4,353 research outputs found
A survey of the UK benefit system
This paper describes all the main benefits in the UK system, giving details of rates and allowances, as well as numbers and types of claimants and levels of expenditure
The Kinematic and Plasma Properties of X-ray Knots in Cassiopeia A from the Chandra HETGS
We present high-resolution X-ray spectra from the young supernova remnant Cas
A using a 70-ks observation taken by the Chandra High Energy Transmission
Grating Spectrometer (HETGS). Line emission, dominated by Si and S ions, is
used for high-resolution spectral analysis of many bright, narrow regions of
Cas A to examine their kinematics and plasma state. These data allow a 3D
reconstruction using the unprecedented X-ray kinematic results: we derive
unambiguous Doppler shifts for these selected regions, with values ranging
between -2500 and +4000 km/s. Plasma diagnostics of these regions, derived from
line ratios of resolved He-like triplet lines and H-like lines of Si, indicate
temperatures largely around 1 keV, which we model as O-rich reverse-shocked
ejecta. The ionization age also does not vary considerably over these regions
of the remnant. The gratings analysis was complemented by the non-dispersed
spectra from the same dataset, which provided information on emission measure
and elemental abundances for the selected Cas A regions. The derived electron
density of X-ray emitting ejecta varies from 20 to 200 cm^{-3}. The measured
abundances of Mg, Si, S and Ca are consistent with O being the dominant element
in the Cas A plasma. With a diameter of 5 arcmin, Cas A is the largest source
observed with the HETGS to date. We, therefore, describe the technique we use
and some of the challenges we face in the HETGS data reduction from such an
extended, complex object.Comment: 26 pages, 16 figures, evised version (minor changes), accepted for
publication in ApJ (Oct 20 2006
Revisiting Multi-Subject Random Effects in fMRI: Advocating Prevalence Estimation
Random Effects analysis has been introduced into fMRI research in order to
generalize findings from the study group to the whole population. Generalizing
findings is obviously harder than detecting activation in the study group since
in order to be significant, an activation has to be larger than the
inter-subject variability. Indeed, detected regions are smaller when using
random effect analysis versus fixed effects. The statistical assumptions behind
the classic random effects model are that the effect in each location is
normally distributed over subjects, and "activation" refers to a non-null mean
effect. We argue this model is unrealistic compared to the true population
variability, where, due to functional plasticity and registration anomalies, at
each brain location some of the subjects are active and some are not. We
propose a finite-Gaussian--mixture--random-effect. A model that amortizes
between-subject spatial disagreement and quantifies it using the "prevalence"
of activation at each location. This measure has several desirable properties:
(a) It is more informative than the typical active/inactive paradigm. (b) In
contrast to the hypothesis testing approach (thus t-maps) which are trivially
rejected for large sample sizes, the larger the sample size, the more
informative the prevalence statistic becomes.
In this work we present a formal definition and an estimation procedure of
this prevalence. The end result of the proposed analysis is a map of the
prevalence at locations with significant activation, highlighting activations
regions that are common over many brains
Asphericity and clumpiness in the winds of Luminous Blue Variables
We present the first systematic spectropolarimetric study of Luminous Blue
Variables (LBVs) in the Galaxy and the Magellanic Clouds, in order to
investigate the geometries of their winds. We find that at least half of our
sample show changes in polarization across the strong H emission line,
indicating that the light from the stars is intrinsically polarized and
therefore that asphericity already exists at the base of the wind. Multi-epoch
spectropolarimetry on four targets reveals variability in their intrinsic
polarization. Three of these, AG Car, HR Car and P Cyg, show a position angle
(PA) of polarization which appears random with time. Such behaviour can be
explained by the presence of strong wind-inhomogeneities, or `clumps' within
the wind. Only one star, R 127, shows variability at a constant PA, and hence
evidence for axi-symmetry as well as clumpiness. However, if viewed at low
inclination, and at limited temporal sampling, such a wind would produce a
seemingly random polarization of the type observed in the other three stars.
Time-resolved spectropolarimetric monitoring of LBVs is therefore required to
determine if LBV winds are axi-symmetric in general.
The high fraction of LBVs ( 50%) showing intrinsic polarization is to be
compared with the lower 20-25 % for similar studies of their
evolutionary neighbours, O supergiants and Wolf-Rayet stars. We anticipate that
this higher incidence is due to the lower effective gravities of the LBVs,
coupled with their variable temperatures within the bi-stability jump regime.
This is also consistent with the higher incidence of wind asphericity that we
find in LBVs with strong H emission and recent (last 10 years)
strong variability.Comment: 20 pages, 12 figures, accepted by A&
Combining Insertion and Deletion in RNA-editing Preserves Regularity
Inspired by RNA-editing as occurs in transcriptional processes in the living
cell, we introduce an abstract notion of string adjustment, called guided
rewriting. This formalism allows simultaneously inserting and deleting
elements. We prove that guided rewriting preserves regularity: for every
regular language its closure under guided rewriting is regular too. This
contrasts an earlier abstraction of RNA-editing separating insertion and
deletion for which it was proved that regularity is not preserved. The
particular automaton construction here relies on an auxiliary notion of slice
sequence which enables to sweep from left to right through a completed rewrite
sequence.Comment: In Proceedings MeCBIC 2012, arXiv:1211.347
Probing the evolving massive star population in Orion with kinematic and radioactive tracers
We assemble a census of the most massive stars in Orion, then use stellar
isochrones to estimate their masses and ages, and use these results to
establish the stellar content of Orion's individual OB associations. From this,
our new population synthesis code is utilized to derive the history of the
emission of UV radiation and kinetic energy of the material ejected by the
massive stars, and also follow the ejection of the long-lived radioactive
isotopes 26Al and 60Fe. In order to estimate the precision of our method, we
compare and contrast three distinct representations of the massive stars. We
compare the expected outputs with observations of 26Al gamma-ray signal and the
extent of the Eridanus cavity. We find an integrated kinetic energy emitted by
the massive stars of 1.8(+1.5-0.4)times 10^52 erg. This number is consistent
with the energy thought to be required to create the Eridanus superbubble. We
also find good agreement between our model and the observed 26Al signal,
estimating a mass of 5.8(+2.7-2.5) times 10^-4 Msol of 26Al in the Orion
region. Our population synthesis approach is demonstrated for the Orion region
to reproduce three different kinds of observable outputs from massive stars in
a consistent manner: Kinetic energy as manifested in ISM excavation, ionization
as manifested in free-free emission, and nucleosynthesis ejecta as manifested
in radioactivity gamma-rays. The good match between our model and the
observables does not argue for considerable modifications of mass loss. If
clumping effects turn out to be strong, other processes would need to be
identified to compensate for their impact on massive-star outputs. Our
population synthesis analysis jointly treats kinematic output and the return of
radioactive isotopes, which proves a powerful extension of the methodology that
constrains feedback from massive stars.Comment: Accepted for publication in A&A, 10 page
Cesarean in the second stage: a possible risk factor for subsequent spontaneous preterm birth.
Fluids with quenched disorder: Scaling of the free energy barrier near critical points
In the context of Monte Carlo simulations, the analysis of the probability
distribution of the order parameter , as obtained in simulation
boxes of finite linear extension , allows for an easy estimation of the
location of the critical point and the critical exponents. For Ising-like
systems without quenched disorder, becomes scale invariant at the
critical point, where it assumes a characteristic bimodal shape featuring two
overlapping peaks. In particular, the ratio between the value of at
the peaks () and the value at the minimum in-between ()
becomes -independent at criticality. However, for Ising-like systems with
quenched random fields, we argue that instead should be observed, where is the
"violation of hyperscaling" exponent. Since is substantially non-zero,
the scaling of with system size should be easily detectable in
simulations. For two fluid models with quenched disorder, versus
was measured, and the expected scaling was confirmed. This provides further
evidence that fluids with quenched disorder belong to the universality class of
the random-field Ising model.Comment: sent to J. Phys. Cond. Mat
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