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Posts: 131433
Date:
 RE: Proplyds Permalink Title: Impact of planet--planet scattering on the formation and survival of debris disks Author: F. Marzari Planet--planet scattering is a major dynamical mechanism able to significantly alter the architecture of a planetary system. In addition to that, it may also affect the formation and retention of a debris disk by the system. A violent chaotic evolution of the planets can easily clear leftover planetesimal belts preventing the ignition of a substantial collisional cascade that can give origin to a debris disk. On the other end, a mild evolution with limited steps in eccentricity and semimajor axis can trigger the formation of a debris disk by stirring an initially quiet planetesimal belt. The variety of possible effects that planet--planet scattering can have on the formation of debris disks is analysed and the statistical probability of the different outcomes is evaluated. This leads to the prediction that systems which underwent an episode of chaotic evolution might have a lower probability of harboring a debris disk. Read more (283kb, PDF) __________________

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Posts: 131433
Date:
 Permalink Mystery of Planet-forming Disks Explained by Magnetism Astronomers say that magnetic storms in the gas orbiting young stars may explain a mystery that has persisted since before 2006. Researchers using NASA's Spitzer Space Telescope to study developing stars have had a hard time figuring out why the stars give off more infrared light than expected. The planet-forming disks that circle the young stars are heated by starlight and glow with infrared light, but Spitzer detected additional infrared light coming from an unknown source. A new theory, based on three-dimensional models of planet-forming disks, suggests the answer: Gas and dust suspended above the disks on gigantic magnetic loops like those seen on the sun absorb the starlight and glow with infrared light. Read more __________________

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Posts: 131433
Date:
 Permalink Title: Explaining millimetre-sized particles in brown dwarf disks Authors: P. Pinilla, T. Birnstiel, M. Benisty, L. Ricci, A. Natta, C. P. Dullemond, C. Dominik, L. Testi Planets have been detected around a variety of stars, including low-mass objects, such as brown dwarfs. However, such extreme cases are challenging for planet formation models. Recent sub-millimetre observations of disks around brown dwarf measured low spectral indices of the continuum emission that suggest that dust grains grow to mm-sizes even in these very low mass environments. To understand the first steps of planet formation in scaled-down versions of T-Tauri disks, we investigate the physical conditions that can theoretically explain the growth from interstellar dust to millimetre-sized grains in disks around brown dwarf. We modelled the evolution of dust particles under conditions of low-mass disks around brown dwarfs. We used coagulation, fragmentation and disk-structure models to simulate the evolution of dust, with zero and non-zero radial drift. For the non-zero radial drift, we considered strong inhomogeneities in the gas surface density profile that mimic long-lived pressure bumps in the disk. We studied different scenarios that could lead to an agreement between theoretical models and the spectral slope found by millimetre observations. We find that fragmentation is less likely and rapid inward drift is more significant for particles in brown dwarf disks than in T-Tauri disks. We present different scenarios that can nevertheless explain millimetre-sized grains. As an example, a model that combines the following parameters can fit the millimetre fluxes measured for brown dwarf disks: strong pressure inhomogeneities of ~40% of amplitude, a small radial extent ~ 15 AU, a moderate turbulence strength alpha_{turb}= 10^{-3}, and average fragmentation velocities for ices v_f = 10 m s^{-1}. Read more (2075kb, PDF) __________________

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Posts: 131433
Date:
 Permalink Title: Hybrid methods in planetesimal dynamics: Formation of protoplanetary systems and the mill condition Authors: Pau Amaro-Seoane, Patrick Glaschke, Rainer Spurzem The formation and evolution of protoplanetary discs remains a challenge from both a theoretical and numerical standpoint. In this work we first perform a series of tests of our new hybrid algorithm presented in Glaschke, Amaro-Seoane and Spurzem 2011 (henceforth Paper I) that combines the advantages of high accuracy of direct-summation N-body methods with a statistical description for the planetesimal disc based on Fokker-Planck techniques. We then address the formation of planets, with a focus on the formation of protoplanets out of planetesimals. We find that the evolution of the system is driven by encounters as well as direct collisions and requires a careful modelling of the evolution of the velocity dispersion and the size distribution over a large range of sizes. The simulations show no termination of the protoplanetary accretion due to gap formation, since the distribution of the planetesimals is only subjected to small fluctuations. We also show that these features are weakly correlated with the positions of the protoplanets. The exploration of different impact strengths indicates that fragmentation mainly controls the overall mass loss, which is less pronounced during the early runaway growth. We prove that the fragmentation in combination with the effective removal of collisional fragments by gas drag sets an universal upper limit of the protoplanetary mass as a function of the distance to the host star, which we refer to as the mill condition. Read more (288kb, PDF) __________________

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Posts: 131433
Date:
 Permalink Dust rings not 'smoking gun' for planets after all There can be smoke without fire. Sharp rings of dust around stars aren't always carved by planets but can form on their own - bad news for those who use the structures to guide them to stars that host planets. The finding also has implications for the existence of a controversial candidate exoplanet. The discs of dust and gas debris surrounding stars sometimes produce sharply defined or elongated rings. These were assumed to be the calling cards of unseen planets, carved by the bodies as they travel through the disc. Read more __________________

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Posts: 131433
Date:
 Permalink Title: A peculiar class of debris disks from Herschel/DUNES - A steep fall off in the far infrared Authors: S. Ertel, S. Wolf, J. P. Marshall, C. Eiroa, J.-C. Augereau, A. V. Krivov, T. Loehne, O. Absil, D. Ardila, M. Arevalo, A. Bayo, G. Bryden, C. del Burgo, J. Greaves, G. Kennedy, J. Lebreton, R. Liseau, J. Maldonado, B. Montesinos, A. Mora, G. L. Pilbratt, J. Sanz-Forcada, K. Stapelfeldt, G. J. White Aims. We present photometric data of debris disks around HIP 103389 (HD 199260), HIP 107350 (HN Peg, HD206860), and HIP 114948 (HD 219482), obtained in the context of our Herschel Open Time Key Program DUNES (DUst around NEarby Stars). Methods. We used Herschel/PACS to detect the thermal emission of the three debris disks with a 3 sigma sensitivity of a few mJy at 100 um and 160 um. In addition, we obtained Herschel/PACS photometric data at 70 um for HIP 103389. Two different approaches are applied to reduce the Herschel data to investigate the impact of data reduction on the photometry. We fit analytical models to the available spectral energy distribution (SED) data. Results. The SEDs of the three disks potentially exhibit an unusually steep decrease at wavelengths > 70 um. We investigate the significance of the peculiar shape of these SEDs and the impact on models of the disks provided it is real. Our modelling reveals that such a steep decrease of the SEDs in the long wavelength regime is inconsistent with a power-law exponent of the grain size distribution -3.5 expected from a standard equilibrium collisional cascade. In contrast, a very distinct range of grain sizes is implied to dominate the thermal emission of such disks. However, we demonstrate that the understanding of the data of faint sources obtained with Herschel is still incomplete and that the significance of our results depends on the version of the data reduction pipeline used. Conclusions. A new mechanism to produce the dust in the presented debris disks, deviations from the conditions required for a standard equilibrium collisional cascade (grain size exponent of -3.5), and/or significantly different dust properties would be necessary to explain the potentially steep SED shape of the three debris disks presented. Read more (524kb, PDF) __________________

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Posts: 131433
Date:
 Protoplanetary Disks Permalink Title: Gaps in Protoplanetary Disks as Signatures of Planets: I. Methodology and Validation Authors: Hannah Jang-Condell, Neal J. Turner We examine the observational consequences of partial gaps being opened by planets in protoplanetary disks. We model the disk using a static alpha-disk model with detailed radiative transfer, parametrising the shape and size of the partially cleared gaps based on the results of hydrodynamic simulations. Shadowing and illumination by stellar irradiation at the surface of the gap leads to increased contrast as the gap trough is deepened by shadowing and cooling and the far gap wall is puffed up by illumination and heating. In calculating observables, we find that multiple scattering is important and derive an approximation to include these effects. A gap produced by a 200 M_Earth (70 M_Earth) planet at 10 AU can lower/raise the midplane temperature of the disk by up to ~-25/+29% (~-11/+19%) by shadowing in the gap trough and illumination on the far shoulder of the gap. At the distance of Taurus, this gap would be resolvable with ~0.01" angular resolution. The gap contrast is most significant in scattered light and at thermal continuum wavelengths characteristic of the surface temperature, reducing or raising the surface brightness by up to order of magnitude. Since gaps sizes are correlated to planet mass, this is a promising way of finding and determining the masses of planets embedded in protoplanetary disks. Read more (2190kb, PDF) __________________

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Posts: 131433
Date:
 Proplyds Permalink Title: The Debris Disk Candidates: Eleven 24 \mu m excess stars in Spitzer SWIRE Fields Authors: Hong Wu (NAOC), Chao-Jian Wu, Chen Cao, Sebastian Wolf, Jing-Yao Hu We present the optical to mid-infrared SEDs of 11 debris disk candidates from Spitzer SWIRE fields. All these candidates are selected from SWIRE 24 \mu m sources matched with both the SDSS star catalogue and the 2MASS point source catalogue. They show an excess in the mid-infrared at 24\mu m (K_S-[24]_{Vega} \ge 0.44) indicating the presence of a circumstellar dust disk. The observed optical spectra show that they are all late type main-sequence stars covering the spectral types of FGKM. Their fractional luminosities are well above 5 x 10^{-5}, even up to the high fractional luminosity of 1 x 10^{-3}. The high galactic latitudes of SWIRE fields indicate that most of these candidates could belong to the oldest stars in the thick disk. Our results indicate that the high fractional luminosity debris disks could exist in the old solar-like star systems, though they are now still quite rare. Their discoveries at high-galactic latitudes will also provide us an excellent opportunity to the further studies of properties and evolution of the debris disk in the ISM poor environments. Read more (1191kb, PDF) __________________

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Posts: 131433
Date:
 Debris Disks Permalink Title: Debris Disks in Kepler Exoplanet Systems Authors: S. M. Lawler, B. Gladman The Kepler Mission recently identified systems hosting candidate extrasolar planets, many of which are super-Earths. Realizing these rocky planetary systems are candidates to host extrasolar asteroid belts, we use mid-infrared data from the Wide-field Infrared Survey Explorer (WISE) to search for emission from dust in these systems. We find excesses around eight stars, indicating the presence of warm to hot dust (~100-500 K), corresponding to orbital distances of 0.1-10 AU for these solar-type stars. The strongest detection, KOI 1099, demands ~500 K dust interior to the orbit of its exoplanet candidate. One star, KOI 904, may host very hot dust (~1200 K, corresponding to 0.02 AU). We find the fraction of these exoplanet-bearing stars with warm excesses (~3%) is consistent with the fraction found for solar-type field stars. It is difficult to explain the presence of dust so close to the host stars, corresponding to dust rings at radii <0.3 AU; both the collisional and Poynting-Robertson drag timescales to remove dust from the system are hundreds of years or less at these distances. Assuming a steady-state for these systems implies large mass consumption rates with these short removal timescales, meaning that the dust production mechanism in these systems must almost certainly be episodic in nature. Possible dust production mechanisms include comets with low perihelia, catastrophic collisions between planetesimals, or even impact ejecta from the exoplanets, which are located at similar orbital distances.Read more (1194kb, PDF) __________________

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Posts: 131433
Date:
 RE: Proplyds Permalink Title: Herschel discovery of a new class of cold, faint debris discs Authors: C. Eiroa, J. P. Marshall, A. Mora, A. V. Krivov, B. Montesinos, O. Absil, D. Ardila, M. Arevalo, J.-Ch.Augereau, A. Bayo, W. Danchi, C. del Burgo, S. Ertel, M. Fridlund, B.M. Gonzalez-Garca, A. M. Heras, J. Lebreton, R. Liseau, J. Maldonado, G. Meeus, D. Montes, G.L. Pilbratt, A. Roberge, J. Sanz-Forcada, K. Stapelfeldt, P. Thebault, G. J. White, S. Wolf We present Herschel PACS 100 and 160 micron observations of the solar-type stars alpha Men, HD 88230 and HD 210277, which form part of the FGK stars sample of the Herschel Open Time Key Programme (OTKP) DUNES (DUst around NEarby S tars). Our observations show small infrared excesses at 160 micron for all three stars. HD 210277 also shows a small excess at 100 micron, while the 100 micron fluxes of alpha Men and HD 88230 agree with the stellar photospheric predictions. We attribute these infrared excesses to a new class of cold, faint debris discs. alpha Men and HD 88230 are spatially resolved in the PACS 160 micron images, while HD 210277 is point-like at that wavelength. The projected linear sizes of the extended emission lie in the range from ~ 115 to ~ 250 AU. The estimated black body temperatures from the 100 and 160 micron fluxes are $\lesssim$ 22 K, while the fractional luminosity of the cold dust is Ldust/Lstar ~ 10E-6, close to the luminosity of the Solar-System's Kuiper belt. These debris discs are the coldest and faintest discs discovered so far around mature stars and cannot easily be explained by invoking "classical" debris disc models. Read more (1598kb, PDF) __________________
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