423 research outputs found
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
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
Debris Disks: Probing Planet Formation
Debris disks are the dust disks found around ~20% of nearby main sequence
stars in far-IR surveys. They can be considered as descendants of
protoplanetary disks or components of planetary systems, providing valuable
information on circumstellar disk evolution and the outcome of planet
formation. The debris disk population can be explained by the steady
collisional erosion of planetesimal belts; population models constrain where
(10-100au) and in what quantity (>1Mearth) planetesimals (>10km in size)
typically form in protoplanetary disks. Gas is now seen long into the debris
disk phase. Some of this is secondary implying planetesimals have a Solar
System comet-like composition, but some systems may retain primordial gas.
Ongoing planet formation processes are invoked for some debris disks, such as
the continued growth of dwarf planets in an unstirred disk, or the growth of
terrestrial planets through giant impacts. Planets imprint structure on debris
disks in many ways; images of gaps, clumps, warps, eccentricities and other
disk asymmetries, are readily explained by planets at >>5au. Hot dust in the
region planets are commonly found (<5au) is seen for a growing number of stars.
This dust usually originates in an outer belt (e.g., from exocomets), although
an asteroid belt or recent collision is sometimes inferred.Comment: Invited review, accepted for publication in the 'Handbook of
Exoplanets', eds. H.J. Deeg and J.A. Belmonte, Springer (2018
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
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
Kuiper belt analogues in nearby M-type planet-host systems
We present the results of a Herschel survey of 21 late-type stars that host planets discovered by the radial velocity technique. The aims were to discover new discs in these systems and to search for any correlation between planet presence and disc properties. In addition to the known disc around GJ 581, we report the discovery of two new discs, in the GJ 433 and GJ 649 systems. Our sample therefore yields a disc detection rate of 14 per cent, higher than the detection rate of 1.2 per cent among our control sample of DEBRIS M-type stars with 98 per cent confidence. Further analysis however shows that the disc sensitivity in the control sample is about a factor of two lower in fractional luminosity than for our survey, lowering the significance of any correlation between planet presence and disc brightness below 98 per cent. In terms of their specific architectures, the disc around GJ 433 lies at a radius somewhere between 1 and 30 au. The disc around GJ 649 lies somewhere between 6 and 30 au, but is marginally resolved and appears more consistent with an edge-on inclination. In both cases the discs probably lie well beyond where the known planets reside (0.06–1.1 au), but the lack of radial velocity sensitivity at larger separations allows for unseen Saturn–mass planets to orbit out to ~5 au, and more massive planets beyond 5 au. The layout of these M-type systems appears similar to Sun-like star + disc systems with low-mass planets.This work was supported by the European Union through ERC grant number 279973 (GMK & MCW)
Kuiper belt structure around nearby super-Earth host stars
We present new observations of the Kuiper belt analogues around HD 38858 and HD 20794,
hosts of super-Earth mass planets within 1 au. As two of the four nearby G-type stars (with
HD 69830 and 61 Vir) that form the basis of a possible correlation between low-mass planets
and debris disc brightness, these systems are of particular interest. The disc around HD 38858
is well resolved with Herschel and we constrain the disc geometry and radial structure. We
also present a probable James Clerk Maxwell Telescope sub-mm continuum detection of the
disc and a CO J = 2–1 upper limit. The disc around HD 20794 is much fainter and appears
marginally resolved with Herschel, and is constrained to be less extended than the discs around
61 Vir and HD 38858. We also set limits on the radial location of hot dust recently detected
around HD 20794 with near-IR interferometry. We present High Accuracy Radial velocity
Planet Searcher upper limits on unseen planets in these four systems, ruling out additional
super-Earths within a fewau, and Saturn-mass planets within 10 au. We consider the disc
structure in the three systems with Kuiper belt analogues (HD 69830 has only a warm dust
detection), concluding that 61 Vir and HD 38858 have greater radial disc extent than HD
20794. We speculate that the greater width is related to the greater minimum planet masses
(10–20 M⊕ versus 3–5 M⊕), arising from an eccentric planetesimal population analogous
to the Solar system’s scattered disc. We discuss alternative scenarios and possible means to
distinguish among them.We thank the referee for a thoughtful review. This work was supported
by the European Union through ERC grant number 279973
(GMK, LM, and MCW). LM also acknowledges support by both
STFC and ESO through graduate studentships. MM, CL, FP, and
SU acknowledge the Swiss National Science Foundation (SNSF)
for the continuous support of the RV research programmes.This is the final published version. It first appeared at http://mnras.oxfordjournals.org/content/449/3/3121.abstract
ALMA Survey of Lupus Class III Stars: Early Planetesimal Belt Formation and Rapid Disk Dispersal
Class III stars are those in star forming regions without large non-photospheric infrared emission, suggesting recent dispersal of their protoplanetary disks. We observed 30 class III stars in the 1-3 Myr Lupus region with ALMA at ∼856μm, resulting in 4 detections that we attribute to circumstellar dust. Inferred dust masses are 0.036 − 0.093M⊕, ∼1 order of magnitude lower than any previous measurements; one disk is resolved with radius ∼80 au. Two class II sources in the field of view were also detected, and 11 other sources, consistent with sub-mm galaxy number counts. Stacking non-detections yields a marginal detection with mean dust mass ∼0.0048M⊕. We searched for gas emission from the CO J=3-2 line, and present its detection to NO Lup inferring a gas mass (4.9 ± 1.1) × 10−5 M⊕ and gas-to-dust ratio 1.0 ± 0.4. Combining our survey with class II sources shows a gap in the disk mass distribution from 0.09 − 2M⊕ for >0.7M⊙ Lupus stars, evidence of rapid dispersal of mm-sized dust from protoplanetary disks. The class III disk mass distribution is consistent with a population model of planetesimal belts that go on to replenish the debris disks seen around main sequence stars. This suggests that planetesimal belt formation does not require long-lived protoplanetary disks, i.e., planetesimals form within ∼2 Myr. While all 4 class III disks are consistent with collisional replenishment, for two the gas and/or mid-IR emission could indicate primordial circumstellar material in the final stages of protoplanetary disk dispersal. Two class III stars without sub-mm detections exhibit hot emission that could arise from ongoing planet formation processes inside ∼1 au
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