295 research outputs found
The Size-Frequency Distribution of the Zodiacal Cloud: Evidence from the Solar System Dust Bands
Recent observations of the size-frequency distribution of zodiacal cloud
particles obtained from the cratering record on the LDEF satellite (Love and
Brownlee 1993) reveal a significant large particle population (100 micron
diameter or greater) near 1 AU. Our previous modeling of the Solar System dust
bands (Grogan et al 1997), features of the zodiacal cloud associated with the
comminution of Hirayama family asteroids, has been limited by the fact that
only small particles (25 micron diameter or smaller) have been considered. This
was due to the prohibitively large amount of computing power required to
numerically analyze the dynamics of larger particles. The recent availability
of cheap, fast processors has finally made this work possible. Models of the
dust bands are created, built from individual dust particle orbits, taking into
account a size-frequency distribution of the material and the dynamical history
of the constituent particles. These models are able to match both the shapes
and amplitudes of the dust band structures observed by IRAS in multiple
wavebands. The size-frequency index, q, that best matches the observations is
approximately 1.4, consistent with the LDEF results in that large particles are
shown to dominate. However, in order to successfully model the `ten degree'
band, which is usually associated with collisional activity within the Eos
family, we find that the mean proper inclination of the dust particle orbits
has to be approximately 9.35 degrees, significantly different to the mean
proper inclination of the Eos family (10.08 degrees).Comment: 49 pages total, including 27 figure pages. Submitted to Icaru
Identification of new genes associated with breast and ovarian cancer risk. Advances of BCAC, CIMBA and OCAC
Embodied Properties of Semantic Knowledge Acquired From Natural Language
The symbol interdependency hypothesis (Louwerse, 2007, 2008) posits that word meaning is dependent upon two sources of information: embodied or grounded knowledge, obtained from observation of and interaction with the physical world, and symbolic or co-occurrence information, gleaned from experience with how words are used together in written and spoken language. This theory assumes that embodied properties of objects influence the statistical structure of language to such an extent that the embodied properties become partially encoded within the structure of language. The work presented in this dissertation provides support for the symbol interdependency hypothesis by demonstrating that grounded knowledge (in the form of physical and behavioural properties of living and non-living objects) can be identified by analyzing word usage in a large body of written text. An automated method of creating high-dimensional vector-based semantic representations is presented. Several demonstrations show that the representations capture word meaning in a way that aligns with intuition and are able to reproduce non-intuitive results of experiments from the psycholinguistic literature. A feedforward neural network was trained to produce a list of physical and behavioural properties of an object in response to the object\u27s high-dimensional vector representation. The resulting network was able to identify features of the concepts on which it was trained with near-perfect accuracy and was able to generalize this ability to novel concepts and identify properties of concepts to which it was not previously exposed. These results indicate that there is sufficient information in word usage to identify embodied properties of concepts, a finding that is consistent with the symbol interdependency hypothesis
Searching for Saturn's Dust Swarm: Limits on the size distribution of Irregular Satellites from km to micron sizes
We describe a search for dust created in collisions between the Saturnian
irregular satellites using archival \emph{Spitzer} MIPS observations. Although
we detected a degree scale Saturn-centric excess that might be attributed to an
irregular satellite dust cloud, we attribute it to the far-field wings of the
PSF due to nearby Saturn. The Spitzer PSF is poorly characterised at such
radial distances, and we expect PSF characterisation to be the main issue for
future observations that aim to detect such dust. The observations place an
upper limit on the level of dust in the outer reaches of the Saturnian system,
and constrain how the size distribution extrapolates from the smallest known
(few km) size irregulars down to micron-size dust. Because the size
distribution is indicative of the strength properties of irregulars, we show
how our derived upper limit implies irregular satellite strengths more akin to
comets than asteroids. This conclusion is consistent with their presumed
capture from the outer regions of the Solar System.Comment: accepted to MNRA
Debris disk size distributions: steady state collisional evolution with P-R drag and other loss processes
We present a new scheme for determining the shape of the size distribution,
and its evolution, for collisional cascades of planetesimals undergoing
destructive collisions and loss processes like Poynting-Robertson drag. The
scheme treats the steady state portion of the cascade by equating mass loss and
gain in each size bin; the smallest particles are expected to reach steady
state on their collision timescale, while larger particles retain their
primordial distribution. For collision-dominated disks, steady state means that
mass loss rates in logarithmic size bins are independent of size. This
prescription reproduces the expected two phase size distribution, with ripples
above the blow-out size, and above the transition to gravity-dominated
planetesimal strength. The scheme also reproduces the expected evolution of
disk mass, and of dust mass, but is computationally much faster than evolving
distributions forward in time. For low-mass disks, P-R drag causes a turnover
at small sizes to a size distribution that is set by the redistribution
function (the mass distribution of fragments produced in collisions). Thus
information about the redistribution function may be recovered by measuring the
size distribution of particles undergoing loss by P-R drag, such as that traced
by particles accreted onto Earth. Although cross-sectional area drops with
1/age^2 in the PR-dominated regime, dust mass falls as 1/age^2.8, underlining
the importance of understanding which particle sizes contribute to an
observation when considering how disk detectability evolves. Other loss
processes are readily incorporated; we also discuss generalised power law loss
rates, dynamical depletion, realistic radiation forces and stellar wind drag.Comment: Accepted for publication by Celestial Mechanics and Dynamical
Astronomy (special issue on EXOPLANETS
Identification of a protein encoded in the EB-viral open reading frame BMRF2
Using monospecific rabbit sera against a peptide derived from a potential antigenic region of the Epstein-Barr viral amino acid sequence encoded in the open reading frame BMRF2 we could identify a protein-complex of 53/55 kDa in chemically induced B95-8, P3HR1 and Raji cell lines. This protein could be shown to be membrane-associated, as predicted by previous computer analysis of the secondary structure and hydrophilicity pattern, and may be a member of EBV-induced membrane proteins in lytically infected cells
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