34,774 research outputs found
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CO J = 3→2 and J = 2→1 mapping and spectroscopy of NGC 7027
We present spectra and mapping for NGC 7072 in the J = 3→2 and J = 2→1 transitions of CO. The central profile at J = 2→1 is shown to be very similar to the J = 1→0 spectrum measured by Thronson (1983), and this implies a source expansion at roughly constant velocity. The J = 3→2 line however appears weaker, with evidence for appreciable quenching of the higher velocity components. Detailed modelling f the source indicates that densities n must vary appreciably with shell radius R(as nα R-a, where α≥2), and this leads to a corresponding steep radial decrease in the radiation temperature TR. In consequence, the source FWHM is found to decrease appreciably iwth increasing transition frequency, a trend which appears also to be confirmed by our central J = 3→2 scans. It is not however possible to constrain gas kinetic tempertures TK, the level of CO thermalisation, or shell mass M with any degree of confidence - both low and high mass models appear capable of replicating our spectra.
Finally, the J = 2→1 spatial velocity map displays evidence for a decrease in velocity width towards the outer regions of the nebula; a feature which is expected of most outflow models. The J = 3→2 map also indicates the presence of a nebular extension to the north-west of the peak emission core, although this is not reproduced in the corresponding J = 1→0 map of Mufson et al. (1975)
Asymptotic solution of a model for bilayer organic diodes and solar cells
The current voltage characteristics of an organic semiconductor diode made by placing together two materials with dissimilar electron affinities and ionisation potentials is analysed using asymptotic methods. An intricate boundary layer structure is examined. We find that there are three regimes for the total current passing through the diode. For reverse bias and moderate forward bias the dependency of the voltage on the current is similar to the behaviour of conventional inorganic semiconductor diodes predicted by the Shockley equation and are governed by recombination at the interface of the materials. There is then a narrow range of currents where the behaviour undergoes a transition. Finally for large forward bias the behaviour is different with the current being linear in voltage and is primarily controlled by drift of charges in the organic layers. The size of the interfacial recombination rate is critical in determining the small range of current where there is rapid transition between the two main regimes. The extension of the theory to organic solar cells is discussed and the analogous current voltage curves derived in the regime of interest
A fast and robust numerical scheme for solving models of charge carrier transport and ion vacancy motion in perovskite solar cells
Drift-diffusion models that account for the motion of both electronic and
ionic charges are important tools for explaining the hysteretic behaviour and
guiding the development of metal halide perovskite solar cells. Furnishing
numerical solutions to such models for realistic operating conditions is
challenging owing to the extreme values of some of the parameters. In
particular, those characterising (i) the short Debye lengths (giving rise to
rapid changes in the solutions across narrow layers), (ii) the relatively large
potential differences across devices and (iii) the disparity in timescales
between the motion of the electronic and ionic species give rise to significant
stiffness. We present a finite difference scheme with an adaptive time step
that is posed on a non-uniform staggered grid that provides second order
accuracy in the mesh spacing. The method is able to cope with the stiffness of
the system for realistic parameters values whilst providing high accuracy and
maintaining modest computational costs. For example, a transient sweep of a
current-voltage curve can be computed in only a few minutes on a standard
desktop computer.Comment: 22 pages, 8 figure
Coboson formalism for Cooper pairs used to derive Richardson's equations
We propose a many-body formalism for Cooper pairs which has similarities to
the one we recently developed for composite boson excitons (coboson in short).
Its Shiva diagram representation evidences that Cooper pairs differ from
single pairs through electron exchange only: no direct coupling exists due
to the very peculiar form of the BCS potential. As a first application, we here
use this formalism to derive Richardson's equations for the exact eigenstates
of Cooper pairs. This gives hints on why the dependence of the
-pair ground state energy we recently obtained by solving Richardson's
equations analytically in the low density limit, stays valid up to the dense
regime, no higher order dependence exists even under large overlap, a
surprising result hard to accept at first. We also briefly question the BCS
wave function ansatz compared to Richardson's exact form, in the light of our
understanding of coboson many-body effects
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Near infrared spectroscopy of W51 IRS-2
Near-infrared spectra at 2.95-3.5 μm and 3.99-10 μm have been obtained towards W51 IRS-2 and its surroundings, in order to investigate the spatial variations in intensity of the 3.28 μm unidentified feature and the 4.05 μm Brackett-α line. The Br-α and 3.28 μm features occupy a broadly similar spatial zone, which is characterised by an unresolved core responsible for most of the emission, and an extended and considerably weaker halo. Grain properties required to excite the 4.28 microns line, the nature of the 3.28 μm emission, and its relation to the source structure are discussed
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A high resolution millimetre and submillimetre study of W3
The continuum bolometer receiver on the James Clerk Maxwell telescope has been used to map the dense core of the star formation region W3 with a spatial resolution of 15-20 arcsec. At 350 and 800 μm, the region appears as two principal peaks around the known IR sources IRS4 and IRS5, while at 1100 μm, a further peak is noted which is interpreted as being due to free-free emission around IRS2. Taking into account the free-free contribution to the intensity, the continuum dust emission from the region is found to be consistent with optically thin emission at all of the three wavelengths considered. Values for the dust optical depth, hydrogen column density, mass, and central density have been obtained for each of the main peaks
Economic evaluation of a nursing-led intermediate care unit
Objectives: The aim of this paper is to examine the costs of introducing a nursing-led ward program together with examining the impact this may have on patients' outcomes. Methods; The study had a sample size of 177 patients with a mean age of 77, and randomized to either a treatment group (care on a nursing-led ward, n = 97) or a control group (standard care usually on a consultant-led acute ward, n = 80). Resource use data including length of stay, tests and investigations performed, and multidisciplinary involvement in care were collected. Results: There were no significant differences in outcome between the two groups. The inpatient costs for the treatment group were significantly higher, due to the longer length of stay in this group. However, the postdischarge costs were significantly lower for the treatment group. Conclusions: The provision of nursing-led intermediate care units has been proposed as a solution to inappropriate use of acute medical wards by patients who require additional nursing rather than medical care. Whether the treatment group is ultimately cost-additive is dependent on how long reductions in postdischarge resource use are maintained
ECONOMIC IMPACTS OF THE 1981 AGRICULTURAL ACT AND THE 1981 TAX ACT ON TEXAS HIGH PLAINS FARMERS
Political Economy,
Particle Acceleration in Relativistic Jets due to Weibel Instability
Shock acceleration is an ubiquitous phenomenon in astrophysical plasmas.
Plasma waves and their associated instabilities (e.g., the Buneman instability,
two-streaming instability, and the Weibel instability) created in the shocks
are responsible for particle (electron, positron, and ion) acceleration. Using
a 3-D relativistic electromagnetic particle (REMP) code, we have investigated
particle acceleration associated with a relativistic jet front propagating
through an ambient plasma with and without initial magnetic fields. We find
only small differences in the results between no ambient and weak ambient
magnetic fields. Simulations show that the Weibel instability created in the
collisionless shock front accelerates particles perpendicular and parallel to
the jet propagation direction. While some Fermi acceleration may occur at the
jet front, the majority of electron acceleration takes place behind the jet
front and cannot be characterized as Fermi acceleration. The simulation results
show that this instability is responsible for generating and amplifying highly
nonuniform, small-scale magnetic fields, which contribute to the electron's
transverse deflection behind the jet head. The ``jitter'' radiation (Medvedev
2000) from deflected electrons has different properties than synchrotron
radiation which is calculated in a uniform magnetic field. This jitter
radiation may be important to understanding the complex time evolution and/or
spectral structure in gamma-ray bursts, relativistic jets, and supernova
remnants.Comment: ApJ, in press, Sept. 20, 2003 (figures with better resolution:
http://gammaray.nsstc.nasa.gov/~nishikawa/apjweib.pdf
Particle Acceleration and Magnetic Field Generation in Electron-Positron Relativistic Shocks
Shock acceleration is an ubiquitous phenomenon in astrophysical plasmas.
Plasma waves and their associated instabilities (e.g., Buneman, Weibel and
other two-stream instabilities) created in collisionless shocks are responsible
for particle (electron, positron, and ion) acceleration. Using a 3-D
relativistic electromagnetic particle (REMP) code, we have investigated
particle acceleration associated with a relativistic electron-positron jet
front propagating into an ambient electron-positron plasma with and without
initial magnetic fields. We find small differences in the results for no
ambient and modest ambient magnetic fields. New simulations show that the
Weibel instability created in the collisionless shock front accelerates jet and
ambient particles both perpendicular and parallel to the jet propagation
direction. Furthermore, the non-linear fluctuation amplitudes of densities,
currents, electric, and magnetic fields in the electron-positron shock are
larger than those found in the electron-ion shock studied in a previous paper
at the comparable simulation time. This comes from the fact that both electrons
and positrons contribute to generation of the Weibel instability. Additionally,
we have performed simulations with different electron skin depths. We find that
growth times scale inversely with the plasma frequency, and the sizes of
structures created by the Weibel instability scale proportional to the electron
skin depth. This is the expected result and indicates that the simulations have
sufficient grid resolution. The simulation results show that the Weibel
instability is responsible for generating and amplifying nonuniform,
small-scale magnetic fields which contribute to the electron's (positron's)
transverse deflection behind the jet head.Comment: 18 pages, 8 figures, revised and accepted for ApJ, A full resolution
of the paper can be found at
http://gammaray.nsstc.nasa.gov/~nishikawa/apjep1.pd
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