T. Kelling, G. Wurm, and M. Köster

Faculty of Physics, University Duisburg-Essen, Lotharstr. 1, D-47057 Duisburg, Germany

For dust aggregates in protoplanetary disks, a transition between sticking and bouncing in individual collisions at mm to cm sizes has been observed in the past. This leads to the notion of a bouncing barrier for which growth gets stalled. Here, we present long-term laboratory experiments on the outcome of repeated aggregate collisions at the bouncing barrier. About 100 SiO₂ dust aggregates 1 mm in size were observed interacting with each other. Collisions occurred within a velocity range from below mm s^-1 up to cm s^-1. Aggregates continuously interacted with each other over a period of 900 s. During this time, more than 10⁵ collisions occurred. Nearly 2000 collisions were analyzed in detail. No temporal stable net growth of larger aggregates was observed even though sticking collision occurred. Larger ensembles of aggregates sticking together were formed but were disassembled again during further collisional evolution. The concept of a bouncing barrier supports the formation of planetesimals by seeded collisional growth, as well as by gravitational instability favoring a significant total mass being limited to certain size ranges. Within our parameter set, the experiments confirm that bouncing barriers are one possible and likely evolutionary limit of self-consistent particle growth.

Reference
Kelling T, Wurm G and Köster M (2014) Experimental Study on Bouncing Barriers in Protoplanetary Disks.  The Astrophysical Journal 783:111.
[doi:10.1088/0004-637X/783/2/111]

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J.P. Emery^a, Y.R. Fernández^b, M.S.P. Kelley^c, K.T. Warden (nèe Crane)^a,1, C. Hergenrother^d, D.S. Lauretta^d, M.J. Drake^d, H. Campins^b, J. Ziffer^e

^aEarth and Planetary Science Dept & Planetary Geosciences Institute, University of Tennessee, Knoxville, TN 37996
^bPhysics Department, University of Central Florida, Orlando, FL 32816
^cDepartment of Astronomy, University of Maryland, College Park, MD 20742-2421
^dDepartment of Planetary Sciences, University of Arizona, Tucson, AZ 85721
^eDepartment of Physics, University of Southern Maine, Portland, ME 04104
¹Department of Earth, Atmospheric, and Planetary Science, Purdue University, West Lafayette, IN 47907.

Near-Earth asteroids (NEAs) have garnered ever increasing attention over the past few years due to the insights they offer into Solar System formation and evolution, the potential hazard they pose, and their accessibility for both robotic and human spaceflight missions. Among the NEAs, carbonaceous asteroids hold particular interest because they may contain clues to how the Earth got its supplies of water and organic materials, and because none has yet been studied in detail by spacecraft. (101955) Bennu is special among NEAs in that it will not only be visited by a spacecraft, but the OSIRIS-REx mission will also return a sample of Bennu’s regolith to Earth for detailed laboratory study. This paper presents analysis of thermal infrared photometry and spectroscopy that test the hypotheses that Bennu is carbonaceous and that its surface is covered in fine-grained (sub-cm) regolith. The Spitzer Space Telescope observed Bennu in 2007, using the Infrared Spectrograph (IRS) to obtain spectra over the wavelength range 5.2 – 38 μm and images at 16 and 22 μm at 10 different longitudes, as well as the Infrared Array Camera (IRAC) to image Bennu at 3.6, 4.5, 5.8, and 8.0 μm, also at 10 different longitudes. Thermophysical analysis, assuming a spherical body with the known rotation period and spin-pole orientation, returns an effective diameter of 484±10 m, in agreement with the effective diameter calculated from the radar shape model at the orientation of the Spitzer observations (492±20 m, Nolan et al. 2013) and a visible geometric albedo of 0.046±0.005 (using H_v=20.51, Hergenrother et al. 2013). Including the radar shape model in the thermal analysis, and taking surface roughness into account, yields a disk-averaged thermal inertia of 310±70 J m^-2K^-1s^-1/2, which is significantly lower than several other NEAs of comparable size. There may be a small variation of thermal inertia with rotational phase (±60 J m^-2K^-1s^-1/2). The spectral analysis is inconclusive in terms of surface mineralogy; the emissivity spectra have a relatively low signal-to-noise ratio and no spectral features are detected. The thermal inertia indicates average regolith grain size on the scale of several millimeters to about a centimeter. This moderate grain size is also consistent with low spectral contrast in the 7.5 – 20 μm spectral range. If real, the rotational variation in thermal inertia would be consistent with a change in average grain size of only about a millimeter. The thermophysical properties of Bennu’s surface appear to be fairly homogeneous longitudinally. A search for a dust coma failed to detect any extended emission, putting an upper limit of about 10⁶ g of dust within 4750 km of Bennu. Three common methodologies for thermal modeling are compared, and some issues to be aware of when interpreting the results of such models are discussed. We predict that the OSIRIS-REx spacecraft will find a low albedo surface with abundant sub-cm sized grains, fairly evenly distributed in longitude.

Reference
Emery JP, Fernández YR, Kelley MSP, Warden (nèe Crane) KT, Hergenrother C, Lauretta DS, Drake MJ, Campins H and Ziffer J (in press) Thermal Infrared Observations and Thermophysical Characterization of OSIRIS-REx Target Asteroid (101955) Bennu. Icarus
[doi:10.1016/j.icarus.2014.02.005]
Copyright Elsevier

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Tsukagoshi¹ et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

¹College of Science, Ibaraki University, Bunkyo 2-1-1, Mito 310-8512, Japan

To reveal the structures of a transition disk around a young stellar object in Lupus, Sz 91 , we have performed aperture synthesis 345 GHz continuum and CO(3-2) observations with the Submillimeter Array (~1”-3” resolution) and high-resolution imaging of polarized intensity at the K_s -band using the HiCIAO instrument on the Subaru Telescope (0.”25 resolution). Our observations successfully resolved the inner and outer radii of the dust disk to be 65 and 170 AU, respectively, which indicates that Sz 91 is a transition disk source with one of the largest known inner holes. The model fitting analysis of the spectral energy distribution reveals an H₂ mass of 2.4 × 10^-3 M_☉ in the cold (T < 30 K) outer part at 65 AU <r < 170 AU by assuming a canonical gas-to-dust mass ratio of 100, although a small amount (>3 × 10^-9 M_☉) of hot (T ~ 180 K) dust possibly remains inside the inner hole of the disk. The structure of the hot component could be interpreted as either an unresolved self-luminous companion body (not directly detected in our observations) or a narrow ring inside the inner hole. Significant CO(3-2) emission with a velocity gradient along the major axis of the dust disk is concentrated on the Sz 91 position, suggesting a rotating gas disk with a radius of 420 AU. The Sz 91 disk is possibly a rare disk in an evolutionary stage immediately after the formation of protoplanets because of the large inner hole and the lower disk mass than other transition disks studied thus far.

Reference
Tsukagoshi et al. (2014) High-resolution Submillimeter and Near-infrared Studies of the Transition Disk around Sz 91.  The Astrophysical Journal 783:90.
[doi:10.1088/0004-637X/783/2/90]

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