221 research outputs found
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
ALMA and Herschel Observations of the Prototype Dusty and Polluted White Dwarf G29-38
ALMA Cycle 0 and Herschel PACS observations are reported for the prototype,
nearest, and brightest example of a dusty and polluted white dwarf, G29-38.
These long wavelength programs attempted to detect an outlying, parent
population of bodies at 1-100 AU, from which originates the disrupted
planetesimal debris that is observed within 0.01 AU and which exhibits L_IR/L =
0.039. No associated emission sources were detected in any of the data down to
L_IR/L ~ 1e-4, generally ruling out cold dust masses greater than 1e24 - 1e25 g
for reasonable grain sizes and properties in orbital regions corresponding to
evolved versions of both asteroid and Kuiper belt analogs. Overall, these null
detections are consistent with models of long-term collisional evolution in
planetesimal disks, and the source regions for the disrupted parent bodies at
stars like G29-38 may only be salient in exceptional circumstances, such as a
recent instability. A larger sample of polluted white dwarfs, targeted with the
full ALMA array, has the potential to unambiguously identify the parent
source(s) of their planetary debris.Comment: 8 pages, 5 figures and 1 table. Accepted to MNRA
Evidence for post-nebula volatilisation in an exo-planetary body
The loss and gain of volatile elements during planet formation is key for
setting their subsequent climate, geodynamics, and habitability. Two broad
regimes of volatile element transport in and out of planetary building blocks
have been identified: that occurring when the nebula is still present, and that
occurring after it has dissipated. Evidence for volatile element loss in
planetary bodies after the dissipation of the solar nebula is found in the high
Mn to Na abundance ratio of Mars, the Moon, and many of the solar system's
minor bodies. This volatile loss is expected to occur when the bodies are
heated by planetary collisions and short-lived radionuclides, and enter a
global magma ocean stage early in their history. The bulk composition of
exo-planetary bodies can be determined by observing white dwarfs which have
accreted planetary material. The abundances of Na, Mn, and Mg have been
measured for the accreting material in four polluted white dwarf systems.
Whilst the Mn/Na abundances of three white dwarf systems are consistent with
the fractionations expected during nebula condensation, the high Mn/Na
abundance ratio of GD362 means that it is not (>3 sigma). We find that heating
of the planetary system orbiting GD362 during the star's giant branch evolution
is insufficient to produce such a high Mn/Na. We, therefore, propose that
volatile loss occurred in a manner analogous to that of the solar system
bodies, either due to impacts shortly after their formation or from heating by
short-lived radionuclides. We present potential evidence for a magma ocean
stage on the exo-planetary body which currently pollutes the atmosphere of
GD362
Stochastic accretion of planetesimals on to white dwarfs: Constraints on the mass distribution of accreted material from atmospheric pollution
This paper explores how the stochastic accretion of planetesimals on to white dwarfs would be manifested in observations of their atmospheric pollution. Archival observations of pollution levels for unbiased samples of DA and non-DA white dwarfs are used to derive the distribution of inferred accretion rates, confirming that rates become systematically lower as sinking time (assumed here to be dominated by gravitational settling) is decreased, with no discernable dependence on cooling age. The accretion rates expected from planetesimals that are all the same mass (i.e., a mono-mass distribution) are explored both analytically and using a Monte Carlo model, quantifying how measured accretion rates inevitably depend on sinking time, since different sinking times probe different times since the last accretion event. However, that dependence is so dramatic that a mono-mass distribution can be excluded within the context of this model. Consideration of accretion from a broad distribution of planetesimal masses uncovers an important conceptual difference: accretion is continuous (rather than stochastic) for planetesimals below a certain mass, and the accretion of such planetesimals determines the rate typically inferred from observations; smaller planetesimals dominate the rates for shorter sinking times. A reasonable fit to the observationally inferred accretion rate distributions is found with model parameters consistent with a collisionally evolved mass distribution up to Pluto-mass, and an underlying accretion rate distribution consistent with that expected from descendants of debris discs of main-sequence A stars. With these parameters, while both DA and non-DA white dwarfs accrete from the same broad planetesimal distribution, this model predicts that the pollution seen in DAs is dominated by the continuous accretion of 35 km objects (though the dominant size varies between stars by around an order of magnitude from this reference value). Furthermore, observations that characterize the dependence of inferred accretion rates on sinking time and cooling age (including a consideration of the effect of thermohaline convection on models used to derive those rates), and the decadal variability of DA accretion signatures, will improve constraints on the mass distribution of accreted material and the lifetime of the disc through which it is accreted
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How to design a planetary system for different scattering outcomes: giant impact sweet spot, maximizing exocomets, scattered discs
This paper considers the dynamics of the scattering of planetesimals or planetary embryos by a planet on a circumstellar orbit. We classify six regions in the planet's mass versus semimajor axis parameter space according to the dominant outcome for scattered objects: ejected, accreted, remaining, escaping, Oort Cloud, and depleted Oort Cloud. We use these outcomes to consider which planetary system architectures maximize the observability of specific signatures, given that signatures should be detected first around systems with optimal architectures (if such systems exist in nature). Giant impact debris is most readily detectable for 0.1–10 M⊕ planets at 1–5 au, depending on the detection method and spectral type. While A stars have putative giant impact debris at 4–6 au consistent with this sweet spot, that of FGK stars is typically ≪1 au contrary to expectations; an absence of 1–3 au giant impact debris could indicate a low frequency of terrestrial planets there. Three principles maximize the cometary influx from exo-Kuiper belts: a chain of closely separated planets interior to the belt, none of which is a Jupiter-like ejector; planet masses not increasing strongly with distance (for a net inward torque on comets); and ongoing replenishment of comets, possibly by embedded low-mass planets. A high Oort Cloud comet influx requires no ejectors and architectures that maximize the Oort Cloud population. Cold debris discs are usually considered classical Kuiper belt analogues. Here we consider the possibility of detecting scattered disc analogues, which could be betrayed by a broad radial profile and lack of small grains, as well as spherical 100–1000 au mini-Oort Clouds. Some implications for escaping planets around young stars, detached planets akin to Sedna, and the formation of super-Earths are also discussed.MCW, AB, and AS acknowledge the support of the European Union through European Research Council grant number 279973. APJ acknowledges support from NASA grant NNX16AI31G. AS is partially supported by funding from the Center for Exoplanets and Habitable Worlds. The Center for Exoplanets and Habitable Worlds is supported by the Pennsylvania State University, the Eberly College of Science, and the Pennsylvania Space Grant Consortium. This research has made use of the NASA Exoplanet Archive, which is operated by the California Institute of Technology, under contract with the National Aeronautics and Space Administration under the Exoplanet Exploration Program
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
Using warm dust to constrain unseen planets
Cold outer debris belts orbit a significant fraction of stars, many of which
are planet-hosts. Radiative forces from the star lead to dust particles leaving
the outer belts and spiralling inwards under Poynting-Robertson drag. We
present an empirical model fitted to N-body simulations that allows the fate of
these dust particles when they encounter a planet to be rapidly calculated.
High mass planets eject most particles, whilst dust passes low mass planets
relatively unperturbed. Close-in, high mass planets (hot Jupiters) are best at
accreting dust. The model predicts the accretion rate of dust onto planets
interior to debris belts, with mass accretions rates of up to hundreds of
kilograms per second predicted for hot Jupiters interior to outer debris belts,
when collisional evolution is also taken into account. The model can be used to
infer the presence and likely masses of as yet undetected planets in systems
with outer belts. The non-detection of warm dust with the Large Binocular
Telescope Interferometer (LBTI) around Vega could be explained by the presence
of a single Saturn mass planet, or a chain of lower mass planets. Similarly,
the detection of warm dust in such systems implies the absence of planets above
a quantifiable level, which can be lower than similar limits from direct
imaging. The level of dust detected with LBTI around beta Leo can be used to
rule out the presence of planets more massive than a few Saturn masses outside
of ~5au
Asteroid Belts in Debris Disk Twins: VEGA and FOMALHAUT
Vega and Fomalhaut, are similar in terms of mass, ages, and global debris
disk properties; therefore, they are often referred as "debris disk twins". We
present Spitzer 10-35 um spectroscopic data centered at both stars, and
identify warm, unresolved excess emission in the close vicinity of Vega for the
first time. The properties of the warm excess in Vega are further characterized
with ancillary photometry in the mid infrared and resolved images in the
far-infrared and submillimeter wavelengths. The Vega warm excess shares many
similar properties with the one found around Fomalhaut. The emission shortward
of ~30 um from both warm components is well described as a blackbody emission
of ~170 K. Interestingly, two other systems, eps Eri and HR 8799, also show
such an unresolved warm dust using the same approach. These warm components may
be analogous to the solar system's zodiacal dust cloud, but of far greater. The
dust temperature and tentative detections in the submillimeter suggest the warm
excess arises from dust associated with a planetesimal ring located near the
water-frost line and presumably created by processes occurring at similar
locations in other debris systems as well. We also review the properties of the
2 um hot excess around Vega and Fomalhaut, showing that the dust responsible
for the hot excess is not spatially associated with the dust we detected in the
warm belt. We suggest it may arise from hot nano grains trapped in the magnetic
field of the star. Finally, the separation between the warm and cold belt is
rather large with an orbital ratio >~10 in all four systems. In light of the
current upper limits on the masses of planetary objects and the large gap, we
discuss the possible implications for their underlying planetary architecture,
and suggest that multiple, low-mass planets likely reside between the two belts
in Vega and Fomalhaut.Comment: 14 pages, accepted for publication in Ap
Are exoplanetesimals differentiated?
Metals observed in the atmospheres of white dwarfs suggest that many have
recently accreted planetary bodies. In some cases, the compositions observed
suggest the accretion of material dominantly from the core (or the mantle) of a
differentiated planetary body. Collisions between differentiated
exoplanetesimals produce such fragments. In this work, we take advantage of the
large numbers of white dwarfs where at least one siderophile (core-loving) and
one lithophile (rock-loving) species have been detected to assess how commonly
exoplanetesimals differentiate. We utilise N-body simulations that track the
fate of core and mantle material during the collisional evolution of planetary
systems to show that most remnants of differentiated planetesimals retain core
fractions similar to their parents, whilst some are extremely core-rich or
mantle-rich. Comparison with the white dwarf data for calcium and iron
indicates that the data are consistent with a model in which
have accreted the remnants of differentiated planetesimals, whilst
have Ca/Fe abundances altered by the effects of heating
(although the former can be as high as , if heating is ignored). These
conclusions assume pollution by a single body and that collisional evolution
retains similar features across diverse planetary systems. These results imply
that both collisions and differentiation are key processes in exoplanetary
systems. We highlight the need for a larger sample of polluted white dwarfs
with precisely determined metal abundances to better understand the process of
differentiation in exoplanetary systems
The ASK Network: developing a virtuous cycle of subsurface data and knowledge exchange
Knowledge of the subsurface is essential in delivering successful construction and regeneration projects. Inadequate understanding of subsurface ground conditions can constrain effective development of urban areas and is a key factor in project delay and overspending. Improving this situation demands much better use, and re-use, of subsurface data and knowledge. The establishment of ASK subsurface data and knowledge exchange network has led to substantial improvements in how urban subsurface data is reported and exchanged between the public and private sectors. Implementation of the GSPEC standardised digital data reporting format has improved the integrity and accessibility of data. ASK and GSPEC are enabling the expansion and exchange of high quality systematic subsurface datasets, improving development of robust 3D ground models which can be used to promote more cost effective and better informed ground engineering investigations, and monitoring and regulation of resources in the urban environment. The work underway in Glasgow is acting as a standard for change, both within the UK and Europe
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