arXiv subjects

Mark C. Wyatt

Publications and source records attributed to Mark C. Wyatt.

At least 109 records · Page 6Linked to original sources

arXiv2013

The Great Escape III: Placing post-main-sequence evolution of planetary and binary systems in a Galactic context

Our improving understanding of the life cycle of planetary systems prompts investigations of the role of the Galactic environment before, during and after Asymptotic Giant Branch (AGB) stellar evolution. Here, we investigate the interplay between stellar mass loss, Galactic tidal perturbations, and stellar flybys for evolving stars which host one planet, smaller body or stellar binary companion and reside in the Milky Way's bulge or disc. We find that the potential evolutionary pathways from a main sequence (MS) to a white dwarf (WD) planetary system are a strong function of Galactocentric distance only with respect to the prevalence of stellar flybys. Planetary ejection and collision with the parent star should be more common towards the bulge. At a given location anywhere in the Galaxy, if the mass loss is adiabatic, then the secondary is likely to avoid close flybys during AGB evolution, and cannot eventually escape the resulting WD because of Galactic tides alone. Partly because AGB mass loss will shrink a planetary system's Hill ellipsoid axes by about 20 to 40 per cent, Oort clouds orbiting WDs are likely to be more depleted and dynamically excited than on the MS.

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arXiv2013

The bright end of the exo-Zodi luminosity function: Disk evolution and implications for exo-Earth detectability

We present the first characterisation of the 12um warm dust ("exo-Zodi") luminosity function around Sun-like stars, focussing on the dustiest systems that can be identified by WISE. We detect six new warm dust candidates, five of which have unknown ages. We show that the dustiest old (>Gyr) systems like BD+20 307 are 1 in 10,000 occurrences. Bright warm dust is more common around young (<120Myr) systems, with a ~1% occurrence rate. We show that a two component in situ model where all stars have initially massive warm disks and in which warm debris is also generated at some random time along the stars' main-sequence lifetime, perhaps due to a collision, can explain the observations. However, if all stars only have initially massive warm disks these would not be visible at Gyr ages, and random collisions on the main-sequence are too infrequent to explain the high disk occurrence rate for young stars. That is, neither component can explain the observations on their own. Despite these conclusions, we cannot rule out an alternative model in which comets are scattered from outer regions because the distribution of systems with the appropriate dynamics is unknown. Our in situ model predicts the fraction of stars with exo-Zodi bright enough to cause problems for future exo-Earth imaging attempts is at least ~10%, and is higher for populations of stars younger than a few Gyr. This prediction of ~10% applies to old stars because bright systems like BD+20 307 imply a population of fainter systems that were once bright but are now decaying through fainter levels. Our prediction should be strongly tested by the Large Binocular Telescope Interferometer, providing valuable input for more detailed evolution models. A detection fraction lower than our prediction could indicate that the hot dust in systems like BD+20 307 has a cometary origin due to the quirks of the planetary dynamics.

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arXiv2013

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.

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arXiv2013

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.

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arXiv2012

The debris disk around gamma Doradus resolved with Herschel

We present observations of the debris disk around gamma Doradus, an F1V star, from the Herschel Key Programme DEBRIS (Disc Emission via Bias-free Reconnaissance in the Infrared/Submillimetre). The disk is well-resolved at 70, 100 and 160 micron, resolved along its major axis at 250 micron, detected but not resolved at 350 micron, and confused with a background source at 500 micron. It is one of our best resolved targets and we find it to have a radially broad dust distribution. The modelling of the resolved images cannot distinguish between two configurations: an arrangement of a warm inner ring at several AU (best-fit 4 AU) and a cool outer belt extending from ~55 to 400 AU or an arrangement of two cool, narrow rings at ~70 AU and ~190 AU. This suggests that any configuration between these two is also possible. Both models have a total fractional luminosity of ~10^{-5} and are consistent with the disk being aligned with the stellar equator. The inner edge of either possible configuration suggests that the most likely region to find planets in this system would be within ~55 AU of the star. A transient event is not needed to explain the warm dust's fractional luminosity.

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arXiv2012

Resolved Debris Discs Around A Stars in the Herschel DEBRIS Survey

The majority of debris discs discovered so far have only been detected through infrared excess emission above stellar photospheres. While disc properties can be inferred from unresolved photometry alone under various assumptions for the physical properties of dust grains, there is a degeneracy between disc radius and dust temperature that depends on the grain size distribution and optical properties. By resolving the disc we can measure the actual location of the dust. The launch of Herschel, with an angular resolution superior to previous far-infrared telescopes, allows us to spatially resolve more discs and locate the dust directly. Here we present the nine resolved discs around A stars between 20 and 40 pc observed by the DEBRIS survey. We use these data to investigate the disc radii by fitting narrow ring models to images at 70, 100 and 160 μm and by fitting blackbodies to full spectral energy distributions. We do this with the aim of finding an improved way of estimating disc radii for unresolved systems. The ratio between the resolved and blackbody radii varies between 1 and 2.5. This ratio is inversely correlated with luminosity and any remaining discrepancies are most likely explained by differences to the minimum size of grain in the size distribution or differences in composition. We find that three of the systems are well fit by a narrow ring, two systems are borderline cases and the other four likely require wider or multiple rings to fully explain the observations, reflecting the diversity of planetary systems.

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arXiv2012

Debris from terrestrial planet formation: the Moon-forming collision

We study the evolution of debris created in the giant impacts expected during the final stages of terrestrial planet formation. The starting point is the debris created in a simulation of the Moon-forming impact. The dynamical evolution is followed for 10 Myr including the effects of Earth, Venus, Mars and Jupiter. The spatial distribution evolves from a clump in the first few months to an asymmetric ring for the first 10 kyr and finally becoming an axisymmetric ring by about 1 Myr after the impact. By 10 Myr after the impact 20% of the particles have been accreted onto Earth and 17% onto Venus, with 8% ejected by Jupiter and other bodies playing minor roles. However, the fate of the debris also depends strongly on how fast it is collisionally depleted, which depends on the poorly constrained size distribution of the impact debris. Assuming that the debris is made up of 30% by mass mm-cm-sized vapour condensates and 70% boulders up to 500 km, we find that the condensates deplete rapidly on ~1000 yr timescales, whereas the boulders deplete predominantly dynamically. By considering the luminosity of dust produced in collisions within the boulder-debris distribution we find that the Moon-forming impact would have been readily detectable around other stars in Spitzer 24 micron surveys for around 25 Myr after the impact, with levels of emission comparable to many known hot dust systems. The vapour condensates meanwhile produce a short-lived, optically thick, spike of emission. We use these surveys to make an estimate of the fraction of stars that form terrestrial planets, F_TPF. Since current terrestrial planet formation models invoke multiple giant impacts, the low fraction of 10-100 Myr stars found to have warm (~150 K) dust implies that F_TPF ~<10%.

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arXiv2012

Debris disks as signposts of terrestrial planet formation. II Dependence of exoplanet architectures on giant planet and disk properties

We present models for the formation of terrestrial planets, and the collisional evolution of debris disks, in planetary systems that contain multiple unstable gas giants. We previously showed that the dynamics of the giant planets introduces a correlation between the presence of terrestrial planets and debris disks. Here we present new simulations that show that this connection is qualitatively robust to changes in: the mass distribution of the giant planets, the width and mass distribution of the outer planetesimal disk, and the presence of gas in the disk. We discuss how variations in these parameters affect the evolution. Systems with equal-mass giant planets undergo the most violent instabilities, and these destroy both terrestrial planets and the outer planetesimal disks that produce debris disks. In contrast, systems with low-mass giant planets efficiently produce both terrestrial planets and debris disks. A large fraction of systems with low-mass outermost giant planets have stable gaps between these planets that are frequently populated by planetesimals. Planetesimal belts between outer giant planets may affect debris disk SEDs. If Earth-mass seeds are present in outer planetesimal disks, the disks radially spread to colder temperatures. We argue that this may explain the very low frequency of > 1 Gyr-old solar-type stars with observed 24 micron excesses. Among the (limited) set of configurations explored, the best candidates for hosting terrestrial planets at ~1 AU are stars older than 0.1-1 Gyr with bright debris disks at 70 micron but with no currently-known giant planets. These systems combine evidence for rocky building blocks, with giant planet properties least likely to undergo destructive dynamical evolution. We predict an anti-correlation between debris disks and eccentric giant planets, and a positive correlation between debris disks and terrestrial planets.

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arXiv2012

The Solar System's Post-Main Sequence Escape Boundary

The Sun will eventually lose about half of its current mass nonlinearly over several phases of post-main sequence evolution. This mass loss will cause any surviving orbiting body to increase its semimajor axis and perhaps vary its eccentricity. Here, we use a range of Solar models spanning plausible evolutionary sequences and assume isotropic mass loss to assess the possibility of escape from the Solar System. We find that the critical semimajor axis in the Solar System within which an orbiting body is guaranteed to remain bound to the dying Sun due to perturbations from stellar mass loss alone is approximately 1,000 AU - 10,000 AU. The fate of objects near or beyond this critical semimajor axis, such as the Oort Cloud, outer scattered disc and specific bodies such as Sedna, will significantly depend on their locations along their orbits when the Sun turns off of the main sequence. These results are applicable to any exoplanetary system containing a single star with a mass, metallicity and age which are approximately equal to the Sun's, and suggest that few extrasolar Oort Clouds could survive post-main sequence evolution intact.

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arXiv2011

Dependence of a planet's chaotic zone on particle eccentricity: the shape of debris disc inner edges

The orbit of a planet is surrounded by a chaotic zone wherein nearby particles' orbits are chaotic and unstable. Wisdom (1980) showed that the chaos is driven by the overlap of mean motion resonances which occurs within a distance (da/a)_chaos = 1.3 mu^2/7 of the planet's orbit. However, the width of mean motion resonances grows with the particles' eccentricity, which will increase the width of the chaotic zone at higher eccentricities. Here we investigate the width of the chaotic zone using the iterated encounter map and N-body integrations. We find that the classical prescription works well for particles on low-eccentricity orbits. However, above a critical eccentricity, dependent upon the mass of the planet, the width of the chaotic zone increases with eccentricity. An extension of Wisdom's analytical arguments then shows that, above the critical eccentricity, the chaotic zone width is given by (da/a)_chaos = 1.8 e^1/5 mu^1/5, which agrees well with the encounter map results. The critical eccentricity is given by e_cr = 0.21 mu^3/7. This extended chaotic zone results in a larger cleared region when a planet sculpts the inner edge of a debris disc comprised of eccentric planetesimals. Hence, the planet mass estimated from the classical chaotic zone may be erroneous. We apply this result to the HR8799 system, showing that the masses of HR8799b inferred from the truncation of the disc may vary by up to 50% depending on the disc particles' eccentricities. With a disc edge at 90AU, the necessary mass of planet b to cause the truncation would be 8-10 Jovian masses if the disc particles have low eccentricities (<0.02), but only 4-8 Jovian masses if the disc particles have higher eccentricities. Our result also has implications for the ability of a planet to feed material into an inner system, a process which may explain metal pollution in White Dwarf atmospheres.

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arXiv2011

Searching for Saturn's Dust Swarm: Limits on the size distribution of Irregular Satellites from km to micron sizes

We describe a search for dust created in collisions between the Saturnian irregular satellites using archival \emph{Spitzer} MIPS observations. Although we detected a degree scale Saturn-centric excess that might be attributed to an irregular satellite dust cloud, we attribute it to the far-field wings of the PSF due to nearby Saturn. The Spitzer PSF is poorly characterised at such radial distances, and we expect PSF characterisation to be the main issue for future observations that aim to detect such dust. The observations place an upper limit on the level of dust in the outer reaches of the Saturnian system, and constrain how the size distribution extrapolates from the smallest known (few km) size irregulars down to micron-size dust. Because the size distribution is indicative of the strength properties of irregulars, we show how our derived upper limit implies irregular satellite strengths more akin to comets than asteroids. This conclusion is consistent with their presumed capture from the outer regions of the Solar System.

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arXiv2011

The Great Escape: How Exoplanets and Smaller Bodies Desert Dying Stars

Mounting discoveries of extrasolar planets orbiting post-main sequence stars motivate studies aimed at understanding the fate of these planets. In the traditional "adiabatic" approximation, a secondary's eccentricity remains constant during stellar mass loss. Here, we remove this approximation, investigate the full two-body point-mass problem with isotropic mass loss, and illustrate the resulting dynamical evolution. The magnitude and duration of a star's mass loss combined with a secondary's initial orbital characteristics might provoke ejection, modest eccentricity pumping, or even circularisation of the orbit. We conclude that Oort clouds and wide-separation planets may be dynamically ejected from 1-7 Solar-mass parent stars during AGB evolution. The vast majority of planetary material which survives a supernova from a 7-20 Solar-mass progenitor will be dynamically ejected from the system, placing limits on the existence of first-generation pulsar planets. Planets around >20 Solar-mass black hole progenitors may easily survive or readily be ejected depending on the core collapse and superwind models applied. Material ejected during stellar evolution might contribute significantly to the free-floating planetary population.

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arXiv2011

Debris disks as signposts of terrestrial planet formation

Circumstantial evidence suggests that most known extra-solar planetary systems are survivors of violent dynamical instabilities. Here we explore how giant planet instabilities affect the formation and survival of terrestrial planets. We simulate planetary system evolution around Sun-like stars from initial conditions that comprise: an inner disk of planetesimals and planetary embryos, three giant planets at Jupiter-Saturn distances, and a massive outer planetesimal disk. We then calculate dust production rates and debris disk SEDs assuming that each planetesimal particle represents an ensemble of smaller bodies in collisional equilibrium. We predict a strong correlation between the presence of terrestrial planets and debris disks, mediated by the giant planets. Strong giant planet instabilities destroy all rocky material - including fully-formed terrestrial planets if the instabilities occur late - along with the icy planetesimals. Stable or weakly unstable systems allow terrestrial planets to accrete and significant dust to be produced in their outer regions. Stars older than ~100 Myr with bright cold dust emission (at ~70 microns) signpost the dynamically calm environments conducive to efficient terrestrial accretion. We predict that while the typical eccentricities of terrestrial planets are small, there should exist a novel class of terrestrial planet system whose single planet undergoes large amplitude oscillations in eccentricity and inclination. By scaling to the observed semimajor axis distribution of giant exoplanets, we estimate that terrestrial exoplanets in the same systems should be a few times more abundant at 0.5 AU than giant or terrestrial exoplanets at 1 AU. Finally, we discuss the Solar System, which appears to be unusual in combining a rich terrestrial planet system with a low dust content.

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arXiv2011

Debris disk size distributions: steady state collisional evolution with P-R drag and other loss processes

We present a new scheme for determining the shape of the size distribution, and its evolution, for collisional cascades of planetesimals undergoing destructive collisions and loss processes like Poynting-Robertson drag. The scheme treats the steady state portion of the cascade by equating mass loss and gain in each size bin; the smallest particles are expected to reach steady state on their collision timescale, while larger particles retain their primordial distribution. For collision-dominated disks, steady state means that mass loss rates in logarithmic size bins are independent of size. This prescription reproduces the expected two phase size distribution, with ripples above the blow-out size, and above the transition to gravity-dominated planetesimal strength. The scheme also reproduces the expected evolution of disk mass, and of dust mass, but is computationally much faster than evolving distributions forward in time. For low-mass disks, P-R drag causes a turnover at small sizes to a size distribution that is set by the redistribution function (the mass distribution of fragments produced in collisions). Thus information about the redistribution function may be recovered by measuring the size distribution of particles undergoing loss by P-R drag, such as that traced by particles accreted onto Earth. Although cross-sectional area drops with 1/age^2 in the PR-dominated regime, dust mass falls as 1/age^2.8, underlining the importance of understanding which particle sizes contribute to an observation when considering how disk detectability evolves. Other loss processes are readily incorporated; we also discuss generalised power law loss rates, dynamical depletion, realistic radiation forces and stellar wind drag.

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arXiv2010

A general model of resonance capture in planetary systems: First and second order resonances

Mean motion resonances are a common feature of both our own Solar System and of extrasolar planetary systems. Bodies can be trapped in resonance when their orbital semi-major axes change, for instance when they migrate through a protoplanetary disc. We use a Hamiltonian model to thoroughly investigate the capture behaviour for first and second order resonances. Using this method, all resonances of the same order can be described by one equation, with applications to specific resonances by appropriate scaling. We focus on the limit where one body is a massless test particle and the other a massive planet. We quantify how the the probability of capture into a resonance depends on the relative migration rate of the planet and particle, and the particle's eccentricity. Resonant capture fails for high migration rates, and has decreasing probability for higher eccentricities. More massive planets can capture particles at higher eccentricities and migration rates. We also calculate libration amplitudes and the offset of the libration centres for captured particles, and the change in eccentricity if capture does not occur. Libration amplitudes are higher for larger initial eccentricity. The model allows for a complete description of a particle's behaviour as it successively encounters several resonances. We discuss implications for several scenarios: (i) Planet migration through gas discs trapping other planets or planetesimals in resonances. (ii) Planet migration through a debris disc. (iii) Dust migration through PR drag. The Hamiltonian model will allow quick interpretation of the resonant properties of extrasolar planets and Kuiper Belt Objects, and will allow synthetic images of debris disc structures to be quickly generated, which will be useful for predicting and interpreting disc images made with ALMA, Darwin/TPF or similar missions. [Abridged]

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arXiv2010

Collisional Evolution of Irregular Satellite Swarms: Detectable Dust around Solar System and Extrasolar Planets

Since the 1980's it has been becoming increasingly clear that the Solar System's irregular satellites are collisionally evolved. We derive a general model for the collisional evolution of an irregular satellite swarm and apply it to the Solar System and extrasolar planets. Our model reproduces the Solar System's complement of observed irregulars well, and suggests that the competition between grain-grain collisions and Poynting-Robertson (PR) drag helps set the fate of the dust. Because swarm collision rates decrease over time the main dust sink can change with time, and may help unravel the accretion history of synchronously rotating regular satellites that show brightness asymmetries. Some level of dust must be present on AU scales around the Solar System's giant planets, which we predict may be at detectable levels. We also predict whether dust produced by extrasolar circumplanetary swarms can be detected. The coronagraphic instruments on JWST will have the ability to detect the dust generated by these swarms, which are most detectable around planets that orbit at tens of AU from the youngest stars. Because the collisional decay of swarms is relatively insensitive to planet mass, swarms can be much brighter than their host planets and allow discovery of Neptune-mass planets that would otherwise remain invisible. This dust may have already been detected. The observations of the planet Fomalhaut b can be explained as scattered light from dust produced by the collisional decay of an irregular satellite swarm around a 10 Earth-mass planet. Such a swarm comprises about 5 Lunar masses worth of irregular satellites. Finally, we consider what happens if Fomalhaut b passes through Fomalhaut's main debris ring, which allows the circumplanetary swarm to be replenished through collisions with ring planetesimals. (abridged)

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arXiv2010

Resolved Imaging of the HD191089 Debris Disc

Two thirds of the F star members of the 12 Myr old Beta Pictoris Moving Group (BPMG) show significant excess emission in the mid-infrared, several million years after the expected dispersal of the protoplanetary disc. Theoretical models of planet formation suggest that this peak in the mid-infrared emission could be due to the formation of Pluto-sized bodies in the disc, which ignite the collisional cascade and enhance the production of small dust. Here we present resolved mid-infrared imaging of the disc of HD191089 (F5V in the BPMG) and consider its implications for the state of planet formation in this system. HD191089 was observed at 18.3 microns using T-ReCS on Gemini South and the images were compared to models of the disc to constrain the radial distribution of the dust. The emission observed at 18.3 microns is shown to be significantly extended beyond the PSF at a position angle of 80 degrees. This is the first time dust emission has been resolved around HD191089. Modelling indicates that the emission arises from a dust belt from 28-90 AU, inclined at 35 degrees from edge on with very little emission from the inner 28AU of the disc, indicating the presence of an inner cavity. The steep slope of the inner edge is more consistent with truncation by a planet than with ongoing stirring. A tentative brightness asymmetry F(W)/F(E)=0.80+/-0.12 (1.8 sigma) between the two sides of the disc could be evidence for perturbations from a massive body on an eccentric orbit in the system.

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arXiv2009

How Common are Extrasolar, Late Heavy Bombardments?

The habitability of planets is strongly affected by impacts from comets and asteroids. Indications from the ages of Moon rocks suggest that the inner Solar System experienced an increased rate of impacts roughly 3.8 Gya known as the Late Heavy Bombardment (LHB). Here we develop a model of how the Solar System would have appeared to a distant observer during its history based on the Nice model of Gomes et al. (2005). We compare our results with observed debris discs. We show that the Solar System would have been amongst the brightest of these systems before the LHB. Comparison with the statistics of debris disc evolution shows that such heavy bombardment events must be rare occurring around less than 12% of Sun-like stars.

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