646 research outputs found
Do Two Temperature Debris Disks Have Multiple Belts?
We present a study of debris disks whose spectra are well modelled by dust
emission at two different temperatures. These disks are typically assumed to be
a sign of multiple belts, which in only a few cases have been confirmed via
high resolution observations. We first compile a sample of two-temperature
disks to derive their properties, summarised by the ratios of the warm and cool
component temperatures and fractional luminosities. The ratio of warm to cool
temperatures is constant in the range 2-4, and the temperatures of both warm
and cool components increases with stellar mass. We then explore whether this
emission can arise from dust in a single narrow belt, with the range of
temperatures arising from the size variation of grain temperatures. This model
can produce two-temperature spectra for Sun-like stars, but is not supported
where it can be tested by observed disk sizes and far-IR/mm spectral slopes.
Therefore, while some two-temperature disks arise from single belts, it is
probable that most have multiple spatial components. These disks are plausibly
similar to the outer Solar System's configuration of Asteroid and
Edgeworth-Kuiper belts separated by giant planets. Alternatively, the inner
component could arise from inward scattering of material from the outer belt,
again due to intervening planets. In either case, we suggest that the ratio of
warm/cool component temperatures is indicative of the scale of outer planetary
systems, which typically span a factor of about ten in radius.Comment: accepted to MNRA
Resolved Imaging of the HD191089 Debris Disc
Two thirds of the F star members of the 12 Myr old Beta Pictoris Moving Group
(BPMG) show significant excess emission in the mid-infrared, several million
years after the expected dispersal of the protoplanetary disc. Theoretical
models of planet formation suggest that this peak in the mid-infrared emission
could be due to the formation of Pluto-sized bodies in the disc, which ignite
the collisional cascade and enhance the production of small dust. Here we
present resolved mid-infrared imaging of the disc of HD191089 (F5V in the BPMG)
and consider its implications for the state of planet formation in this system.
HD191089 was observed at 18.3 microns using T-ReCS on Gemini South and the
images were compared to models of the disc to constrain the radial distribution
of the dust. The emission observed at 18.3 microns is shown to be significantly
extended beyond the PSF at a position angle of 80 degrees. This is the first
time dust emission has been resolved around HD191089. Modelling indicates that
the emission arises from a dust belt from 28-90 AU, inclined at 35 degrees from
edge on with very little emission from the inner 28AU of the disc, indicating
the presence of an inner cavity. The steep slope of the inner edge is more
consistent with truncation by a planet than with ongoing stirring. A tentative
brightness asymmetry F(W)/F(E)=0.80+/-0.12 (1.8 sigma) between the two sides of
the disc could be evidence for perturbations from a massive body on an
eccentric orbit in the system.Comment: 11 Pages Accepted to MNRA
Hamiltonian model of capture into mean motion resonance
Mean motion resonances are a common feature of both our own Solar System and
of extrasolar planetary systems. Bodies can be trapped in resonance when their
orbital semi-major axes change, for instance when they migrate through a
protoplanetary disc. We use a Hamiltonian model to thoroughly investigate the
capture behaviour for first and second order resonances. Using this method, all
resonances of the same order can be described by one equation, with
applications to specific resonances by appropriate scaling. We focus on the
limit where one body is a massless test particle and the other a massive
planet. We quantify how the the probability of capture into a resonance depends
on the relative migration rate of the planet and particle, and the particle's
eccentricity. Resonant capture fails for high migration rates, and has
decreasing probability for higher eccentricities, although for certain
migration rates, capture probability peaks at a finite eccentricity. We also
calculate libration amplitudes and the offset of the libration centres for
captured particles, and the change in eccentricity if capture does not occur.
Libration amplitudes are higher for larger initial eccentricity. The model
allows for a complete description of a particle's behaviour as it successively
encounters several resonances. The model is applicable to many scenarios,
including (i) Planet migration through gas discs trapping other planets or
planetesimals in resonances; (ii) Planet migration through a debris disc; (iii)
Dust migration through PR drag. Full details can be found in
\cite{2010submitted}. (Abridged)Comment: 4 pages, Proceedings of IAUS276 "The Astrophysics of Planetary
Systems: Formation, Structure, and Dynamical Evolution
Constraining the orbits of sub-stellar companions imaged over short orbital arcs
Imaging a star's companion at multiple epochs over a short orbital arc
provides only four of the six coordinates required for a unique orbital
solution. Probability distributions of possible solutions are commonly
generated by Monte Carlo (MCMC) analysis, but these are biased by priors and
may not probe the full parameter space. We suggest alternative methods to
characterise possible orbits, which compliment the MCMC technique. Firstly the
allowed ranges of orbital elements are prior-independent, and we provide means
to calculate these ranges without numerical analyses. Hence several interesting
constraints (including whether a companion even can be bound, its minimum
possible semi-major axis and its minimum eccentricity) may be quickly computed
using our relations as soon as orbital motion is detected. We also suggest an
alternative to posterior probability distributions as a means to present
possible orbital elements, namely contour plots of elements as functions of
line of sight coordinates. These plots are prior-independent, readily show
degeneracies between elements and allow readers to extract orbital solutions
themselves. This approach is particularly useful when there are other
constraints on the geometry, for example if a companion's orbit is assumed to
be aligned with a disc. As examples we apply our methods to several imaged
sub-stellar companions including Fomalhaut b, and for the latter object we show
how different origin hypotheses affect its possible orbital solutions. We also
examine visual companions of A- and G-type main sequence stars in the
Washington Double Star Catalogue, and show that per cent must be
unbound.Comment: Accepted for publication in MNRA
- …