Resolving the terrestrial planet forming regions of HD113766 and HD172555 with MIDI

We present new MIDI interferometric and VISIR spectroscopic observations of HD113766 and HD172555. Additionally we present VISIR 11um and 18um imaging observations of HD113766. These sources represent the youngest (16Myr and 12Myr old respectively) debris disc hosts with emission on <<10AU scales. We find that the disc of HD113766 is partially resolved on baselines of 42-102m, with variations in resolution with baseline length consistent with a Gaussian model for the disc with FWHM of 1.2-1.6AU (9-12mas). This is consistent with the VISIR observations which place an upper limit of 0."14 (17AU) on the emission, with no evidence for extended emission at larger distances. For HD172555 the MIDI observations are consistent with complete resolution of the disc emission on all baselines of lengths 56-93m, putting the dust at a distance of >1AU (>35mas). When combined with limits from TReCS imaging the dust at ~10um is constrained to lie somewhere in the region 1-8AU. Observations at ~18um reveal extended disc emission which could originate from the outer edge of a broad disc, the inner parts of which are also detected but not resolved at 10um, or from a spatially distinct component. These observations provide the most accurate direct measurements of the location of dust at 1-8AU that might originate from the collisions expected during terrestrial planet formation. These observations provide valuable constraints for models of the composition of discs at this epoch and provide a foundation for future studies to examine in more detail the morphology of debris discs.Comment: 22 pages, 19 figures, accepted for publication in MNRA

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.Comment: 11 Pages Accepted to MNRA

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.Comment: Accepted by Ap

Warm dusty discs: Exploring the A star 24um debris population

(Abridged) Studies of debris discs have shown that most systems are analogous to the EKB. In this study we aim to determine how many IRAS 25um excesses towards A stars are real, and investigate where the dust lies. We observe with TIMMI2, VISIR, Michelle and TReCS a sample of A and B-type main sequence stars reported as having mid-IR excess. We constrain the location of the debris through combined modelling of the emission spectrum and a modelling technique designed to constrain the radial extent of emission in mid-IR imaging. We independently confirm the presence of warm dust around 3 of the candidates: HD3003, HD80950 and eta Tel. For the binary HD3003 a stability analysis indicates the dust is either circumstellar and lying at ~4 AU with the binary orbiting at >14AU, or the dust lies in an unstable location; there is some evidence for temporal evolution of its excess emission on a ~20 year timescale. For 7 of the targets we present quantitative limits on the location of dust around the star. We demonstrate that the disc around HD71155 must have spatially distinct components at 2 and 60AU. We model the limits of current instrumentation and show that most of the known A star debris discs which could be readily resolved at 18um on 8m instruments have been resolved. Limits from unresolved imaging can help distinguish between competing models of the disc emission, but resolved imaging is key to the determination of the disc location. Modelling of the detection limits for extended emission can be useful for targeting future observational campaigns. MIRI on the JWST will be able to resolve most of the known A star debris disc population. METIS on the E-ELT will provide the opportunity to explore the hot disc population more thoroughly by detecting extended emission where calibration accuracy limits disc detection through photometry, reaching levels below 1 zodi for stars at <10pc.Comment: Accepted for publication in Astronomy and Astrophysic

Resolving the hot dust around HD69830 and eta Corvi with MIDI and VISIR

Most of the known debris discs exhibit cool dust in regions analogous to the Edgeworth-Kuiper Belt. However, a rare subset show hot excess from within a few AU, which is often inferred to be transient. We examine 2 such sources to place limits on their location to help distinguish between different interpretations for their origin. We use MIDI on the VLTI to observe the debris discs around eta Corvi and HD69830 using baseline lengths from 44-130m. New VISIR observations of HD69830 at 18.7um are also presented. These observations are compared with disc models to place limits on disc size. The visibility functions measured with MIDI for both sources show significant variation with wavelength across 8-13um in a manner consistent with the disc flux being well resolved, notably with a dip at 10-11.5um due to the silicate emission feature. The average ratio of visibilities measured between 10-11.5um and 8-9um is 0.934+/-0.015 for HD69830 and 0.880+/-0.013 for eta Corvi over the 4 baselines for each source, a departure of 4 and 9sigma from that expected if the discs were unresolved. HD69830 is unresolved by VISIR at 18.7um. The combined limits from MIDI and 8m imaging constrain the warm dust to lie within 0.05-2.4AU for HD69830 and 0.16-2.98AU for eta Corvi. These results represent the first resolution in the mid-IR of dust around main sequence stars. The constraints placed on the location of the dust are consistent with radii predicted by SED modelling. Tentative evidence for a common position angle for the dust at 1.7AU with that at 150AU around eta Corvi, which might be expected if the hot dust is fed from the outer disc, demonstrates the potential of this technique for constraining the origin of the dust and more generally for the study of dust in the terrestrial regions of main sequence stars.Comment: Accepted for publication in Astronomy and Astrophysic

On the observability of resonant structures in planetesimal disks due to planetary migration

We present a thorough study of the impact of a migrating planet on a planetesimal disk, by exploring a broad range of masses and eccentricities for the planet. We discuss the sensitivity of the structures generated in debris disks to the basic planet parameters. We perform many N-body numerical simulations, using the symplectic integrator SWIFT, taking into account the gravitational influence of the star and the planet on massless test particles. A constant migration rate is assumed for the planet. The effect of planetary migration on the trapping of particles in mean motion resonances is found to be very sensitive to the initial eccentricity of the planet and of the planetesimals. A planetary eccentricity as low as 0.05 is enough to smear out all the resonant structures, except for the most massive planets. The planetesimals also initially have to be on orbits with a mean eccentricity of less than than 0.1 in order to keep the resonant clumps visible. This numerical work extends previous analytical studies and provides a collection of disk images that may help in interpreting the observations of structures in debris disks. Overall, it shows that stringent conditions must be fulfilled to obtain observable resonant structures in debris disks. Theoretical models of the origin of planetary migration will therefore have to explain how planetary systems remain in a suitable configuration to reproduce the observed structures.Comment: 16 pages, 13 figures. Accepted for publication in A&

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.Comment: 16 pages, accepted by ApJ, removed HD128400 as hot dust candidat

Collisional evolution of eccentric planetesimal swarms

Models for the steady state collisional evolution of low eccentricity planetesimal belts identify debris disks with hot dust at 1AU, like eta Corvi and HD69830, as anomalous since collisional processing should have removed most of the planetesimal mass over their >1 Gyr lifetimes. This paper looks at the effect of large planetesimal eccentricities (e>>0.3) on their collisional lifetime and the amount of mass that can remain at late times M_{late}. For an axisymmetric planetesimal disk with common pericentres and eccentricities e, we find that M_{late} \propto e^{-5/3}(1+e)^{4/3}(1-e)^{-3}. For a scattered disk-like population (i.e., common pericentres), in the absence of dynamical evolution, the mass evolution at late times would be as if only planetesimals with the largest eccentricity were present. Despite the increased remaining mass, higher eccentricities do not increase the hot emission from the collisional cascade until e>0.99, partly because most collisions occur near pericentre thus increasing the dust blow-out diameter. However, at high eccentricities (e>0.97) the blow-out population extending out from pericentre may be detectable above the collisional cascade; higher eccentricities also increase the probability of witnessing a recent collision. All of the imaging and spectroscopic constraints for eta Corvi can be explained with a single planetesimal population with pericentre at 0.75AU, apocentre at 150AU, and mass 5M_\oplus; however, the origin of such a high eccentricity population remains challenging. The mid-IR excess to HD69830 can be explained by the ongoing destruction of a debris belt produced in a recent collision in an eccentric planetesimal belt, but the lack of far-IR emission requires small bound grains to be absent from the parent planetesimal belt, possibly due to sublimation.Comment: MNRAS in pres

Are debris disks self-stirred?

This paper considers the evidence that debris disks are self-stirred by the formation of Plutos. A model for the dust produced during self-stirring is applied to statistics for A stars. As there is no significant difference between excesses of A-stars <50Myr old, we focus on reproducing the broad trends, the "rise and fall" of the fraction of stars with excesses. Using a population model, we find that the statistics and trends can be reproduced with a self-stirring model of planetesimal belts with radii distributed between 15-120AU. Disks must have this 15AU minimum radius to show a peak in disk fraction, rather than a monotonic decline. Populations of extended disks with fixed inner and/or outer radii fail to fit the statistics, due mainly to the slow 70um evolution as stirring moves further out in the disk. This conclusion, that debris disks are narrow belts, is independent of the significance of 24um trends for young A-stars. We show that the statistics can also be reproduced with a model in which disks are stirred by secular perturbations from a nearby eccentric planet. Detailed imaging is therefore the best way to characterise the stirring mechanism. From a more detailed look at beta Pictoris Moving Group and TW Hydrae Association A-stars we find that the disk around beta Pictoris is likely the result of secular stirring by the proposed planet at ~10AU; the structure of the HR 4796A disk also points to sculpting by a planet. The two other stars with disks, HR 7012 and eta Tel, possess transient hot dust, though the outer eta Tel disk is consistent with a self-stirred origin. Planet formation provides a natural explanation for the belt-like nature of debris disks, with inner regions cleared by planets that may also stir the disk, and the outer edges set by where planetesimals can form. [abridged]Comment: Accepted to MNRA

The nature of mid-infrared excesses from hot dust around Sun-like stars

(ABRIDGED) Studies of debris disks have shown that most systems are analogous to the Edgeworth-Kuiper Belt. However a rare subset of sun-like stars possess dust which lies in the terrestrial planet region. In this study we aim to determine how many sources with apparent mid-IR excess are truly hosts of warm dust, and investigate where the dust must lie. We observed using mid-IR imaging with TIMMI2, VISIR and MICHELLE a sample of FGK main sequence stars reported to have hot dust. A new modelling approach was developed to determine the constraints that can be set on the radial extent of excess emission. We confirm the presence of warm dust around 3 of the candidates (eta Corvi, HD145263 and HD202406), and present constraints on the emitting dust regions. Of 2 alternative models for the eta Corvi excess emission, we find that a model with 1 hot dust component at <3 AU (combined with the known submm dust population) fits the data better at the 2.6sigma level than an alternative model with 2 populations of dust in the mid-IR. We identify several systems which have a companion (HD65277 and HD79873) or background object (HD53246, HD123356 and HD128400) responsible for their mid-infrared excess, and for 3 other systems we were able to rule out a point-like source near the star at the level of excess observed in lower resolution observations (HD12039, HD69830 and HD191089). Hot dust sources are either young and possibly primordial or transitional, or have relatively small radius steady-state planetesimal belts, or they are old and luminous with transient emission. High resolution imaging can be used to constrain the location of the disk and help to discriminate between different models of disk emission. For some small disks, interferometry is needed to resolve the disk location.Comment: Accepted for publication in Astronomy & Astrophysic