43,812 research outputs found
SCUBA observations of the Horsehead Nebula - what did the horse swallow?
We present observations taken with SCUBA on the JCMT of the Horsehead Nebula
in Orion (B33), at wavelengths of 450 and 850 \mum. We see bright emission from
that part of the cloud associated with the photon-dominated region (PDR) at the
`top' of the horse's head, which we label B33-SMM1. We characterise the
physical parameters of the extended dust responsible for this emission, and
find that B33-SMM1 contains a more dense core than was previously suspected. We
compare the SCUBA data with data from the Infrared Space Observatory (ISO) and
find that the emission at 6.75-\mum is offset towards the west, indicating that
the mid-infrared emission is tracing the PDR while the submillimetre emission
comes from the molecular cloud core behind the PDR. We calculate the virial
balance of this core and find that it is not gravitationally bound but is being
confined by the external pressure from the HII region IC434, and that it will
either be destroyed by the ionising radiation, or else may undergo triggered
star formation. Furthermore we find evidence for a lozenge-shaped clump in the
`throat' of the horse, which is not seen in emission at shorter wavelengths. We
label this source B33-SMM2 and find that it is brighter at submillimetre
wavelengths than B33-SMM1. SMM2 is seen in absorption in the 6.75-\mum ISO
data, from which we obtain an independent estimate of the column density in
excellent agreement with that calculated from the submillimetre emission. We
calculate the stability of this core against collapse and find that it is in
approximate gravitational virial equilibrium. This is consistent with it being
a pre-existing core in B33, possibly pre-stellar in nature, but that it may
also eventually undergo collapse under the effects of the HII region.Comment: 11 pages, 6 figures, accepted by MNRA
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Theoretical calculation of the electromagnetic response of a radially layered model moon Technical report
Theoretical calculation of electromagnetic response of radially layered moon mode
Weyl corrections to holographic conductivity
For conformal field theories which admit a dual gravitational description in
anti-de Sitter space, electrical transport properties, such as conductivity and
charge diffusion, are determined by the dynamics of a U(1) gauge field in the
bulk and thus obey universality relations at the classical level due to the
uniqueness of the Maxwell action. We analyze corrections to these transport
parameters due to higher-dimension operators in the bulk action, beyond the
leading Maxwell term, of which the most significant involves a coupling to the
bulk Weyl tensor. We show that the ensuing corrections to conductivity and the
diffusion constant break the universal relation with the U(1) central charge
observed at leading order, but are nonetheless subject to interesting bounds
associated with causality in the boundary CFT.Comment: 15 pages, v2: references adde
First Observations of the Magnetic Field Geometry in Pre-stellar Cores
We present the first published maps of magnetic fields in pre-stellar cores,
to test theoretical ideas about the way in which the magnetic field geometry
affects the star formation process. The observations are JCMT-SCUBA maps of 850
micron thermal emission from dust. Linear polarizations at typically ten or
more independent positions in each of three objects, L1544, L183 and L43 were
measured, and the geometries of the magnetic fields in the plane of the sky
were mapped from the polarization directions. The observed polarizations in all
three objects appear smooth and fairly uniform. In L1544 and L183 the mean
magnetic fields are at an angle of around 30 degrees to the minor axes of the
cores. The L43 B-field appears to have been influenced in its southern half,
such that it is parallel to the wall of a cavity produced by a CO outflow from
a nearby T Tauri star, whilst in the northern half the field appears less
disturbed and has an angle of 44 degrees to the core minor axis. We briefly
compare our results with published models of magnetized cloud cores and
conclude that no current model can explain these observations simultaneously
with previous ISOCAM data.Comment: 13 pages, 3 figs, to appear in ApJ Letter
Asymptotic Methods for Metal Oxide Semiconductor Field Effect Transistor Modeling
The behavior of metal oxide semiconductor field effect transistors (MOSFETs) with small aspect ratio and large doping levels is analyzed using formal perturbation techniques. Specifically, the influence of interface layers in the potential on the averaged channel conductivity is closely examined. The interface and internal layers that occur in the potential are resolved in the limit of large doping using the method of matched asymptotic expansions. This approach, together with other asymptotic techniques, provides both a pointwise description of the state variables as well as lumped current-voltage relations that vary uniformly across the various bias regimes. These current-voltage relations are derived for a variable doping model respresenting a particular class of devices
Bounding inconsistency using a novel threshold metric for dead reckoning update packet generation
Human-to-human interaction across distributed applications requires that sufficient consistency be maintained among participants in the face of network characteristics such as latency and limited bandwidth. The level of inconsistency arising from the network is proportional to the network delay, and thus a function of bandwidth consumption. Distributed simulation has often used a bandwidth reduction technique known as dead reckoning that combines approximation and estimation in the communication of entity movement to reduce network traffic, and thus improve consistency. However, unless carefully tuned to application and network characteristics, such an approach can introduce more inconsistency than it avoids. The key tuning metric is the distance threshold. This paper questions the suitability of the standard distance threshold as a metric for use in the dead reckoning scheme. Using a model relating entity path curvature and inconsistency, a major performance related limitation of the distance threshold technique is highlighted. We then propose an alternative time—space threshold criterion. The time—space threshold is demonstrated, through simulation, to perform better for low curvature movement. However, it too has a limitation. Based on this, we further propose a novel hybrid scheme. Through simulation and live trials, this scheme is shown to perform well across a range of curvature values, and places bounds on both the spatial and absolute inconsistency arising from dead reckoning
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