8,765 research outputs found
Radio emission models of Colliding-Wind Binary Systems
We present calculations of the spatial and spectral distribution of the radio
emission from a wide WR+OB colliding-wind binary system based on
high-resolution hydrodynamical simulations and solutions to the radiative
transfer equation. We account for both thermal and synchrotron radio emission,
free-free absorption in both the unshocked stellar wind envelopes and the
shocked gas, synchrotron self-absorption, and the Razin effect. The
applicability of these calculations to modelling radio images and spectra of
colliding-wind systems is demonstrated with models of the radio emission from
the wide WR+OB binary WR147. Its synchrotron spectrum follows a power-law
between 5 and 15 GHz but turns down to below this at lower and higher
frequencies. We find that while free-free opacity from the circum-binary
stellar winds can potentially account for the low-frequency turnover, models
that also include a combination of synchrotron self-absorption and Razin effect
are favoured. We argue that the high-frequency turn down is a consequence of
inverse-Compton cooling. We present our resulting spectra and intensity
distributions, along with simulated MERLIN observations of these intensity
distributions. From these we argue that the inclination of the WR147 system to
the plane of the sky is low. We summarise by considering extensions of the
current model that are important for models of the emission from closer
colliding wind binaries, in particular the dramatically varying radio emission
of WR140.Comment: 18 pages, 18 figures; Accepted by Astronomy and Astrophysics, July 8,
200
Cardiovascular disease in a cohort exposed to the 1940-45 Channel Islands occupation
BACKGROUND
To clarify the nature of the relationship between food deprivation/undernutrition during pre- and postnatal development and cardiovascular disease (CVD) in later life, this study examined the relationship between birth weight (as a marker of prenatal nutrition) and the incidence of hospital admissions for CVD from 1997–2005 amongst 873 Guernsey islanders (born in 1923–1937), 225 of whom had been exposed to food deprivation as children, adolescents or young adults (i.e. postnatal undernutrition) during the 1940–45 German occupation of the Channel Islands, and 648 of whom had left or been evacuated from the islands before the occupation began.
METHODS
Three sets of Cox regression models were used to investigate (A) the relationship between birth weight and CVD, (B) the relationship between postnatal exposure to the occupation and CVD and (C) any interaction between birth weight, postnatal exposure to the occupation and CVD. These models also tested for any interactions between birth weight and sex, and postnatal exposure to the occupation and parish of residence at birth (as a marker of parish residence during the occupation and related variation in the severity of food deprivation).
RESULTS
The first set of models (A) found no relationship between birth weight and CVD even after adjustment for potential confounders (hazard ratio (HR) per kg increase in birth weight: 1.12; 95% confidence intervals (CI): 0.70 – 1.78), and there was no significant interaction between birth weight and sex (p = 0.60). The second set of models (B) found a significant relationship between postnatal exposure to the occupation and CVD after adjustment for potential confounders (HR for exposed vs. unexposed group: 2.52; 95% CI: 1.54 – 4.13), as well as a significant interaction between postnatal exposure to the occupation and parish of residence at birth (p = 0.01), such that those born in urban parishes (where food deprivation was worst) had a greater HR for CVD than those born in rural parishes. The third model (C) found no interaction between birth weight and exposure to the occupation (p = 0.43).
CONCLUSION
These findings suggest that the levels of postnatal undernutrition experienced by children, adolescents and young adults exposed to food deprivation during the 1940–45 occupation of the Channel Islands were a more important determinant of CVD in later life than the levels of prenatal undernutrition experienced in utero prior to the occupatio
Cosmic Ray Acceleration at the Forward Shock in Tycho's Supernova Remnant: Evidence from Chandra X-ray Observations
We present evidence for cosmic ray acceleration at the forward shock in
Tycho's supernova remnant (SNR) from three X-ray observables: (1) the proximity
of the contact discontinuity to the forward shock, or blast wave, (2) the
morphology of the emission from the rim of Tycho, and (3) the spectral nature
of the rim emission. We determine the locations of the blast wave (BW), contact
discontinuity (CD), and reverse shock (RS) around the rim of Tycho's supernova
remnant using a principal component analysis and other methods applied to new
Chandra data. The azimuthal-angle-averaged radius of the BW is 251". For the CD
and RS we find average radii of 241" and 183", respectively. Taking account of
projection effects, we find ratios of 1:0.93:0.70 (BW:CD:RS). We show these
values to be inconsistent with adiabatic hydrodynamical models of SNR
evolution. The CD:BW ratio can be explained if cosmic ray acceleration of ions
is occurring at the forward shock. The RS:BW ratio, as well as the strong Fe Ka
emission from the Tycho ejecta, imply that the RS is not accelerating cosmic
rays. We also extract radial profiles from ~34% of the rim of Tycho and compare
them to models of surface brightness profiles behind the BW for a purely
thermal plasma with an adiabatic shock. The observed morphology of the rim is
much more strongly peaked than predicted by the model, indicating that such
thermal emission is implausible here. Spectral analysis also implies that the
rim emission is non-thermal in nature, lending further support to the idea that
Tycho's forward shock is accelerating cosmic rays.Comment: 39 pages, 10 figures, accepted by Ap
Stereoscopic electron spectroscopy of solar hard X-ray flares with a single spacecraft
Hard X-ray (HXR) spectroscopy is the most direct method of diagnosing
energetic electrons in solar flares. Here we present a technique which allows
us to use a single HXR spectrum to determine an effectively stereoscopic
electron energy distribution. Considering the Sun's surface to act as a
'Compton mirror' allows us to look at emitting electrons also from behind the
source, providing vital information on downward-propagating particles. Using
this technique we determine simultaneously the electron spectra of downward and
upward directed electrons for two solar flares observed by the Ramaty High
Energy Solar Spectroscopic Imager (RHESSI). The results reveal surprisingly
near-isotropic electron distributions, which contrast strongly with the
expectations from the standard model which invokes strong downward beaming,
including collisional thick-target model.Comment: 7 pages, 3 figures, accepted to Astrophysical Journal Letter
The effect of galaxy mass ratio on merger--driven starbursts
We employ numerical simulations of galaxy mergers to explore the effect of
galaxy mass ratio on merger--driven starbursts. Our numerical simulations
include radiative cooling of gas, star formation, and stellar feedback to
follow the interaction and merger of four disk galaxies. The galaxy models span
a factor of 23 in total mass and are designed to be representative of typical
galaxies in the local Universe. We find that the merger--driven star formation
is a strong function of merger mass ratio, with very little, if any, induced
star formation for large mass ratio mergers. We define a burst efficiency that
is useful to characterize the merger--driven star formation and test that it is
insensitive to uncertainties in the feedback parameterization. In accord with
previous work we find that the burst efficiency depends on the structure of the
primary galaxy. In particular, the presence of a massive stellar bulge
stabilizes the disk and suppresses merger--driven star formation for large mass
ratio mergers. Direct, co--planar merging orbits produce the largest tidal
disturbance and yield that most intense burst of star formation. Contrary to
naive expectations, a more compact distribution of gas or an increased gas
fraction both decrease the burst efficiency. Owing to the efficient feedback
model and the newer version of SPH employed here, the burst efficiencies of the
mergers presented here are smaller than in previous studies.Comment: 26 pages, 21 figures, submitted to MNRA
Many Roads to Synchrony: Natural Time Scales and Their Algorithms
We consider two important time scales---the Markov and cryptic orders---that
monitor how an observer synchronizes to a finitary stochastic process. We show
how to compute these orders exactly and that they are most efficiently
calculated from the epsilon-machine, a process's minimal unifilar model.
Surprisingly, though the Markov order is a basic concept from stochastic
process theory, it is not a probabilistic property of a process. Rather, it is
a topological property and, moreover, it is not computable from any
finite-state model other than the epsilon-machine. Via an exhaustive survey, we
close by demonstrating that infinite Markov and infinite cryptic orders are a
dominant feature in the space of finite-memory processes. We draw out the roles
played in statistical mechanical spin systems by these two complementary length
scales.Comment: 17 pages, 16 figures:
http://cse.ucdavis.edu/~cmg/compmech/pubs/kro.htm. Santa Fe Institute Working
Paper 10-11-02
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