Context. Debris discs are a consequence of the planet formation process and constitute the fingerprints of planetesimal systems. Their counterparts in the solar system are the asteroid and ...Edgeworth-Kuiper belts. Aims. The aim of this paper is to provide robust numbers for the incidence of debris discs around FGK stars in the solar neighbourhood. Methods. The full sample of 177 FGK stars with d ≤ 20 pc proposed for the DUst around NEarby Stars (DUNES) survey is presented. Herschel/PACS observations at 100 and 160 μm were obtained, and were complemented in some cases with data at 70 μm and at 250, 350, and 500 μm SPIRE photometry. The 123 objects observed by the DUNES collaboration were presented in a previous paper. The remaining 54 stars, shared with the Disc Emission via a Bias-free Reconnaissance in IR and Sub-mm (DEBRIS) consortium and observed by them, and the combined full sample are studied in this paper. The incidence of debris discs per spectral type is analysed and put into context together with other parameters of the sample, like metallicity, rotation and activity, and age. Results. The subsample of 105 stars with d ≤ 15 pc containing 23 F, 33 G, and 49 K stars is complete for F stars, almost complete for G stars, and contains a substantial number of K stars from which we draw solid conclusions on objects of this spectral type. The incidence rates of debris discs per spectral type are 0.26+0.21-0.14 (6 objects with excesses out of 23 F stars), 0.21+0.17-0.11 (7 out of 33 G stars), and 0.200.14-0.09 (10 out of 49 K stars); the fraction for all three spectral types together is 0.22+0.08-0.07 (23 out of 105 stars). The uncertainties correspond to a 95% confidence level. The medians of the upper limits of Ldust/L∗ for each spectral type are 7.8 × 10-7 (F), 1.4 × 10-6 (G), and 2.2 × 10-6 (K); the lowest values are around 4.0 × 10-7. The incidence of debris discs is similar for active (young) and inactive (old) stars. The fractional luminosity tends to drop with increasing age, as expected from collisional erosion of the debris belts.
Context. Since circumstellar dust in debris disks is short-lived, dust-replenishing requires the presence of a reservoir of planetesimals. These planetesimals in the parent belt of debris disks orbit ...their host star and continuously supply the disk with fine dust through their mutual collisions. Aims. We aim to understand effects of different collisional parameters on the observational appearance of eccentric debris disks. These parameters are the eccentricity of the planetesimal belt, dynamical excitation, and the material strength. Methods. The collisional evolution of selected debris disk configurations was simulated with the numerical code ACE. Subsequently, selected observable quantities are simulated with our newly developed code DMS. The impact of the eccentricity, dynamical excitation, and the material strength is discussed with respect to the grain size distribution, the spectral energy distribution, and spatially resolved images of debris disk systems. Results. The most recognizable features in different collisional evolutions are as follows. First, both the increase of dynamical excitation in the eccentric belt of the debris disk system and the decrease of the material strength of dust particles result in a higher production rate of smaller particles. This reduces the surface brightness differences between the periastron and the apastron sides of the disks. For very low material strengths, the “pericenter glow” phenomenon is reduced and eventually even replaced by the opposite effect, the “apocenter glow”. In contrast, higher material strengths and lower dynamical excitation of the system result in an enhancement of asymmetries in the surface brightness distribution. Second, it is possible to constrain the level of collisional activity from the appearance of the disk, for example, the wavelength-dependent apocenter-to-pericenter flux ratio. Within the considered parameter space, the impact of the material strength on the appearance of the disk is stronger than that of dynamical excitation of the system. Finally, we find that the impact of the collisional parameters on the net spectral energy distribution is weak.
We model a typical debris disk, treated as an idealized ensemble of dust particles, exposed to stellar gravity and direct radiation pressure and experiencing fragmenting collisions. Applying the ...kinetic method of statistical physics, written in orbital elements, we calculate size and spatial distibutions expected in a steady-state disk, investigate timescales needed to reach the steady state, and calculate mass loss rates. Particular numerical examples are given for the debris disk around Vega. The disk should comprise a population of larger grains in bound orbits and a population of smaller particles in hyperbolic orbits. The cross section area is dominated by the smallest grains that still can stay in bound orbits, for Vega about 10 ${\rm \mu m}$ in radius. The size distribution is wavy, implying secondary peaks in the size distribution at larger sizes. The radial profile of the pole-on surface density or the optical depth in the steady-state disk has a power-law index between about -1 and -2. It cannot be much steeper even if dust production is confined to a narrow planetesimal belt, because collisional grinding produces smaller and smaller grains, and radiation pressure pumps up their orbital eccentricities and spreads them outward, which flattens the radial profile. The timescales to reach a steady state depend on grain sizes and distance from the star. For Vega, they are about 1 Myr for grains up to some hundred ${\rm \mu m}$ at 100 AU. The total mass of the Vega disk needed to produce the observed amount of micron and submillimeter-sized dust does not exceed several earth masses for an upper size limit of parent bodies of about 1 km. The collisional depletion of the disk occurs on Gyr timescales.
Context. High-resolution images of circumstellar debris discs reveal off-centred rings that indicate past or ongoing perturbation, possibly caused by secular gravitational interaction with unseen ...stellar or substellar companions. The purely dynamical aspects of this departure from radial symmetry are well understood. However, the observed dust is subject to additional forces and effects, most notably collisions and drag. Aims. To complement the studies of dynamics, we therefore aim to understand how the addition of collisional evolution and drag forces creates new asymmetries and strengthens or overrides existing ones. Methods. We augmented our existing numerical code Analysis of Collisional Evolution (ACE) by an azimuthal dimension, the longitude of periapse. A set of fiducial discs with global eccentricities ranging from 0 to 0.4 was evolved over gigayear timescales. Size distribution and spatial variation of dust were analysed and interpreted. We discuss the basic impact of belt eccentricity on spectral energy distributions and images. Results. We find features imposed on characteristic timescales. First, radiation pressure defines size cut-offs that differ between periapse and apoapse, resulting in an asymmetric halo. The differences in size distribution make the observable asymmetry of the halo depend on wavelength. Second, collisional equilibrium prefers smaller grains on the apastron side of the parent belt, reducing the effect of pericentre glow and the overall asymmetry. Third, Poynting–Robertson drag fills the region interior to an eccentric belt such that the apastron side is more tenuous. Interpretation and prediction of the appearance in scattered light is problematic when spatial and size distribution are coupled.
In contrast to all other debris disks, where the dust can be seen via an infrared excess over the stellar photosphere, the dust emission of the Edgeworth-Kuiper belt (EKB) eludes remote detection ...because of the strong foreground emission of the zodiacal cloud. We accessed the expected EKB dust disk properties by modeling. We treated the debiased population of the known trans-Neptunian objects (TNOs) as parent bodies and generated the dust with our collisional code. The resulting dust distributions were modified to take into account the influence of gravitational scattering and resonance trapping by planets on migrating dust grains as well as the effect of sublimation. A difficulty with the modeling is that the amount and distribution of dust are largely determined by sub-kilometer-sized bodies. These are directly unobservable, and their properties cannot be accessed by collisional modeling, because objects larger than (10...60) m in the present-day EKB are not in a collisional equilibrium. To place additional constraints, we used in-situ measurements of the New Horizons spacecraft within 20 AU. We show that to sustain a dust disk consistent with these measurements, the TNO population has to have a break in the size distribution at s ≲ 70 km. However, even this still leaves us with several models that all correctly reproduce nearly constant dust impact rates in the region of giant planet orbits and do not violate the constraints from the non-detection of the EKB dust thermal emission by the COBE spacecraft. The modeled EKB dust disks, which conform to the observational constraints, can either be transport-dominated or intermediate between the transport-dominated and collision-dominated regime. The in-plane optical depth of such disks is τ∥(r > 10 AU) ~ 10-6 and their fractional luminosity is fd ~ 10-7. Planets and sublimation are found to have little effect on dust impact fluxes and dust thermal emission. The spectral energy distribution of an EKB analog as seen from 10 pc distance peaks at wavelengths of (40...50) μm at F ≈ 0.5 mJy, which is less than 1% of the photospheric flux at those wavelengths. Therefore, EKB analogs cannot be detected with present-day instruments such as Herschel/PACS.
The archetypical debris disk around Vega has been observed intensively over the past 25 years. It has been argued that the resulting photometric data and images may be in contradiction with a ...standard, steady-state collisional scenario of the disk evolution. In particular, the emission in the mid-infrared (mid-IR) appears to be in excess of what is expected from a 'Kuiper belt' at ~100 AU, which is evident in the submillimeter images and inferred from the majority of photometric points. Here we re-address the question of whether or not the Vega disk observations are compatible with a continuous dust production through a collisional cascade. Instead of seeking a size and spatial distribution of dust that provide the best fit to observations, our approach involves physical modeling of the debris disk 'from the sources.' We assume that dust is maintained by a belt of parent planetesimals, and employ our collisional and radiative transfer codes to consistently model the size and radial distribution of the disk material and then thermal emission of dust. In doing so, we vary a broad set of parameters, including the stellar properties, the exact location, extension, and dynamical excitation of the planetesimal belt, chemical composition of solids, and the collisional prescription. We are able to reproduce the spectral energy distribution in the entire wavelength range from the near-IR to millimeter, as well as the mid-IR and submillimeter radial brightness profiles of the Vega disk. Thus, our results suggest that the Vega disk observations are not in contradiction with a steady-state collisional dust production, and we put important constraints on the disk parameters and physical processes that sustain it. The total disk mass in 100 km-sized bodies is estimated to be ~10 Earth masses. Provided that collisional cascade has been operating over much of the Vega age of ~350 Myr, the disk must have lost a few Earth masses of solids during that time. We also demonstrate that using an intermediate luminosity of the star between the pole and the equator, as derived from its fast rotation, is required to reproduce the debris disk observations. Finally, we show that including cratering collisions into the model is mandatory.
The Edgeworth-Kuiper debris disk Vitense, Ch; Krivov, A. V.; Löhne, T.
Astronomy & astrophysics,
09/2010, Volume:
520
Journal Article
Peer reviewed
Open access
The Edgeworth-Kuiper belt (EKB) and its presumed dusty debris is a natural reference for extrsolar debris disks. We re-analyze the current database of known transneptunian objects (TNOs) and employ a ...new algorithm to eliminate the inclination and the distance selection effects in the known TNO populations to derive expected parameters of the “true” EKB. Its estimated mass is MEKB = 0.12 $M_\oplus$, which is by a factor of ~15 larger than the mass of the EKB objects detected so far. About a half of the total EKB mass is in classical and resonant objects and another half is in scattered ones. Treating the debiased populations of EKB objects as dust parent bodies, we then “generate” their dust disk with our collisional code. Apart from accurate handling of destructive and cratering collisions and direct radiation pressure, we include the Poynting-Robertson (P-R) drag. The latter is known to be unimportant for debris disks around other stars detected so far, but cannot be ignored for the EKB dust disk because of its much lower optical depth. We find the radial profile of the normal optical depth to peak at the inner edge of the classical belt, ≈40 AU. Outside the classical EKB, it approximately follows τ $\propto$ r-2 which is roughly intermediate between the slope predicted analytically for collision-dominated (r-1.5) and transport-dominated (r-2.5) disks. The size distribution of dust is less affected by the P-R effect. The cross section-dominating grain size still lies just above the blowout size (~ 1 $\dots$ 2μm), as it would if the P-R effect was ignored. However, if the EKB were by one order of magnitude less massive, its dust disk would have distinctly different properties. The optical depth profile would fall off as τ $\propto$ r-3, and the cross section-dominating grain size would shift from ~ 1 $\dots$ 2μm to ~ 100μm. These properties are seen if dust is assumed to be generated only by known TNOs without applying the debiasing algorithm. An upper limit of the in-plane optical depth of the EKB dust set by our model is τ = 2 × 10-5 outside 30 AU. If the solar system were observed from outside, the thermal emission flux from the EKB dust would be about two orders of magnitude lower than for solar-type stars with the brightest known infrared excesses observed from the same distance. Herschel and other new-generation facilities should reveal extrasolar debris disks nearly as tenuous as the EKB disk. We estimate that the Herschel/PACS instrument should be able to detect disks at a ~ 1 $\dots$ 2MEKB level.
Context. The nearby K2 V star ε Eridani hosts one known inner planet, an outer Kuiper belt analog, and an inner disk of warm dust. Spitzer/IRS measurements indicate that the warm dust is present at ...distances as close as a few AU from the star. Its origin is puzzling, since an “asteroid belt” that could produce this dust would be unstable because of the known inner planet. Aims. Here we test a hypothesis that the observed warm dust is generated by collisions in the outer belt and is transported inward by Poynting-Robertson drag and strong stellar winds. Methods. We simulated a steady-state distribution of dust particles outside 10 AU with a collisional code and in the inner region (r < 10 AU) with single-particle numerical integrations. By assuming homogeneous spherical dust grains composed of ice and astrosilicate, we calculated the thermal emission of the dust and compared it with observations. We investigated two different orbital configurations for the inner planet inferred from radial velocity measurements, one with a highly eccentric orbit of e = 0.7 and another one with a moderate eccentricity of e = 0.25. We also produced a simulation without a planet. Results. Our models can reproduce the shape and magnitude of the observed spectral energy distribution from mid-infrared to submillimeter wavelengths, as well as the Spitzer/MIPS radial brightness profiles. The best-fit dust composition includes both water ice and silicates. The results are similar for the two possible planetary orbits and without a planet. Conclusions. The observed warm dust in the ε Eridani system can indeed stem from the outer “Kuiper belt” and be transported inward by Poynting-Robertson and stellar wind drag. The inner planet has little effect on the distribution of dust, so that the planetary orbit could not be constrained. Reasonable agreement between the model and observations can only be achieved by relaxing the assumption of purely silicate dust and assuming a mixture of silicate and water ice in comparable amounts.
AU Microscopii’s debris disc is one of the most famous and best-studied debris discs and one of only two resolved debris discs around M stars. We perform in-depth collisional modelling of the AU Mic ...disc including stellar radiative and corpuscular forces (stellar winds), aiming at a comprehensive understanding of the dust production and the dust and planetesimal dynamics in the system. Our models are compared to a suite of observational data for thermal and scattered light emission, ranging from the ALMA radial surface brightness profile at 1.3 mm to spatially resolved polarisation measurements in the visible. Most of the data are shown to be reproduced with dust production in a belt of planetesimals with an outer edge at around 40 au and subsequent inward transport of dust by stellar winds. A low dynamical excitation of the planetesimals with eccentricities up to 0.03 is preferred. The radial width of the planetesimal belt cannot be constrained tightly. Belts that are 5 au and 17 au wide, as well as a broad 44 au-wide belt, are consistent with observations. All models show surface density profiles that increase with distance from the star up to ≈40 au, as inferred from observations. The best model is achieved by assuming a stellar mass loss rate that exceeds the solar one by a factor of 50. The models reproduce the spectral energy distribution and the shape of the ALMA radial profile well, but deviate from the scattered light observations more strongly. The observations show a bluer disc colour and a lower degree of polarisation for projected distances <40 au than predicted by the models. These deviations may be reduced by taking irregularly shaped dust grains which have scattering properties different from the Mie spheres used in this work. From tests with a handful of selected dust materials, we favour mixtures of silicate, carbon, and ice of moderate porosity. We also address the origin of the unresolved central excess emission detected by ALMA and show that it cannot stem from an additional inner belt alone. Instead, it should derive, at least partly, from the chromosphere of the central star.
Context. Debris discs are thought to be formed through the collisional grinding of planetesimals, and then can be considered as the outcome of planet formation. Understanding the properties of gas ...and dust in debris discs can help us comprehend the architecture of extrasolar planetary systems. Herschel Space Observatory far-infrared (IR) photometry and spectroscopy have provided a valuable dataset for the study of debris discs gas and dust composition. This paper is part of a series of papers devoted to the study of Herschel-PACS observations of young stellar associations. Aims. This work aims at studying the properties of discs in the beta Pictoris moving group (BPMG) through far-IR PACS observations of dust and gas. Methods. We obtained Herschel-PACS far-IR photometric observations at 70, 100, and 160 μm of 19 BPMG members, together with spectroscopic observations for four of them. These observations were centred at 63.18 μm and 157 μm, aiming to detect OI and CII emission. We incorporated the new far-IR observations in the SED of BPMG members and fitted modified blackbody models to better characterise the dust content. Results. We have detected far-IR excess emission towards nine BPMG members, including the first detection of an IR excess towards HD 29391.The star HD 172555, shows OI emission, while HD 181296 shows CII emission, expanding the short list of debris discs with a gas detection. No debris disc in BPMG is detected in both OI and CII. The discs show dust temperatures in the range 55–264 K, with low dust masses (<6.6 × 10-5 M⊕ to 0.2 M⊕) and radii from blackbody models in the range 3 to ~82 AU. All the objects with a gas detection are early spectral type stars with a hot dust component.
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