4,112 research outputs found
Radiation shielding calculations for MuCool Test Area at Fermilab
The MuCool Test Area (MTA) is an intense primary beam facility derived
directly from the Fermilab Linac to test heat deposition and other technical
concerns associated with the liquid hydrogen targets being developed for
cooling intense muon beams. In this shielding study the results of Monte Carlo
radiation shielding calculations performed using the MARS14 code for the MuCool
Test Area and including the downstream portion of the target hall and berm
around it, access pit, service building, and parking lot are presented and
discussed within the context of the proposed MTA experimental configuration.Comment: 17 pages, 13 figure
The cuticle
The nematode cuticle is an extremely flexible and resilient exoskeleton that permits locomotion via
attachment to muscle, confers environmental protection and allows growth by molting. It is synthesised five
times, once in the embryo and subsequently at the end of each larval stage prior to molting. It is a highly
structured extra-cellular matrix (ECM), composed predominantly of cross-linked collagens, additional
insoluble proteins termed cuticlins, associated glycoproteins and lipids. The cuticle collagens are encoded by a large gene family that are subject to strict patterns of temporal regulation. Cuticle collagen biosynthesis
involves numerous co- and post-translational modification, processing, secretion and cross-linking steps that
in turn are catalysed by specific enzymes and chaperones. Mutations in individual collagen genes and their
biosynthetic pathway components can result in a range of defects from abnormal morphology (dumpy and
blister) to embryonic and larval death, confirming an essential role for this structure and highlighting its
potential as an ECM experimental model system
Stellar Winds on the Main-Sequence II: the Evolution of Rotation and Winds
Aims: We study the evolution of stellar rotation and wind properties for
low-mass main-sequence stars. Our aim is to use rotational evolution models to
constrain the mass loss rates in stellar winds and to predict how their
properties evolve with time on the main-sequence.
Methods: We construct a rotational evolution model that is driven by observed
rotational distributions of young stellar clusters. Fitting the free parameters
in our model allows us to predict how wind mass loss rate depends on stellar
mass, radius, and rotation. We couple the results to the wind model developed
in Paper I of this series to predict how wind properties evolve on the
main-sequence.
Results: We estimate that wind mass loss rate scales with stellar parameters
as . We
estimate that at young ages, the solar wind likely had a mass loss rate that is
an order of magnitude higher than that of the current solar wind. This leads to
the wind having a higher density at younger ages; however, the magnitude of
this change depends strongly on how we scale wind temperature. Due to the
spread in rotation rates, young stars show a large range of wind properties at
a given age. This spread in wind properties disappears as the stars age.
Conclusions: There is a large uncertainty in our knowledge of the evolution
of stellar winds on the main-sequence, due both to our lack of knowledge of
stellar winds and the large spread in rotation rates at young ages. Given the
sensitivity of planetary atmospheres to stellar wind and radiation conditions,
these uncertainties can be significant for our understanding of the evolution
of planetary environments.Comment: 26 pages, 14 figures, 2 tables, to be published in A&
Space use by passerine birds : a study of territory economics in robins Erithacus rubecula and dippers Cinclus cinclus
1. Cost constraints in models of territory size are based on time/activity/laboratory
estimates that predict birds using larger territories will incur higher energy costs. The predicted form of the cost constraint may be linear, accelerating or decelerating depending on assumptions inherent in the models. The aim of this study was to assess the reality and form of the cost constraint by making direct measurements of the energy costs of territory use in birds that occupy territories of different size and shape; polygonal territories represented by the
robin Erithacus rubecula, and linear by the dipper Cinclus cinclus. Free-living energy
expenditure was measured using the doubly-labelled water technique, whilst simultaneously recording patterns of territory use by radio-tracking.
2. Territorial robins concentrated their activity in one or more foraging patches located in bushes. Range polygons containing all the foraging patches used by an individual provided estimates of territory area, and were generally of high eccentricity. A small proportion of robins
was classified as non-territorial based on range polygon areas. Furthermore, while territorial robins showed high fidelity to ranges over the short term (days), non-territorial individuals were nomadic. Over the longer term (months), however, some territorial robins showed range drift.
Dippers similarly used preferred core regions within ranges, although there was no selection for particular habitat features.
3. Because robins occupied territory polygons which varied from polygonal to highly linear, work was focused on this species to allow intra-specific comparison. Robins tended to commute between foraging patches by flying. It was appropriate, therefore, to describe territories in terms of a number of patches linked by a network of flight paths. This generated two further measures of territory size; the number of patches used and the total flight distance
between patches.
4. The robins exploited a renewing food supply. Predictions were tested concerning the temporal scheduling of visits to foraging patches within territories. Patches tended to be separated by flight paths of similar lengths, and were visited in a regular sequence. Although the number of foraging patches used varied, all territories had similar total core areas. Robins using many small foraging patches commuted between patches more often and covered a larger total flight distance during each foraging circuit of the territory. The configurations of foraging
patches were used in a highly linear manner. This was true even if the territory containing them was of low eccentricity.
5. Changes in structure and pattern of use varied predictably with territory size, and could be described mathematically. Based on this and published time/activity budgets, a suite of models was developed to predict how energy costs would vary with number of patches used and total flight distance between patches. Models were tested by directly measuring the energy expenditure of robins using different territories. The number of patches used and total flight
distance between patches were both significantly correlated with energy expenditure, while territory area was not. One of the models showed a significant fit to the observed data, and suggested that the form of the energy cost constraint on territory size was linear. The effect of
territory shape on energy costs was minimal. The implications of these results for models of territory size are discussed.
6. The slope and elevation of the energy cost constraint varied with the morphology of
territory occupants. Based on this, an association of morphology with territory size was predicted; robins of lower mass and wing-loading using larger territories. The observed data supported these predictions, and suggested a possible genetic predisposition to particular patterns of territory occupancy in the robin
Stellar Winds on the Main-Sequence I: Wind Model
Aims: We develop a method for estimating the properties of stellar winds for
low-mass main-sequence stars between masses of 0.4 and 1.1 solar masses at a
range of distances from the star.
Methods: We use 1D thermal pressure driven hydrodynamic wind models run using
the Versatile Advection Code. Using in situ measurements of the solar wind, we
produce models for the slow and fast components of the solar wind. We consider
two radically different methods for scaling the base temperature of the wind to
other stars: in Model A, we assume that wind temperatures are fundamentally
linked to coronal temperatures, and in Model B, we assume that the sound speed
at the base of the wind is a fixed fraction of the escape velocity. In Paper II
of this series, we use observationally constrained rotational evolution models
to derive wind mass loss rates.
Results: Our model for the solar wind provides an excellent description of
the real solar wind far from the solar surface, but is unrealistic within the
solar corona. We run a grid of 1200 wind models to derive relations for the
wind properties as a function of stellar mass, radius, and wind temperature.
Using these results, we explore how wind properties depend on stellar mass and
rotation.
Conclusions: Based on our two assumptions about the scaling of the wind
temperature, we argue that there is still significant uncertainty in how these
properties should be determined. Resolution of this uncertainty will probably
require both the application of solar wind physics to other stars and detailed
observational constraints on the properties of stellar winds. In the final
section of this paper, we give step by step instructions for how to apply our
results to calculate the stellar wind conditions far from the stellar surface.Comment: 24 pages, 13 figures, 2 tables, Accepted for publication in A&
Testing in High-Dimensional Spiked Models
We consider the five classes of multivariate statistical problems identified by James (1964), which together cover much of classical multivariate analysis, plus a simpler limiting case, symmetric matrix denoising. Each of James' problems involves the eigenvalues of {code} where H and E are proportional to high dimensional Wishart matrices. Under the null hypothesis, both Wisharts are central with identity covariance. Under the alternative, the non-centrality or the covariance parameter of H has a single eigenvalue, a spike, that stands alone. When the spike is smaller than a case-specific phase transition threshold, none of the sample eigenvalues separate from the bulk, making the testing problem challenging. Using a unified strategy for the six cases, we show that the log likelihood ratio processes parameterized by the value of the sub-critical spike converge to Gaussian processes with logarithmic correlation. We then derive asymptotic power envelopes for tests for the presence of a spike
A multi-criteria performance study of an integrated demand/supply energy system for low and zero carbon technologies within domestic building design
When low carbon and renewable energy (RE) systems are adopted in a building, matching the outputs from RE systems (e.g. photovoltaic, solar collectors, small scale wind turbines and heat pumps) to demand has to be taken into account to fully realise the potential of the hybrid energy system. Considering the varying demand profiles due to different building design options (e.g. orientation, construction types etc), it is necessary to evaluate key technology elements in an integrated context and establish appropriate strategies for simultaneously meeting heating and electricity loads as well as matching demand and supply. This paper presents a new approach to evaluate the interactive effects of low carbon technologies and demand reduction measures in the early design stage of a new building. A case study of a sustainable domestic building project (PLUS 50), was implemented on the basis of the proposed design approach
High-Dimensional Inference with the generalized Hopfield Model: Principal Component Analysis and Corrections
We consider the problem of inferring the interactions between a set of N
binary variables from the knowledge of their frequencies and pairwise
correlations. The inference framework is based on the Hopfield model, a special
case of the Ising model where the interaction matrix is defined through a set
of patterns in the variable space, and is of rank much smaller than N. We show
that Maximum Lik elihood inference is deeply related to Principal Component
Analysis when the amp litude of the pattern components, xi, is negligible
compared to N^1/2. Using techniques from statistical mechanics, we calculate
the corrections to the patterns to the first order in xi/N^1/2. We stress that
it is important to generalize the Hopfield model and include both attractive
and repulsive patterns, to correctly infer networks with sparse and strong
interactions. We present a simple geometrical criterion to decide how many
attractive and repulsive patterns should be considered as a function of the
sampling noise. We moreover discuss how many sampled configurations are
required for a good inference, as a function of the system size, N and of the
amplitude, xi. The inference approach is illustrated on synthetic and
biological data.Comment: Physical Review E: Statistical, Nonlinear, and Soft Matter Physics
(2011) to appea
Viscous diffusion and photoevaporation of stellar disks
The evolution of a stellar disk under the influence of viscous evolution,
photoevaporation from the central source, and photoevaporation by external
stars is studied. We take the typical parameters of TTSs and the Trapezium
Cluster conditions. The photoionizing flux from the central source is assumed
to arise both from the quiescent star and accretion shocks at the base of
stellar magnetospheric columns, along which material from the disk accretes.
The accretion flux is calculated self-consistently from the accretion mass loss
rate. We find that the disk cannot be entirely removed using only viscous
evolution and photoionization from the disk-star accretion shock. However, when
FUV photoevaporation by external massive stars is included the disk is removed
in 10^6 -10^7yr; and when EUV photoevaporation by external massive stars is
included the disk is removed in 10^5 - 10^6yr.
An intriguing feature of photoevaporation by the central star is the
formation of a gap in the disk at late stages of the disk evolution. As the gap
starts forming, viscous spreading and photoevaporation work in resonance.
There is no gap formation for disks nearby external massive stars because the
outer annuli are quickly removed by the dominant EUV flux. On the other hand,
at larger, more typical distances (d>>0.03pc) from the external stars the flux
is FUV dominated. As a consequence, the disk is efficiently evaporated at two
different locations; forming a gap during the last stages of the disk
evolution.Comment: 27 pages, 11 figures, accepted for publication in Ap
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