arXiv subjects

M. C. Wyatt

Publications and source records attributed to M. C. Wyatt.

106 records · Page 6Linked to original sources

arXiv2007

Steady-state evolution of debris disks around A stars

In this paper a simple analytical model for the steady-state evolution of debris disks due to collisions is confronted with Spitzer observations of main sequence A stars. All stars are assumed to have planetesimal belts with a distribution of initial masses and radii. In the model disk mass is constant until the largest planetesimals reach collisional equilibrium whereupon the mass falls off oc 1/t. We find that the detection statistics and trends seen at both 24 and 70um can be fitted well by the model. While there is no need to invoke stochastic evolution or delayed stirring to explain the statistics, a moderate rate of stochastic events is not ruled out. Potentially anomalous systems are identified by a high dust luminosity compared with the maximum permissible in the model (HD3003, HD38678, HD115892, HD172555). Their planetesimals may have unusual properties (high strength or low eccentricity) or this dust could be transient. While transient phenomena are also favored for a few systems in the literature, the overall success of our model, which assumes planetesimals in all belts have the same strength, eccentricity and maximum size, suggests a large degree of uniformity in the outcome of planet formation. The distribution of planetesimal belt radii, once corrected for detection bias, follows N(r) oc r^{-0.8+-0.3} for 3-120AU. Since the inner edge is often attributed to an unseen planet, this provides a unique constraint on the planetary systems of A stars. It is also shown that P-R drag may sculpt the inner edges of A star disks close to the Spitzer detection threshold (HD2262, HD19356, HD106591, HD115892). This model can be readily applied to the interpretation of future surveys, and predictions are made for the upcoming SCUBA-2 survey, including that >17% of A stars should be detectable at 850um.

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arXiv2006

On the Nature of the Dust in the Debris Disk Around HD69830

We have used the infrared mineralogical model derived from the Spitzer IRS observations of the Deep Impact experiment to study the nature of the dust in the debris found around the K0V star HD69830 (Beichman et al. 2005). Using a robust approach to determine the bulk average mineralogical composition of the dust, we show it to be substantially different from that found for comet 9P/Tempel 1 (Lisse et al. 2006) or C/Hale-Bopp 1995 O1 and comet-dominated YSO HD100546 (Lisse et al. 2007). Lacking in carbonaceous and ferrous materials but including small icy grains, the composition of the HD 69830 dust resembles that of a disrupted P or D-type asteroid. The amount of mass responsible for the observed emission is the equivalent of a 30 km radius, 2500 kg m-3 sphere, while the radiative temperature of the dust implies that the bulk of the observed material is at ~1.0 AU from the central source, coincident with the 2:1 and 5:2 mean motion resonances of the outermost of 3 Neptune-sized planets detected by Lovis et al. (2006). In our solar system, P and D-type asteroids are both large and numerous in the outer main belt and near Jupiter (e.g. the Hildas and Trojans) and have undergone major disruptive events to produce debris disk-like structures (c.f. the Karin and Veritas families 5-8 Myrs ago). The short-lived nature of the small and icy dust implies that the disruption occurred within the last year, or that replenishment due to ongoing collisional fragmentation is occurring.

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arXiv2006

Transience of hot dust around sun-like stars

There is currently debate over whether the dust content of planetary systems is stochastically regenerated or originates in planetesimal belts evolving in steady state. In this paper a simple model for the steady state evolution of debris disks due to collisions is developed and confronted with the properties of the emerging population of 7 sun-like stars that have hot dust <10AU. The model shows there is a maximum possible disk mass at a given age, since more massive primordial disks process their mass faster. The corresponding maximum dust luminosity is f_max=0.00016r^(7/3)/t_age. The majority (4/7) of the hot disks exceed this limit by >1000 and so cannot be the products of massive asteroid belts, rather the following systems must be undergoing transient events characterized by an unusually high dust content near the star: eta Corvi, HD69830, HD72905 and BD+20307. It is also shown that the hot dust cannot originate in a recent collision in an asteroid belt, since there is also a maximum rate at which collisions of sufficient magnitude to reproduce a given dust luminosity can occur. Further it is shown that the planetesimal belt feeding the dust in these systems must be located further from the star than the dust, typically at >2AU. Other notable properties of the 4 hot dust systems are: two also have a planetesimal belt at >10AU (eta Corvi and HD72905); one has 3 Neptune mass planets at <1AU (HD69830); all exhibit strong silicate features in the mid-IR. We consider the most likely origin for the dust in these systems to be a dynamical instability which scattered planetesimals inwards from a more distant planetesimal belt in an event akin to the Late Heavy Bombardment in our own system, the dust being released from such planetesimals in collisions and possibly also sublimation.

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arXiv2006

Search for Cold Debris Disks around M-dwarfs

Debris disks are believed to be related to planetesimals left over around stars after planet formation has ceased. The frequency of debris disks around M-dwarfs which account for 70% of the stars in the Galaxy is unknown while constrains have already been found for A- to K-type stars. We have searched for cold debris disks around 32 field M-dwarfs by conducting observations at lambda = 850 microns with the SCUBA bolometerarray camera at the JCMT and at lambda = 1.2mm with the MAMBO array at the IRAM 30-m telescopes. This is the first survey of a large sample of M-dwarfs conducted to provide statistical constraints on debris disks around this type of stars. We have detected a new debris disk around the M0.5 dwarf GJ842.2 at lambda = 850 microns, providing evidence for cold dust at large distance from this star (~ 300AU). By combining the results of our survey with the ones of Liu et al. (2004), we estimate for the first time the detection rate of cold debris disks around field M-dwarfs with ages between 20 and 200 Myr. This detection rate is 13^{+6}_{-8} % and is consistent with the detection rate of cold debris disks (9 - 23 %) around A- to K-type main sequence stars of the same age. This is an indication that cold disks may be equally prevalent across stellar spectral types.

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arXiv2006

Evolution of Circumstellar Disks Around Normal Stars: Placing Our Solar System in Context

Over the past 10 years abundant evidence has emerged that many (if not all) stars are born with circumstellar disks. Understanding the evolution of post-accretion disks can provide strong constraints on theories of planet formation and evolution. In this review, we focus on developments in understanding: a) the evolution of the gas and dust content of circumstellar disks based on observational surveys, highlighting new results from the Spitzer Space Telescope; b) the physical properties of specific systems as a means to interpret the survey results; c) theoretical models used to explain the observations; d) an evolutionary model of our own solar system for comparison to the observations of debris disks around other stars; and e) how these new results impact our assessment of whether systems like our own are common or rare compared to the ensemble of normal stars in the disk of the Milky Way.

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arXiv2005

Dust in Resonant Extrasolar Kuiper Belts - Grain Size and Wavelength Dependence of Disk Structure

This paper considers the distribution of dust which originates in the break-up of planetesimals that are trapped in resonance with a planet. There are three grain populations with different spatial distributions: (I) large grains have the clumpy resonant distribution of the planetesimals; (II) moderate sized grains are no longer in resonance and have an axisymmetric distribution; (III) small grains are blown out of the system by radiation pressure and have a distribution which falls off oc 1/r, however these grains can be further divided into subclasses: (IIIa) grains produced from pop I that exhibit trailing spiral structure emanating from the clumps; and (IIIb) grains produced from pop II that have an axisymmetric distribution. Since observations in different wavebands are sensitive to different sized dust grains, multi-wavelength imaging can be used to test models for the origin of debris disk structure. For example, a disk with no blow-out grains would appear clumpy in the sub-mm, but smooth at mid- to far-IR wavelengths. The wavelength of the transition is indicative of the mass of the perturbing planet. The size distribution of Vega's disk is modeled in the light of the recent Spitzer observations. The origin of the large quantities of pop III grains seen by Spitzer must be in the destruction of the grains seen in the sub-mm, and so at high resolution and sensitivity the mid- to far-IR structure of Vega's disk is predicted to include spiral structure emanating from the sub-mm clumps.

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arXiv2005

Spiral Structure when Setting up Pericentre Glow: Possible Giant Planets at Hundreds of AU in the HD141569 Disk

This paper discusses the impact of introducing a planet on an eccentric orbit into a planetesimal disk. That planet's secular perturbations cause the orbits of the planetesimals to evolve in such a way that at any one time planetesimals at the same distance from the star have common pericentres and eccentricities. This causes the surface density distribution of an extended planetesimal disk to exhibit two spirals, one exterior the other interior to the planet's orbit. These two spirals unwind in different directions and their structure is described by two parameters: the time since the planet was introduced and the planet's eccentricity. At late times the spirals become tightly wound and the offset centre of symmetry of the pericentre glow approximation is recovered. Comparison with spiral structure seen in the HD141569 disk shows that its spiral at 325 AU is similar to that caused by introducing a planet 5 Myr ago with a mass 0.2-2M_Jup orbiting at 235-250 AU with an eccentricity of 0.05-0.2; likewise a Saturn mass planet at 150 AU would cause structure like that seen at 200 AU. More definitive statements about any planets orbiting HD141569 from this model could be made once the effect of the binary companion on the disk is known, and once the disk's structure has been better characterised down to 100 AU, including the location of the star within the disk. The relatively young age of this system (~5 Myr) means that if giant planets really do exist at hundreds of AU from HD141569, this provides a unique opportunity to set constraints on the mechanism by which those planets came to be at such large distances.

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arXiv2005

The Vega Debris Disk -- A Surprise from Spitzer

We present high spatial resolution mid- and far-infrared images of the Vega debris disk obtained with the Multiband Imaging Photometer for Spitzer (MIPS). The disk is well resolved and its angular size is much larger than found previously. The radius of the disk is at least 43" (330 AU), 70"(543 AU), and 105" (815 AU) in extent at 24, 70 and 160 um, respectively. The disk images are circular, smooth and without clumpiness at all three wavelengths. The radial surface brightness profiles imply an inner boundary at a radius of 11"+/-2" (86 AU). Assuming an amalgam of amorphous silicate and carbonaceous grains, the disk can be modeled as an axially symmetric and geometrically thin disk, viewed face-on, with the surface particle number density following an r^-1 power law. The disk radiometric properties are consistent with a range of models using grains of sizes ~1 to ~50 um. We find that a ring, containing grains larger than 180 um and at radii of 86-200 AU from the star, can reproduce the observed 850 um flux, while its emission does not violate the observed MIPS profiles. This ring could be associated with a population of larger asteroidal bodies analogous to our own Kuiper Belt. Cascades of collisions starting with encounters amongthese large bodies in the ring produce the small debris that is blown outward by radiation pressure to much larger distances where we detect its thermal emission. The dust production rate is >~10^15 g/s based on the MIPS results. This rate would require a very massive asteroidal reservoir for the dust to be produced in a steady state throughout Vega's life. Instead, we suggest that the disk we imaged is ephemeral and that we are witnessing the aftermath of a large and relatively recent collisional event, and subsequent collisional cascade.

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arXiv2005

The Insignificance of P-R Drag in Detectable Extrasolar Planetesimal Belts

This paper considers a simple model in which dust produced in a planetesimal belt migrates in toward the star due to P-R drag suffering destructive collisions with other dust grains on the way. Assuming the dust is all of the same size, the resulting surface density distribution can be derived analytically and depends only on the parameter eta0=5000tau*sqrt(Mstar/r)/beta; this parameter can be determined observationally with the hypothesis that beta=0.5. For massive belts in which eta0>>1 dust is confined to the planetesimal belt, while the surface density of more tenuous belts, in which eta0<<1, is constant with distance from the star. The emission spectrum of dust from planetesimal belts at different distances from different mass stars shows that the dust belts which have been detected to date should have eta0>>1; dust belts with eta0<<1 are hard to detect as they are much fainter than the stellar photosphere. This is confirmed for a sample of 37 debris disk candidates for which eta0 was determined to be >10. This means that these disks are so massive that mutual collisions prevent dust from reaching the inner regions of these systems and P-R drag can be ignored when studying their dynamics. Models for the formation of structure in debris disks by the trapping of particles into planetary resonances by P-R drag should be reconsidered. However, since collisions do not halt 100% of the dust, this means that in the absence of planetary companions debris disk systems should be populated by small quantities of hot dust which may be detectable in the mid-IR. Even in disks with eta0<<1 the temperature of dust emission is shown to be a reliable tracer of the planetesimal distribution meaning that inner holes in the dust distribution imply a lack of colliding planetesimals in the inner regions.

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arXiv2004

Sub-millimeter images of a dusty Kuiper belt around eta Corvi

We present sub-millimeter and mid-infrared images of the circumstellar disk around the nearby F2V star eta Corvi. The disk is resolved at 850um with a size of ~100AU. At 450um the emission is found to be extended at all position angles, with significant elongation along a position angle of 130+-10deg; at the highest resolution (9.3") this emission is resolved into two peaks which are to within the uncertainties offset symmetrically from the star at 100AU projected separation. Modeling the appearance of emission from a narrow ring in the sub-mm images shows the observed structure cannot be caused by an edge-on or face-on axisymmetric ring; the observations are consistent with a ring of radius 150+-20AU seen at 45+-25deg inclination. More face-on orientations are possible if the dust distribution includes two clumps similar to Vega; we show how such a clumpy structure could arise from the migration over 25Myr of a Neptune mass planet from 80-105AU. The inner 100AU of the system appears relatively empty of sub-mm emitting dust, indicating that this region may have been cleared by the formation of planets, but the disk emission spectrum shows that IRAS detected an additional hot component with a characteristic temperature of 370+-60K (implying a distance of 1-2AU). At 11.9um we found the emission to be unresolved with no background sources which could be contaminating the fluxes measured by IRAS. The age of this star is estimated to be ~1Gyr. It is very unusual for such an old main sequence star to exhibit significant mid-IR emission. The proximity of this source makes it a perfect candidate for further study from optical to mm wavelengths to determine the distribution of its dust.

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arXiv2003

Sub-mm observations and modelling of Vega type stars

We present new sub-mm observations and modelling of Vega excess stars, using realistic dust grain models. For resolved disks, we find that different objects require very different dust grain properties in order to simultaneously fit the image data and SED. Fomalhaut and Vega require solid dust grains, whilst HR4796 and HD141569 can only be fitted using porous grains. The older stars tend to have less porous grains than younger stars, which may indicate that collisions have compacted the dust grains. Eps Eri appears to be deficient in small dust grains compared to our best fitting model. This may be due to factors which affect the size distribution of grains close to the radiation pressure blowout limit. Alternatively, this discrepancy may be due to some external influence on the disk (e.g. a planet). When the model is applied to unresolved targets, an estimate of the disk size can be made. However, the large diversity in dust composition for the resolved disks means that we cannot make a reliable assumption as to the composition of the grains in an unresolved disk, and there is corresponding uncertainty in the disk size. In addition, the poor fit for Eps Eri shows that the model cannot always account for the SED even if the disk size is known. These two factors mean that it may not be possible to determine a disk's size without actually resolving it.

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arXiv2003

Resonant Trapping of Planetesimals by Planet Migration: Debris Disk Clumps and Vega's Similarity to the Solar System

This paper describes a model which can explain the observed clumpy structures of debris disks. Clumps arise because after a planetary system forms its planets migrate due to angular momentum exchange with the remaining planetesimals. Outward migration of the outermost planet traps planetesimals outside its orbit into its resonances and resonant forces cause azimuthal structure in their distribution. The model is based on numerical simulations of planets of different masses, Mpl, migrating at different rates, dapl/dt, through a dynamically cold (e<0.01) planetesimal disk initially at a semimajor axis a. Trapping probabilities and the resulting azimuthal structures are presented for a planet's 2:1, 5:3, 3:2, and 4:3 resonances. Seven possible dynamical structures are identified from migrations defined by mu=Mpl/Mstar and theta=dapl/dt*sqrt(a/Mstar). Application of this model to the 850um image of Vega's disk shows its two clumps of unequal brightness can be explained by the migration of a Neptune-mass planet from 40 to 65AU over 56Myr; tight constraints are set on possible ranges of these parameters. The clumps are caused by planetesimals in the 3:2 and 2:1 resonances; the asymmetry arises because of the overabundance of planetesimals in the 2:1(u) over the 2:1(l) resonance. The similarity of this migration to that proposed for our own Neptune hints that Vega's planetary system may be much more akin to the solar system than previously thought. Predictions are made which would substantiate this model, such as the orbital motion of the clumpy pattern, the location of the planet, and the presence of lower level clumps.

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arXiv2003

SCUBA Observations of Dust around Lindroos Stars: Evidence for a Substantial Submillimetre Disc Population

We have observed 22 Lindroos stars with SCUBA on the JCMT to search for evidence of dust discs. Stars in this sample are the less massive companions of B-type primaries and have ages of 10-170Myr. Dust was detected around three of these stars (HD74067, HD112412 and HD99803B). The emission around HD74067 is centrally peaked and is approximately symmetrically distributed out to ~70". This emission either arises from a two component disc, one circumstellar and the other circumbinary with dust masses of 0.3 and >27Mearth respectively, or an unrelated background object. The other two detections we attribute to circumsecondary discs with masses of 0.04 and 0.3Mearth; a circumprimary disc is also present around HD112413 with a similar mass to that around the companion HD112412. Cross-correlation of our sample with the IRAS catalogs only showed evidence for dust emission at 25um and 60um toward one star (HD1438); none of the sub-mm detections were evident in the far-IR data implying that these discs are cold (<40K assuming beta=1). Our sub-mm detections are some of the first of dust discs surrounding evolved stars that were not detected by IRAS or ISO and imply that 9-14% of stars could harbour previously undetected dust discs that await discovery in unbiased sub-mm surveys. If these discs are protoplanetary remnants, rather than secondary debris discs, dust lifetime arguments show that they must be devoid of small <0.1mm grains. Thus it may be possible to determine the origin of these discs from their spectral energy distributions. The low dust masses for this sample support the picture that protoplanetary dust discs are depleted to the levels of the brightest debris discs (~1Mearth) within 10Myr.

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arXiv2002

Collisional Processes in Extrasolar Planetsimal Disks - Dust Clumps in Fomalhaut's Debris Disk

This paper presents a model for the outcome of collisions between planetesimals in a debris disk and assesses the impact of collisional processes on the structure and size distribution of the disk. The model is presented by its application to Fomalhaut's collisionally replenished dust disk; a recent 450 micron image of this disk shows a clump embedded within it with a flux ~5 per cent of the total. The following conclusions are drawn: (i) SED modelling is consistent with Fomalhaut's disk having a collisional cascade size distribution extending from bodies 0.2 m in diameter down to 7 micron-sized dust. (ii) Collisional lifetime arguments imply that the cascade starts with planetesimals 1.5-4 km in diameter. Any larger bodies must be predominantly primordial. (iii) Constraints on the timescale for the ignition of the cascade are consistent with these primordial planetesimals having a distribution that extends up to 1000km, resulting in a disk mass of 5-10 times the minimum mass solar nebula. (iv) The debris disk is expected to be intrinsically clumpy, since planetesimal collisions result in dust clumps. The intrinsic clumpiness of Fomalhaut's disk is below current detection limits, but could be detectable by future observatories such as the ALMA, and could provide the only way of determining the primordial planetesimal population. (v) The observed clump could have originated in a collision between two runaway planetesimals, both larger than 1400 km diameter. It is unlikely that we should witness such an event unless both the formation of these runaways and the ignition of the collisional cascade occurred within the last ~10 Myr. (vi) Another explanation for Fomalhaut's clump is that ~5 per cent of the planetesimals in the ring are trapped in 1:2 resonance with a planet orbiting at 80 AU.

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arXiv1999

Deep 10 and 18 micron Imaging of the HR 4796A Circumstellar Disk: Transient Dust Particles & Tentative Evidence for a Brightness Asymmetry

We present new 10.8 and 18.2 micron images of HR 4796A, a young A0V star that was recently discovered to have a spectacular, nearly edge-on, circumstellar disk prominent at ~20 microns (Jayawardhana et al. 1998; Koerner et al. 1998). These new images, obtained with OSCIR at Keck II, show that the disk's size at 10 microns is comparable to its size at 18 microns. Therefore, the 18 micron-emitting dust may also emit some, or all, of the 10 micron radiation. Using these multi-wavelength images, we determine a "characteristic" diameter of 2-3 microns for the mid-infrared-emitting dust particles if they are spherical and composed of astronomical silicates. Particles this small are expected to be blown out of the system by radiation pressure in a few hundred years, and therefore these particles are unlikely to be primordial. Dynamical modeling of the disk (Wyatt et al. 2000) indicates that the disk surface density is relatively sharply peaked near 70 AU, which agrees with the mean annular radius deduced by Schneider et al. (1999) from their NICMOS images. We present evidence (~1.8 sigma significance) for a brightness asymmetry that may result from the presence of the hole and the gravitational perturbation of the disk particle orbits by the low-mass stellar companion or a planet. This "pericenter glow," which must still be confirmed, results from a very small (a few AU) shift of the disk's center of symmetry relative to the central star HR 4796A; one side of the inner boundary of the annulus is shifted towards HR 4796A, thereby becoming warmer and more infrared-emitting. The possible detection of pericenter glow implies that the detection of even complex dynamical effects of planets on disks is within reach.

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arXiv1999

How Observations of Circumstellar Disk Asymmetries Can Reveal Hidden Planets: Pericenter Glow and its Application to the HR 4796 Disk

Recent images of the disks of dust around the young stars HR 4796A and Fomalhaut show, in each case, a double-lobed feature that may be asymmetric (one lobe may be brighter than the other). A symmetric double-lobed structure is that expected from a disk of dust with a central hole that is observed nearly edge-on (i.e., close to the plane of the disk). This paper shows how the gravitational influence of a second body in the system with an eccentric orbit would cause a brightness asymmetry in such a disk by imposing a "forced eccentricity" on the orbits of the constituent dust particles, thus shifting the center of symmetry of the disk away from the star and causing the dust near the forced pericenter of the perturbed disk to glow. Dynamic modeling of the HR 4796 disk shows that its 5% brightness asymmetry could be the result of a forced eccentricity as small as 0.02 imposed on the disk by either the binary companion HR 4796B, or by an unseen planet close to the inner edge of the disk. Since it is likely that a forced eccentricity of 0.01 or higher would be imposed on a disk in a system in which there are planets, but no binary companion, the corresponding asymmetry in the disk's structure could serve as a sensitive indicator of these planets that might otherwise remain undetected.

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