15,029 research outputs found
Resonant Trapping of Planetesimals by Planet Migration: Debris Disk Clumps and Vega's Similarity to the Solar System
This paper describes a model which can explain the observed clumpy structures
of debris disks. Clumps arise because after a planetary system forms its
planets migrate due to angular momentum exchange with the remaining
planetesimals. Outward migration of the outermost planet traps planetesimals
outside its orbit into its resonances and resonant forces cause azimuthal
structure in their distribution. The model is based on numerical simulations of
planets of different masses, Mpl, migrating at different rates, dapl/dt,
through a dynamically cold (e<0.01) planetesimal disk initially at a semimajor
axis a. Trapping probabilities and the resulting azimuthal structures are
presented for a planet's 2:1, 5:3, 3:2, and 4:3 resonances. Seven possible
dynamical structures are identified from migrations defined by mu=Mpl/Mstar and
theta=dapl/dt*sqrt(a/Mstar). Application of this model to the 850um image of
Vega's disk shows its two clumps of unequal brightness can be explained by the
migration of a Neptune-mass planet from 40 to 65AU over 56Myr; tight
constraints are set on possible ranges of these parameters. The clumps are
caused by planetesimals in the 3:2 and 2:1 resonances; the asymmetry arises
because of the overabundance of planetesimals in the 2:1(u) over the 2:1(l)
resonance. The similarity of this migration to that proposed for our own
Neptune hints that Vega's planetary system may be much more akin to the solar
system than previously thought. Predictions are made which would substantiate
this model, such as the orbital motion of the clumpy pattern, the location of
the planet, and the presence of lower level clumps.Comment: 30 pages, accepted by Ap
The Insignificance of P-R Drag in Detectable Extrasolar Planetesimal Belts
This paper considers a simple model in which dust produced in a planetesimal
belt migrates in toward the star due to P-R drag suffering destructive
collisions with other dust grains on the way. Assuming the dust is all of the
same size, the resulting surface density distribution can be derived
analytically and depends only on the parameter eta0=5000tau*sqrt(Mstar/r)/beta;
this parameter can be determined observationally with the hypothesis that
beta=0.5. For massive belts in which eta0>>1 dust is confined to the
planetesimal belt, while the surface density of more tenuous belts, in which
eta0<<1, is constant with distance from the star. The emission spectrum of dust
from planetesimal belts at different distances from different mass stars shows
that the dust belts which have been detected to date should have eta0>>1; dust
belts with eta0<<1 are hard to detect as they are much fainter than the stellar
photosphere. This is confirmed for a sample of 37 debris disk candidates for
which eta0 was determined to be >10. This means that these disks are so massive
that mutual collisions prevent dust from reaching the inner regions of these
systems and P-R drag can be ignored when studying their dynamics. Models for
the formation of structure in debris disks by the trapping of particles into
planetary resonances by P-R drag should be reconsidered. However, since
collisions do not halt 100% of the dust, this means that in the absence of
planetary companions debris disk systems should be populated by small
quantities of hot dust which may be detectable in the mid-IR. Even in disks
with eta0<<1 the temperature of dust emission is shown to be a reliable tracer
of the planetesimal distribution meaning that inner holes in the dust
distribution imply a lack of colliding planetesimals in the inner regions.Comment: 6 pages. Accepted by A&
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
Resolving the terrestrial planet forming regions of HD113766 and HD172555 with MIDI
We present new MIDI interferometric and VISIR spectroscopic observations of
HD113766 and HD172555. Additionally we present VISIR 11um and 18um imaging
observations of HD113766. These sources represent the youngest (16Myr and 12Myr
old respectively) debris disc hosts with emission on <<10AU scales. We find
that the disc of HD113766 is partially resolved on baselines of 42-102m, with
variations in resolution with baseline length consistent with a Gaussian model
for the disc with FWHM of 1.2-1.6AU (9-12mas). This is consistent with the
VISIR observations which place an upper limit of 0."14 (17AU) on the emission,
with no evidence for extended emission at larger distances. For HD172555 the
MIDI observations are consistent with complete resolution of the disc emission
on all baselines of lengths 56-93m, putting the dust at a distance of >1AU
(>35mas). When combined with limits from TReCS imaging the dust at ~10um is
constrained to lie somewhere in the region 1-8AU. Observations at ~18um reveal
extended disc emission which could originate from the outer edge of a broad
disc, the inner parts of which are also detected but not resolved at 10um, or
from a spatially distinct component. These observations provide the most
accurate direct measurements of the location of dust at 1-8AU that might
originate from the collisions expected during terrestrial planet formation.
These observations provide valuable constraints for models of the composition
of discs at this epoch and provide a foundation for future studies to examine
in more detail the morphology of debris discs.Comment: 22 pages, 19 figures, accepted for publication in MNRA
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