* Astronomy

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RE: Proplyds

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Title: Breaking the Ice: Planetesimal Formation at the Snowline
Authors: Guillem Aumatell, Gerhard Wurm

Recently Saito & Sirono (2011) proposed that large ice aggregates which drift in- wards in protoplanetary disks break up during sublimation, ejecting embedded silicate particles. This would lead to a concentration of small solid particles close to the snow- line. In view of this model we carried out laboratory experiments where we observed freely levitating ice aggregates sublimating. We find that frequent break up is indeed very common. Scaled to a 10 cm aggregate about 2x10^4 small silicate aggregates might result. This supports the idea that sublimation of drifting ice aggregates might locally increase the density of small dust (silicate) particles which might more easily be swept up by larger dust aggregates or trigger gravitational instability. Either way this might locally boost the formation of planetesimals at the snowline.

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Title: Planetesimals in Debris Disks of Sun-like Stars
Authors: Andrew B. Shannon, Yanqin Wu

Observations of dusty debris disks can be used to test theories of planetesimal coagulation. Planetesimals of sizes up to a couple thousand kms are embedded in these disks and their mutual collisions generate the small dust grains that are observed. The dust luminosities, when combined with information on the dust spatial extent and the system age, can be used to infer initial masses in the planetesimal belts. Carrying out such a procedure for a sample of debris disks around Sun-like stars, we reach the following two conclusions. First, if we assume that colliding planetesimals satisfy a primordial size spectrum of the form dn/ds ~ s^{-q}, observed disks strongly favour a value of q between 3.5 and 4, while both current theoretical expectations and statistics of Kuiper belt objects favour a somewhat larger value. Second, number densities of planetesimals are two to three orders of magnitude higher in detected disks than in the Kuiper belt, for comparably-sized objects. This is a surprise for the coagulation models. It would require a similar increase in the solid surface density of the primordial disk over that of the Minimum Mass Solar Nebula, which is unreasonable. Both of our conclusions are driven by the need to explain the presence of bright debris disks at a few Gyrs of age.

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Title: The Physics of Protoplanetesimal Dust Agglomerates. Vi. Erosion of Large Aggregates and its Consequences for the Dust-Size Distribution
Authors: Rainer Schräpler, Jürgen Blum

Observed protoplanetary disks consist of a large amount of micrometer-sized particles. Dullemond and Dominik (2005) pointed out for the first time the difficulty in explaining the strong mid-IR excess of classical T-Tauri stars without any dust-retention mechanisms. Because high relative velocities in between micrometer-sized and macroscopic particles exist in protoplanetary disks, we present experimental results on the erosion of macroscopic agglomerates consisting of micrometer-sized spherical particles via the impact of micrometer-sized particles. We find that after an initial phase, in which an impacting particle erodes up to 10 particles of an agglomerate, the impacting particles compress the agglomerate's surface, which partly passivates the agglomerates against erosion. Due to this effect the erosion halts within our error bars for impact velocities up to ~30 m/s. For larger velocities, the erosion is reduced by an order of magnitude. This outcome is explained and confirmed by a numerical model. In a next step we build an analytical disk model and implement the experimentally found erosive effect. The model shows that erosion is a strong source of micrometer-sized particles in a protoplanetary disk. Finally we use the stationary solution of this model to explain the amount of micrometer-sized particles in observational infrared data of Furlan et al. (2006).

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Direct Images of Disks Unravel Mystery of Planet Formation

The fruits of the SEEDS Project, led by Motohide Tamura at NAOJ , are accumulating. Composed of over 100 scientists and 25 institutions, the international consortium of researchers supporting the project has announced another set of stunning findings obtained with the recently commissioned Subaru instrument HiCIAO , an upgraded version of its predecessor CIAO. Their initial announcement of a significant discovery came in December, 2009: an exoplanet candidate around a Sun-like star. Now they are announcing another remarkable discovery: direct and sharp images of the protoplanetary disks of two young stars that reveal how planets may have formed within them. No other telescopes, whether ground-based or in space, have ever penetrated so close to a central star, showing the details of its disk.
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Disks of dust point to cosmic births

By carving 'gaps' in the disks of dust that create and enshroud them, newborn planets are giving astronomers clues to locating possible new worlds.
An international research team, led by Swinburne University's Associate Professor Sarah Maddison, is studying the disks of dust that enfold newborn planets in order to better understand cosmic birth.
Because dust obscures optical light, astronomers need to look for other ways to identify the presence of unseen planets in the dusty disks around young stars. A new planet's existence can be inferred from the behaviour of the dust (and gas) around it; much as a ship's presence might be inferred from its wake on the ocean by an observer flying high above.
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Title: A Spitzer IRS Study of Debris Disks Around Planet-Host Stars
Authors: Sarah E. Dodson-Robinson (1), C. A. Beichman (2,3), John M. Carpenter (3), Geoffrey Bryden (4) ((1) University of Texas, (2) NASA Exoplanet Science Institute, (3) Caltech, (4) Jet Propulsion Laboratory)

Since giant planets scatter planetesimals within a few tidal radii of their orbits, the locations of existing planetesimal belts indicate regions where giant planet formation failed in bygone protostellar disks. Infrared observations of circumstellar dust produced by colliding planetesimals are therefore powerful probes of the formation histories of known planets. Here we present new Spitzer IRS spectrophotometry of 111 Solar-type stars, including 105 planet hosts. Our observations reveal 11 debris disks, including two previously undetected debris disks orbiting HD 108874 and HD 130322. Combining our 32 micron spectrophotometry with previously published MIPS photometry, we find that the majority of debris disks around planet hosts have temperatures in the range 60 < T < 100 K. Assuming a dust temperature T = 70 K, which is representative of the nine debris disks detected by both IRS and MIPS, we find that debris rings surrounding Sunlike stars orbit between 15 and 240 AU, depending on the mean particle size. Our observations imply that the planets detected by radial-velocity searches formed within 240 AU of their parent stars. If any of the debris disks studied here have mostly large, blackbody emitting grains, their companion giant planets must have formed in a narrow region between the ice line and 15 AU.

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Protoplanetary Disks

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RE: Proplyds

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Title: Locating the planetesimals belts in the multiple-planet systems HD 128311, HD 202206, HD 82943 and HR 8799
Authors: Amaya Moro-Martin, Renu Malhotra, Geoffrey Bryden, George H. Rieke, Kate Y. L. Su, Charles A. Beichman, Samantha M. Lawler

In addition to the Sun, six other stars are known to harbour multiple planets and debris disks: HD 69830, HD 38529, HD 128311, HD 202206, HD 82943 and HR 8799. In this paper we set constraints on the location of the dust-producing planetesimals around the latter four systems. We use a radiative transfer model to analyse the spectral energy distributions of the dust disks (including two new Spitzer IRS spectra presented in this paper), and a dynamical model to assess the long-term stability of the planetesimals' orbits. As members of a small group of stars that show evidence of harbouring a multiple planets and planetesimals, their study can help us learn about the diversity of planetary systems.

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