2,544 research outputs found
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Nature or nurture of coplanar tatooines: The aligned circumbinary Kuiper belt analogue around HD 131511
A key discovery of the Kepler mission is of the circumbinary planets known as ‘Tatooines’, which appear to be well aligned with their host stars’ orbits. Whether this alignment is due to initially coplanar circumbinary planet-forming discs (i.e. nature), or subsequent alignment of initially misaligned discs by warping the inner disc or torquing the binary (i.e. nurture), is not known. Tests of which scenario dominates may be possible by observing circumbinary Kuiper belt analogues (‘debris discs’), which trace the plane of the primordial disc. Here, the 140 au diameter circumbinary debris disc around HD 131511 is shown to be aligned to within 10° of the plane of the near edge-on 0.2 au binary orbit. The stellar equator is also consistent with being in this plane. If the primordial disc was massive enough to pull the binary into alignment, this outcome should be common and distinguishing nature versus nurture will be difficult. However, if only the inner disc becomes aligned with the binary, the HD 131511 system was never significantly misaligned. Given an initial misalignment, the ∼Gyr mainsequence lifetime of the star allows secular perturbations to align the debris disc out to 100 au at the cost of an increased scaleheight. The observed debris disc scaleheight limits any misalignment to less than 25°. With only a handful known, many more such systems need to be characterized to help test whether the alignment of circumbinary planets is nature or nurture.GMKis supported by the European Union through ERC grant number 279973. Thanks to Stefano Facchini for helpful discussions on primordial circumbinary disc warping and to the referee for a swift and helpful review.This article has been accepted for publication in Monthly Notices Letters of the Royal Astronomical Society ©: 2014 Grant M. Kennedy Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved. GM Kennedy, MNRAS, 2015, 447 L75-L79 doi:10.1093/mnrasl/slu19
Warm exo-Zodi from cool exo-Kuiper belts: The significance of P-R drag and the inference of intervening planets
Poynting-Robertson drag has been considered an ineffective mechanism for delivering
dust to regions interior to the cool Kuiper belt analogues seen around other Sun-like
stars. This conclusion is however based on the very large contrast in dust optical depth
between the parent belt and the interior regions that results from the dominance of collisions
over drag in systems with detectable cool belts. Here, we show that the levels
of habitable zone dust arising from detectable Kuiper belt analogues can be tens to a
few hundreds of times greater than the optical depth in the Solar Zodiacal cloud. Dust
enhancements of more than a few tens of ‘zodi’ are expected to hinder future Earthimaging
missions, but relatively few undetectable Kuiper belts result in such levels,
particularly around stars older than a few Gyr. Thus, current mid to far-IR photometric
surveys have already identified most of the 20-25% of nearby stars where P-R drag
from outer belts could seriously impact Earth-imaging. The LBTI should easily detect
such warm dust around many nearby stars with outer belts, and will provide insight
into currently unclear details of the competition between P-R drag and collisions. Given
sufficient confidence in future models, the inevitability of P-R drag means that the nondetection
of warm dust where detectable levels were expected could be used to infer
additional dust removal process, the most likely being the presence of intervening planets.We thank Rik van Lieshout and Mark Wyatt for useful discussions,
Bertrand Mennesson for sharing the KIN results ahead of
publication, the reviewer for thoughtful and constructive comments,
and the LBTI Science and Instrument teams for providing
some of the motivation for this work. GMK is supported
by the European Union through ERC grant number 279973,
and AP gratefully acknowledges support from an Undergraduate
Research Bursary from the Royal Astronomical Society.This is the final published version. It first appeared at http://mnras.oxfordjournals.org/content/449/3/2304.abstract
Five steps in the evolution from protoplanetary to debris disk
The protoplanetary disks of Herbig Ae stars eventually dissipate leaving a
tenuous debris disk comprised of planetesimals and dust, as well as possibly
gas and planets. This paper uses the properties of 10-20Myr A star debris disks
to consider the protoplanetary to debris disk transition. The physical
distinction between these two classes is argued to rest on the presence of
primordial gas in sufficient quantities to dominate the motion of small dust
grains (not the secondary nature of the dust or its level of stirring). This
motivates an observational classification based on the dust spectrum,
empirically defined so that A star debris disks require fractional excesses <3
at 12um and <2000 at 70um. We also propose a hypothesis to test, that the main
sequence planet/planetesimal structures are already in place (but obscured)
during the protoplanetary disk phase. This may be only weakly true if planetary
architectures change until frozen during disk dispersal, or completely false if
planets and planetesimals form during disk dispersal. Five steps in the
transition are discussed: (i) carving an inner hole to form a transition disk;
(ii) depletion of mm-sized dust in outer disk, noting the importance of
determining whether this mass ends up in planetesimals or is collisionally
depleted; (iii) final clearing of inner regions, noting that many mechanisms
replenish moderate hot dust levels at later phases, and likely also operate in
protoplanetary disks; (iv) disappearence of gas, noting recent discoveries of
primordial and secondary gas in debris disks that highlight our ignorance and
its impending enlightenment by ALMA; (v) formation of ring-like planetesimal
structures, noting these are shaped by interactions with planets, and that the
location of planetesimals in protoplanetary disks may be unrelated to the dust
concentrations therein that are set by gas interactions.The authors are grateful for
support from the European Union through ERC grant
number 279973.This is the author accepted manuscript. The final version is available via Springer at http://link.springer.com/article/10.1007/s10509-015-2315-6/fulltext.html
Shaping HR8799's outer dust belt with an unseen planet
HR8799 is a benchmark system for direct imaging studies. It hosts two debris
belts, which lie internally and externally to four giant planets. This paper
considers how the four known planets and a possible fifth planet, interact with
the external population of debris through N-body simulations. We find that when
only the known planets are included, the inner edge of the outer belt predicted
by our simulations is much closer to the outermost planet than recent ALMA
observations suggest. We subsequently include a fifth planet in our simulations
with a range of masses and semi-major axes, which is external to the outermost
known planet. We find that a fifth planet with a mass and semi-major axis of
0.1 and 138au predicts an outer belt that agrees well with ALMA
observations, whilst remaining stable for the lifetime of HR8799 and lying
below current direct imaging detection thresholds. We also consider whether
inward scattering of material from the outer belt can input a significant
amount of mass into the inner belt. We find that for the current age of HR8799,
only 1\% of the mass loss rate of the inner disk can be replenished by
inward scattering. However we find that the higher rate of inward scattering
during the first 10Myr of HR8799 would be expected to cause warm dust
emission at a level similar to that currently observed, which may provide an
explanation for such bright emission in other systems at Myr ages.Comment: 16 pages, 13 figures. Accepted for publication in MNRA
An Empirical Planetesimal Belt Radius-Stellar Luminosity Relation
Resolved observations of millimetre-sized dust, tracing larger planetesimals,
have pinpointed the location of 26 Edgeworth-Kuiper belt analogs. We report
that a belt's distance to its host star correlates with the star's
luminosity , following with a low
intrinsic scatter of 17%. Remarkably, our Edgeworth-Kuiper belt in the
Solar System and the two CO snow lines imaged in protoplanetary disks lie close
to this - relation, suggestive of an intrinsic relationship
between protoplanetary disk structures and belt locations. To test the effect
of bias on the relation, we use a Monte Carlo approach and simulate
uncorrelated model populations of belts. We find that observational bias could
produce the slope and intercept of the - relation, but is unable
to reproduce its low scatter. We then repeat the simulation taking into account
the collisional evolution of belts, following the steady state model that fits
the belt population as observed through infrared excesses. This significantly
improves the fit by lowering the scatter of the simulated -
relation; however, this scatter remains only marginally consistent with the one
observed. The inability of observational bias and collisional evolution alone
to reproduce the tight relationship between belt radius and stellar luminosity
could indicate that planetesimal belts form at preferential locations within
protoplanetary disks. The similar trend for CO snow line locations would then
indicate that the formation of planetesimals and/or planets in the outer
regions of planetary systems is linked to the volatility of their building
blocks, as postulated by planet formation models
ALMA observations of the narrow HR 4796A debris ring
The young A0V star HR 4796A is host to a bright and narrow ring of dust, thought to originate in collisions between planetesimals within a belt analogous to the Solar system’s Edgeworth–Kuiper belt. Here we present high spatial resolution 880 μm continuum images from the Atacama Large Millimeter Array. The 80 au radius dust ring is resolved radially with a characteristic width of 10 au, consistent with the narrow profile seen in scattered light. Our modelling consistently finds that the disc is also vertically resolved with a similar extent. However, this extent is less than the beam size, and a disc that is dynamically very cold (i.e. vertically thin) provides a better theoretical explanation for the narrow scattered light profile, so we remain cautious about this conclusion. We do not detect 12CO J=3–2 emission, concluding that unless the disc is dynamically cold the CO+CO2 ice content of the planetesimals is of order a few per cent or less. We consider the range of semi-major axes and masses of an interior planet supposed to cause the ring’s eccentricity, finding that such a planet should be more massive than Neptune and orbit beyond 40 au. Independent of our ALMA observations, we note a conflict between mid-IR pericentre-glow and scattered light imaging interpretations, concluding that models where the spatial dust density and grain size vary around the ring should be explored
Analysis of the Herschel DEBRIS Sun-like star sample
This paper presents a study of circumstellar debris around Sun-like stars using data from the Herschel DEBRIS Key Programme. DEBRIS is an unbiased survey comprising the nearest ∼90 stars of each spectral type A-M. Analysis of the 275 F-K stars shows that excess emission from a debris disc was detected around 47 stars, giving a detection rate of 17.1
+2.6−2.3
per cent, with lower rates for later spectral types. For each target a blackbody spectrum was fitted to the dust emission to determine its fractional luminosity and temperature. The derived underlying distribution of fractional luminosity versus blackbody radius in the population showed that most detected discs are concentrated at f ∼ 10−5 and at temperatures corresponding to blackbody radii 7–40 au, which scales to ∼40 au for realistic dust properties (similar to the current Kuiper belt). Two outlying populations are also evident; five stars have exceptionally bright emission ( f > 5 × 10−5), and one has unusually hot dust <4 au. The excess emission distributions at all wavelengths were fitted with a steady-state evolution model, showing that these are compatible with all stars being born with a narrow belt that then undergoes collisional grinding. However, the model cannot explain the hot dust systems – likely originating in transient events – and bright emission systems – arising potentially from atypically massive discs or recent stirring. The emission from the present-day Kuiper belt is predicted to be close to the median of the population, suggesting that half of stars have either depleted their Kuiper belts (similar to the Solar system) or had a lower planetesimal formation efficiency.This work was supported by the European Union through European Research Council grant number 279973 (MCW, GMK). GMK was also supported by the Royal Society as a Royal Society University Research Fellow
Predictions for the secondary CO, C and O gas content of debris discs from the destruction of volatile-rich planetesimals
This paper uses observations of dusty debris discs, including a growing number of gas detections in these systems, to test our understanding of the origin and evolution of this gaseous component. It is assumed that all debris discs with icy planetesimals create second generation CO, C and O gas at some level, and the aim of this paper is to predict that level and assess its observability. We present a new semi-analytical equivalent of the numerical model of Kral et al. allowing application to large numbers of systems. That model assumes CO is produced from volatile-rich solid bodies at a rate that can be predicted from the debris discs fractional luminosity. CO photodissociates rapidly into C and O that then evolve by viscous spreading. This model provides a good qualitative explanation of all current observations, with a few exceptional systems that likely have primordial gas. The radial location of the debris and stellar luminosity explain some non-detections, e.g. close-in debris (like HD 172555) is too warm to retain CO, while high stellar luminosities (like η Tel) result in short CO lifetimes. We list the most promising targets for gas detections, predicting >15 CO detections and >30 C i detections with ALMA, and tens of C ii and O i detections with future far-IR missions. We find that CO, C i, C ii and O i gas should be modelled in non-LTE for most stars, and that CO, C i and O i lines will be optically thick for the most gas-rich systems. Finally, we find that radiation pressure, which can blow out C i around early-type stars, can be suppressed by self-shielding.QK, LM and MCW acknowledge support from the European Union through ERC grant number 279973. QK and MCW acknowledge funding from STFC via the Institute of Astronomy, Cambridge Consolidated Grant. GMK is supported by the Royal Society as a Royal Society University Research Fellow. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA
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An automated search for transiting exocomets
This paper discusses an algorithm for detecting single transits in
photometric time-series data. Specifically, we aim to identify asymmetric
transits with ingress that is more rapid than egress, as expected for cometary
bodies with a significant tail. The algorithm is automated, so can be applied
to large samples and only a relatively small number of events need to be
manually vetted. We applied this algorithm to all long cadence light curves
from the Kepler mission, finding 16 candidate transits with significant
asymmetry, 11 of which were found to be artefacts or symmetric transits after
manual inspection. Of the 5 remaining events, four are the 0.1% depth events
previously identified for KIC 3542116 and 11084727. We identify HD 182952 (KIC
8027456) as a third system showing a potential comet transit. All three stars
showing these events have H-R diagram locations consistent with
100Myr-old open cluster stars, as might be expected given that cometary
source regions deplete with age, and giving credence to the comet hypothesis.
If these events are part of the same population of events as seen for KIC
8462852, the small increase in detections at 0.1% depth compared to 10% depth
suggests that future work should consider whether the distribution is naturally
flat, or if comets with symmetric transits in this depth range remain
undiscovered. Future searches relying on asymmetry should be more successful if
they focus on larger samples and young stars, rather than digging further into
the noise.GMK is supported by the Royal Society as a Royal Society University Research Fellow. STH acknowledges support from an STFC Consolidated Grant
Unlocking the secrets of the midplane gas and dust distribution in the young hybrid disc HD 141569
Context. HD 141569 is a pre-main sequence star with a disc uniquely placed between protoplanetary and debris discs, similar to the older "hybrid" type discs.
Aims: This work aims to place the mass and spatial structure of the disc midplane in the context of the debris, hybrid and protoplanetary discs.
Methods: We observed HD 141569 with ALMA in 1.3 mm continuum and 13CO (2-1). This is the first detection and image of the optically thin gas emission from the midplane of this disc.
Results: In continuum emission, we detect a combination of an unresolved central peak and a ring of millimetre emission at 220 ± 10 au, slightly interior to one of the rings discovered in scattered light. The minimum dust mass of the ring is 0.13 ± 0.02 M⊕ while the unresolved millimetre peak at the stellar location is predominantly thermal emission due to a minimum of 1.2 ± 0.2 M⊕ of dust. 13CO is distributed asymmetrically around the stellar position with a peak at 1ʺ&dotbelow;1 distance and a PA of -33°. The gas is detected as far as 220 ± 10 au, a radial separation the same as that of the mm ring. Assuming optically thin emission and standard ISM abundances, we used our 13CO data to derive the gas mass in the disc of (6.0 ± 0.9) × 10-4M⊙. Comparison to published 12CO data shows that 12CO is optically thick, explaining why estimates based on 12CO underestimated the gas mass
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