There is currently debate over whether the dust content of planetary systems is stochastically regenerated or originates in planetesimal belts evolving in quasi-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 at < 10 AU. The model shows that 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 = 0.16 × 10 −3 r 7/3 t −1age , where r is disk radius in AU and t age is system age in Myr. The majority (4/7) of the hot disks exceed this limit by a factor ≫ 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: η 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 in a disk of a given age. For the 4 transient disks, there is at best a 1:10 5 chance of witnessing such an event compared with 2% of stars showing this phenomenon. 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 ≫ 2 AU. Other notable properties of the 4 hot dust systems are: two also have a planetesimal belt at > 10 AU (η Corvi and HD72905); one has 3 Neptune mass planets at < 1 AU (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. Further detailed study of the planet, planetesimal and dust populations in these rare objects has the potential to uncover the chaotic evolutionary history of these systems and to shed light on the history of the solar system.
This paper confronts a simple analytical model for the steady state evolution of debris disks due to collisions with Spitzer observations of dust around main-sequence A stars. It is assumed that every star has a planetesimal belt, the initial mass and radius of which are drawn from distributions. In the model disk mass is constant until the largest planetesimals reach collisional equilibrium, whereupon mass falls /t À1 age . We find that the detection statistics and trends seen at 24 and 70 m 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 (HD 3003, HD 38678, HD 115892, HD 172555); their planetesimals may have unusual properties ( high strength or low eccentricity), or this dust could be transient. The overall success of our model, which assumes planetesimals in all belts have the same strength, eccentricity, and maximum size, suggests the outcome of planet formation is reasonably uniform. The distribution of planetesimal belt radii, once corrected for detection bias, follows N (r) / r À0:8AE0:3 for 3Y120 AU. Since belt boundaries may be attributed to unseen planets, this provides a unique constraint on A star planetary systems. It is also shown that P-R drag may sculpt the inner edges of A star disks close to the Spitzer detection threshold (HD 2262, HD 19356, HD 106591, HD 115892). This model can be readily applied to the interpretation of future surveys, and predictions for the upcoming SCUBA-2 survey include that 17% of A star disks should be detectable at 850 m.
Aims. 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 moreover is often inferred to be transient from models for planetesimal belt evolution. In this paper we examine 2 such sources to place limits on their location to help distinguish between different interpretations for their origin. Methods. We use MIDI on the VLTI to observe the debris discs around η Corvi and HD69830 using baseline lengths from 44-130 m. New VISIR observations of HD69830 at 18.7 μm are also presented. These observations are compared with disc models to place limits on disc size. Results. The visibility functions measured with MIDI for both sources show significant variation with wavelength across 8-13 μm in a manner consistent with the disc flux being well resolved, notably with a dip at 10-11.5 μm due to the silicate emission feature. The average ratio of visibilities measured between 10-11.5 μm and 8-9 μm is 0.934 ± 0.015 for HD69830 and 0.880 ± 0.013 for η Corvi over the four baselines for each source, a departure of 4 and 9σ from that expected if the discs were unresolved. HD69830 is unresolved by VISIR at 18.7 μm. The combined limits from MIDI and 8 m imaging constrain the warm dust to lie within 0.05-2.4 AU for HD69830 and 0.16-2.98 AU for η Corvi. Conclusions. These results represent the first resolution of dust around main sequence stars using mid-infrared interferometry. The constraints placed on the location of the dust are consistent with radii predicted by SED modelling (1.0 AU for HD69830 and 1.7 AU for η Corvi). Tentative evidence for a common position angle for the dust at 1.7 AU with that at 150 AU around η 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.
We present new MIDI interferometric and VISIR spectroscopic observations of HD 113766 and HD 172555. Additionally, we present VISIR 11‐m and 18‐m imaging observations of HD 113766. These sources represent the youngest (16 and 12 Myr old, respectively) debris disc hosts with emission on ≪10 au scales. We find that the disc of HD 113766 is partially resolved on baselines of 42–102 m, with variations in resolution with baseline length consistent with a Gaussian model for the disc with a full width at half‐maximum (FWHM) of 1.2–1.6 au (9–12 mas). This is consistent with the VISIR observations which place an upper limit of 0.14 arcsec (17 au) on the emission, with no evidence for extended emission at larger distances. For HD 172555, the MIDI observations are consistent with complete resolution of the disc emission on all baselines of lengths 56–93 m, putting the dust at a distance of >1 au (>35 mas). When combined with limits from TReCS imaging, the dust at ∼10 m is constrained to lie somewhere in the region of 1–8 au. Observations at ∼18 m 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 10 m, or from a spatially distinct component. These observations provide the most accurate direct measurements of the location of the dust at 1–8 au 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.
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