Kevin R. Grazier^a, Julie.C. Castillo-Rogez^a and Philip W. Sharp^b

^aJet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, United States
^bDepartment of Mathematics, University of Auckland, New Zealand

We quantify the relative contribution of volatiles supplied from outer Solar System planetesimal reservoirs to large wet asteroids during the first few My after the beginning of the Solar System. To that end, we simulate the fate of planetesimals originating within different regions of the Solar System–and thus characterized by different chemical inventories–using a highly accurate integrator tuned to handle close planet/planetesimal encounters. The fraction of icy planetesimals crossing the Asteroid Belt was relatively significant, and our simulations show that planetesimals originating from the Jupiter/Saturn region were orders of magnitude more abundant than those stemming from the Uranus and Neptune regions when the planets were just embryos. As the planets reached their full masses the Jupiter/ Saturn and Saturn/Uranus regions contributed similar fractions of planetesimals for any material remaining in these reservoirs late in the stage of planetary formation. This implies that large asteroids like Ceres accreted very little material enriched in low-eutectic volatiles (e.g., methanol, nitrogen and methane ices, etc.) and clathrate hydrates expected to condense at the very low temperatures predicted for beyond Saturn’s orbit in current early Solar nebula models. Further, a large fraction of the content in organics of Ceres and neighboring ice-rich objects originates from the outer Solar System.

Reference
Grazier KR, Castillo-Rogez JC and Sharp PW (in press) Dynamical Delivery of Volatiles to the Outer Main Belt. Icarus 780:154.
[doi:10.1088/0004-637X/780/2/154]
Copyright Elsevier

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Laura Vican¹ and Adam Schneider^2,3

¹Department of Physics and Astronomy, University of California, Los Angeles, CA 90095, USA
²Department of Physics and Astronomy, University of Georgia, Athens, GA 30602, USA
³Current Address: Department of Physics and Astronomy, The University of Toledo, Toledo, OH 43606, USA.

We used chromospheric activity to determine the ages of 2820 field stars. We searched these stars for excess emission at 22 μm with the Wide-Field Infrared Survey Explorer. Such excess emission is indicative of a dusty debris disk around a star. We investigated how disk incidence trends with various stellar parameters, and how these parameters evolve with time. We found 22 μm excesses around 98 stars (a detection rate of 3.5%). Of these 98 excess sources, 74 are presented here for the first time. We also measured the abundance of lithium in eight dusty stars in order to test our stellar age estimates.

Reference
Vican L and Schneider A (2014) The evolution of dusty debris disks around solar type stars. The Astrophysical Journal 780:154.
[doi:10.1088/0004-637X/780/2/154]

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Tilman Birnstiel and Sean M. Andrews

Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA

The expectation that aerodynamic drag will force the solids in a gas-rich protoplanetary disk to spiral in toward the host star on short timescales is one of the fundamental problems in planet formation theory. The nominal efficiency of this radial drift process is in conflict with observations, suggesting that an empirical calibration of solid transport mechanisms in a disk is highly desirable. However, the fact that both radial drift and grain growth produce a similar particle size segregation in a disk (such that larger particles are preferentially concentrated closer to the star) makes it difficult to disentangle a clear signature of drift alone. We highlight a new approach, by showing that radial drift leaves a distinctive “fingerprint” in the dust surface density profile that is directly accessible to current observational facilities. Using an analytical framework for dust evolution, we demonstrate that the combined effects of drift and (viscous) gas drag naturally produce a sharp outer edge in the dust distribution (or, equivalently, a sharp decrease in the dust-to-gas mass ratio). This edge feature forms during the earliest phase in the evolution of disk solids, before grain growth in the outer disk has made much progress, and is preserved over longer timescales when both growth and transport effects are more substantial. The key features of these analytical models are reproduced in detailed numerical simulations, and are qualitatively consistent with recent millimeter-wave observations that find gas/dust size discrepancies and steep declines in dust continuum emission in the outer regions of protoplanetary disks.

Reference
T Birnstiel and Andrews SM (2014) On the outer edges of protoplanetary dust disks. The Astrophysical Journal 780:153.
[doi:10.1088/0004-637X/780/2/153]

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P. R. Brook^1,5, A. Karastergiou¹, S. Buchner^2,6, S. J. Roberts³, M. J. Keith^4,7, S. Johnston⁴ and R. M. Shannon⁴

¹Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
²Hartebeesthoek Radio Astronomy Observatory, P.O. Box 443, Krugersdorp 1740, South Africa
³Information Engineering, University of Oxford, Parks Road, Oxford OX1 3PJ, UK
⁴CSIRO Astronomy and Space Science, Australia Telescope National Facility, P.O. Box 76, Epping, NSW 1710, Australia
⁵CSIRO Astronomy and Space Science, Australia Telescope National Facility, P.O. Box 76, Epping, NSW 1710, Australia
⁶School of Physics, University of Witwatersrand, Johannesburg, South Africa
⁷Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK

Debris disks and asteroid belts are expected to form around young pulsars due to fallback material from their original supernova explosions. Disk material may migrate inward and interact with a pulsar’s magnetosphere, causing changes in torque and emission. Long-term monitoring of PSR J0738–4042 reveals both effects. The pulse shape changes multiple times between 1988 and 2012. The torque, inferred via the derivative of the rotational period, changes abruptly from 2005 September. This change is accompanied by an emergent radio component that drifts with respect to the rest of the pulse. No known intrinsic pulsar processes can explain these timing and radio emission signatures. The data lead us to postulate that we are witnessing an encounter with an asteroid or in-falling debris from a disk.

Reference
Brook PR, Karastergiou A, Buchner S, Roberts SJ, Keith MJ, Johnston S and Shannon RM (2014) Evidence of an Asteroid Encountering a Pulsar. The Astrophysical Journal – Letters 780:L31.
[doi:10.1088/2041-8205/780/2/L31]

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