the inner-ring/external-planet case, planetary perturbations cannot
prevent collision-produced small fragments from populating the regions beyond
the ring: The radial luminosity profile exterior to the ring is close to the
one it should have in the absence of the planet. However, a Jovian planet on a
circular orbit leaves precessing azimutal structures that can be used to
indirectly infer its presence. For a planet on an eccentric orbit, the ring is
elliptic and the pericentre glow effect is visible despite of collisions and
radiation pressure, but detecting such features in real discs is not an
unambiguous indicator of the presence of an outer planet.Comment: Accepted for Publication in A&A (NOTE: Abridged abstract and
(very)LowRes Figures. Better version, with High Res figures and full abstract
can be found at http://lesia.obspm.fr/perso/philippe-thebault/planpapph.pdf
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
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