59 research outputs found
A wide binary trigger for white dwarf pollution
Metal pollution in white dwarf atmospheres is likely to be a signature of
remnant planetary systems. Most explanations for this pollution predict a sharp
decrease in the number of polluted systems with white dwarf cooling age.
Observations do not confirm this trend, and metal pollution in old (1-5 Gyr)
white dwarfs is difficult to explain. We propose an alternative,
time-independent mechanism to produce the white dwarf pollution. The orbit of a
wide binary companion can be perturbed by Galactic tides, approaching close to
the primary star for the first time after billions of years of evolution on the
white dwarf branch. We show that such a close approach perturbs a planetary
system orbiting the white dwarf, scattering planetesimals onto star-grazing
orbits, in a manner that could pollute the white dwarf's atmosphere. Our
estimates find that this mechanism is likely to contribute to metal pollution,
alongside other mechanisms, in up to a few percent of an observed sample of
white dwarfs with wide binary companions, independent of white dwarf age. This
age independence is the key difference between this wide binary mechanism and
others mechanisms suggested in the literature to explain white dwarf pollution.
Current observational samples are not large enough to assess whether this
mechanism makes a significant contribution to the population of polluted white
dwarfs, for which better constraints on the wide binary population are
required, such as those that will be obtained in the near future with Gaia.Comment: MNRAS accepted 10 page
Vega's hot dust from icy planetesimals scattered inward by an outward-migrating planetary system
Vega has been shown to host multiple dust populations, including both hot
exo-zodiacal dust at sub-AU radii and a cold debris disk extending beyond 100
AU. We use dynamical simulations to show how Vega's hot dust can be created by
long-range gravitational scattering of planetesimals from its cold outer
regions. Planetesimals are scattered progressively inward by a system of 5-7
planets from 30-60 AU to very close-in. In successful simulations the outermost
planets are typically Neptune-mass. The back-reaction of planetesimal
scattering causes these planets to migrate outward and continually interact
with fresh planetesimals, replenishing the source of scattered bodies. The most
favorable cases for producing Vega's exo-zodi have negative radial mass
gradients, with sub-Saturn- to Jupiter-mass inner planets at 5-10 AU and outer
planets of 2.5 to 20 Earth masses. The mechanism fails if a Jupiter-sized
planet exists beyond ~15 AU because the planet preferentially ejects
planetesimals before they can reach the inner system. Direct-imaging planet
searches can therefore directly test this mechanism.Comment: Updated references. Accepted to MNRAS Letters. 5 pages, 4 figures.
Blog post about the paper at
http://planetplanet.net/2014/03/31/vega-a-planetary-poem
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
The unseen planets of double belt debris disk systems
The gap between two component debris disks is often taken to be carved by
intervening planets scattering away the remnant planetesimals. We employ N-body
simulations to determine how the time needed to clear the gap depends on the
location of the gap and the mass of the planets. We invert this relation, and
provide an equation for the minimum planet mass, and another for the expected
number of such planets, that must be present to produce an observed gap for a
star of a given age. We show how this can be combined with upper limits on the
planetary system from direct imaging non-detections (such as with GPI or
SPHERE) to produce approximate knowledge of the planetary system.Comment: Accepted to MNRA
Simulations of two-planet systems through all phases of stellar evolution: implications for the instability boundary and white dwarf pollution
Exoplanets have been observed at many stages of their host star's life,
including the main sequence (MS), subgiant and red giant branch stages. Also,
polluted white dwarfs (WDs) likely represent dynamically active systems at late
times. Here, we perform 3-body simulations which include realistic post-MS
stellar mass loss and span the entire lifetime of exosystems with two massive
planets, from the endpoint of formation to several Gyr into the WD phase of the
host star. We find that both MS and WD systems experience ejections and
star-planet collisions (Lagrange instability) even if the planet-planet
separation well-exceeds the analytical orbit-crossing (Hill instability)
boundary. Consequently, MS-stable planets do not need to be closely-packed to
experience instability during the WD phase. This instability may pollute the WD
directly through collisions, or, more likely, indirectly through increased
scattering of smaller bodies such as asteroids or comets. Our simulations show
that this instability occurs predominately between tens of Myr to a few Gyrs of
WD cooling.Comment: Accepted for publication in MNRAS; 24 pages, 19 figure
Herschel Observations of Debris Discs Orbiting Planet-hosting Subgiants
Debris discs are commonly detected orbiting main-sequence stars, yet little
is known regarding their fate as the star evolves to become a giant. Recent
observations of radial velocity detected planets orbiting giant stars highlight
this population and its importance for probing, for example, the population of
planetary systems orbiting intermediate mass stars. Our Herschel survey
observed a subset of the Johnson et al program subgiants, finding that 4/36
exhibit excess emission thought to indicate debris, of which 3/19 are
planet-hosting stars and 1/17 are stars with no current planet detections.
Given the small numbers involved, there is no evidence that the disc detection
rate around stars with planets is different to that around stars without
planets. Our detections provide a clear indication that large quantities of
dusty material can survive the stars' main-sequence lifetime and be detected on
the subgiant branch, with important implications for the evolution of planetary
systems and observations of polluted or dusty white dwarfs. Our detection rates
also provide an important constraint that can be included in models of debris
disc evolution.Comment: 12 pages, MNRAS, accepte
Asynchronous accretion can mimic diverse white dwarf pollutants I: core and mantle fragments
Polluted white dwarfs serve as astrophysical mass spectrometers - their
photospheric abundances are used to infer the composition of planetary objects
that accrete onto them. We show that due to asymmetries in the accretion
process, the composition of the material falling onto a star may vary with time
during the accretion of a single planetary body. Consequently, the
instantaneous photospheric abundances of white dwarfs do not necessarily
reflect the bulk composition of their pollutants, especially when their
diffusion timescales are short. In particular, we predict that when an asteroid
with an iron core tidally disrupts around a white dwarf, a larger share of its
mantle is ejected, and that the core/mantle fraction of the accreting material
varies with time during the event. Crucially, this implies that the core
fraction of differentiated pollutants cannot be determined for white dwarfs
with short diffusion timescales, which sample only brief episodes of longer
accretion processes. The observed population of polluted white dwarfs backs up
the proposed theory. More white dwarfs have accreted material with high Fe/Ca
than low Fe/Ca relative to stellar abundance ratios, indicating the ejection of
mantle material. Additionally, we find tentative evidence that the accretion
rate of iron decreases more rapidly than that of magnesium or calcium, hinting
at variability of the accreted composition. Further corroboration of the
proposed theory will come from the up-coming analysis of large samples of young
white dwarfs.Comment: Accepted for publication in MNRAS. Part one of a series of two
papers. Comments and questions welcom
Can comets deliver prebiotic molecules to rocky exoplanets?
In this work we consider the potential of cometary impacts to deliver complex
organic molecules and the prebiotic building blocks required for life to rocky
exoplanets. Numerical experiments have demonstrated that for these molecules to
survive, impacts at very low velocities are required. This work shows that for
comets scattered from beyond the snow-line into the habitable zone, the minimum
impact velocity is always lower for planets orbiting Solar-type stars than
M-dwarfs. Using both an analytical model and numerical N-body simulations, we
show that the lowest velocity impacts occur onto planets in tightly-packed
planetary systems around high-mass (i.e. Solar-mass) stars, enabling the intact
delivery of complex organic molecules. Impacts onto planets around low-mass
stars are found to be very sensitive to the planetary architecture, with the
survival of complex prebiotic molecules potentially impossible in
loosely-packed systems. Rocky planets around M-dwarfs also suffer significantly
more high velocity impacts, potentially posing unique challenges for life on
these planets. In the scenario that cometary delivery is important for the
origins of life, this study predicts the presence of biosignatures will be
correlated with i) decreasing planetary mass (i.e. escape velocity), ii)
increasing stellar-mass, and iii) decreasing planetary separation (i.e.
exoplanets in tightly-packed systems).Comment: Accepted by Proceedings A of the Royal Society. 17 pages, 5 figure
Spatially Resolved Images of Dust Belt(s) Around the Planet-hosting Subgiant Kappa CrB
We present Herschel spatially resolved images of the debris disc orbiting the
subgiant Kappa CrB. Not only are these the first resolved images of a debris
disc orbiting a subgiant, but Kappa CrB is a rare example of an intermediate
mass star where a detailed study of the structure of the planetary system can
be made, including both planets and planetesimal belt(s). The only way to
discover planets around such stars using the radial velocity technique is to
observe 'retired' A stars, which are cooler and slower rotators compared to
their main-sequence counterparts. A planetary companion has already been
detected orbiting the subgiant Kappa CrB, with revised parameters of m sin i =
2.1MJ and apl = 2.8AU (Johnson et al. 2008a). We present additional Keck I
HIRES radial velocity measurements that provide evidence for a second planetary
companion, alongside Keck II AO imaging that places an upper limit on the mass
of this companion. Modelling of our Herschel images shows that the dust is
broadly distributed, but cannot distinguish between a single wide belt (from 20
to 220AU) or two narrow dust belts (at around 40 and 165AU). Given the
existence of a second planetary companion beyond approximately 3AU it is
possible that the absence of dust within approximately 20AU is caused by
dynamical depletion, although the observations are not inconsistent with
depletion of these regions by collisional erosion, which occurs at higher rates
closer to the star.Comment: Updated abstrac
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