Sociology & Anthropology News

Annual Persico Bear Roast 5/3 3-7PM Sociological Society End of the Year Celebration Honors Day 201

'Occupational Therapy'

E. Casson, reprinted from the Report of Conference on Welfare of Cripples and Invalid Children, held at Drapers' Hall London

Resonant Trapping of Planetesimals by Planet Migration: Debris Disk Clumps and Vega's Similarity to the Solar System

This paper describes a model which can explain the observed clumpy structures of debris disks. Clumps arise because after a planetary system forms its planets migrate due to angular momentum exchange with the remaining planetesimals. Outward migration of the outermost planet traps planetesimals outside its orbit into its resonances and resonant forces cause azimuthal structure in their distribution. The model is based on numerical simulations of planets of different masses, Mpl, migrating at different rates, dapl/dt, through a dynamically cold (e<0.01) planetesimal disk initially at a semimajor axis a. Trapping probabilities and the resulting azimuthal structures are presented for a planet's 2:1, 5:3, 3:2, and 4:3 resonances. Seven possible dynamical structures are identified from migrations defined by mu=Mpl/Mstar and theta=dapl/dt*sqrt(a/Mstar). Application of this model to the 850um image of Vega's disk shows its two clumps of unequal brightness can be explained by the migration of a Neptune-mass planet from 40 to 65AU over 56Myr; tight constraints are set on possible ranges of these parameters. The clumps are caused by planetesimals in the 3:2 and 2:1 resonances; the asymmetry arises because of the overabundance of planetesimals in the 2:1(u) over the 2:1(l) resonance. The similarity of this migration to that proposed for our own Neptune hints that Vega's planetary system may be much more akin to the solar system than previously thought. Predictions are made which would substantiate this model, such as the orbital motion of the clumpy pattern, the location of the planet, and the presence of lower level clumps.Comment: 30 pages, accepted by Ap

Using the surface profiles of modern ice masses to inform palaeo-glacier reconstructions

Morphometric study of modern ice masses is useful because many reconstructions of glaciers traditionally draw on their shape for guidance Here we analyse data derived from the surface profiles of 200 modern ice masses-valley glaciers icefields ice caps and ice sheets with length scales from 10(0) to 10(3) km-from different parts of the world Four profile attributes are investigated relief span and two parameters C* and C that result from using Nye s (1952) theoretical parabola as a profile descriptor C* and C respectively measure each profile s aspect ratio and steepness and are found to decrease in size and variability with span This dependence quantifies the competing influences of unconstrained spreading behaviour of ice flow and bed topography on the profile shape of ice masses which becomes more parabolic as span Increases (with C* and C tending to low values of 2 5-3 3 m(1/2)) The same data reveal coherent minimum bounds in C* and C for modern ice masses that we develop into two new methods of palaeo glacier reconstruction In the first method glacial limits are known from moraines and the bounds are used to constrain the lowest palaeo ice surface consistent with modern profiles We give an example of applying this method over a three-dimensional glacial landscape in Kamchatka In the second method we test the plausibility of existing reconstructions by comparing their C* and C against the modern minimum bounds Of the 86 published palaeo ice masses that we put to this test 88% are found to be plausible The search for other morphometric constraints will help us formalise glacier reconstructions and reduce their uncertainty and subjectiveness (C) 2010 Elsevier Ltd All rights reserve