71,487 research outputs found
Graded Lie algebras with finite polydepth
If A is a graded connected algebra then we define a new invariant, polydepth
A, which is finite if for some A-module M of at most
polynomial growth. Theorem 1: If f : X \to Y is a continuous map of finite
category, and if the orbits of H_*(\Omega Y) acting in the homology of the
homotopy fibre grow at most polynomially, then H_*(\Omega Y) has finite
polydepth. Theorem 2: If L is a graded Lie algebra and polydepth UL is finite
then either L is solvable and UL grows at most polynomially or else for some
integer d and all r, , some
Understanding the effects of geometry and rotation on pulsar intensity profiles
We have developed a method to compute the possible distribution of radio
emission regions in a typical pulsar magnetosphere, taking into account the
viewing geometry and rotational effects of the neutron star. Our method can
estimate the emission altitude and the radius of curvature of particle
trajectory as a function of rotation phase for a given inclination angle,
impact angle, spin-period, Lorentz factor, field line constant and the
observation frequency. Further, using curvature radiation as the basic emission
mechanism, we simulate the radio intensity profiles that would be observed from
a given distribution of emission regions, for different values of radio
frequency and Lorentz factor. We show clearly that rotation effects can
introduce significant asymmetries into the observed radio profiles. We
investigate the dependency of profile features on various pulsar parameters. We
find that the radiation from a given ring of field lines can be seen over a
large range of pulse longitudes, originating at different altitudes, with
varying spectral intensity. Preferred heights of emission along discrete sets
of field lines are required to reproduce realistic pulsar profiles, and we
illustrate this for a known pulsar. Finally, we show how our model provides
feasible explanations for the origin of core emission, and also for one-sided
cones which have been observed in some pulsars.Comment: 21 pages, 11 figures, accepted for publication in MNRA
Extended Red Emission and the evolution of carbonaceaous nanograins in NGC 7023
Extended Red Emission (ERE) was recently attributed to the photo-luminescence
of either doubly ionized Polycyclic Aromatic Hydrocarbons (PAH), or
charged PAH dimers. We analysed the visible and mid-infrared (mid-IR) dust
emission in the North-West and South photo-dissociation regions of the
reflection nebula NGC 7023.Using a blind signal separation method, we extracted
the map of ERE from images obtained with the Hubble Space Telescope, and at the
Canada France Hawaii Telescope. We compared the extracted ERE image to the
distribution maps of the mid-IR emission of Very Small Grains (VSGs), neutral
and ionized PAHs (PAH and PAH) obtained with the Spitzer Space
Telescope and the Infrared Space Observatory. ERE is dominant in transition
regions where VSGs are being photo-evaporated to form free PAH molecules, and
is not observed in regions dominated by PAH. Its carrier makes a minor
contribution to the mid-IR emission spectrum. These results suggest that the
ERE carrier is a transition species formed during the destruction of VSGs.
Singly ionized PAH dimers appear as good candidates but PAH molecules
seem to be excluded.Comment: Accepted for publication in A&
Gamma-ray emission from globular clusters
Over the last few years, the data obtained using the Large Area Telescope
(LAT) aboard the Fermi Gamma-ray Space Telescope has provided new insights on
high-energy processes in globular clusters, particularly those involving
compact objects such as Millisecond Pulsars (MSPs). Gamma-ray emission in the
100 MeV to 10 GeV range has been detected from more than a dozen globular
clusters in our galaxy, including 47 Tucanae and Terzan 5. Based on a sample of
known gamma-ray globular clusters, the empirical relations between gamma-ray
luminosity and properties of globular clusters such as their stellar encounter
rate, metallicity, and possible optical and infrared photon energy densities,
have been derived. The measured gamma-ray spectra are generally described by a
power law with a cut-off at a few gigaelectronvolts. Together with the
detection of pulsed gamma-rays from two MSPs in two different globular
clusters, such spectral signature lends support to the hypothesis that
gamma-rays from globular clusters represent collective curvature emission from
magnetospheres of MSPs in the clusters. Alternative models, involving
Inverse-Compton (IC) emission of relativistic electrons that are accelerated
close to MSPs or pulsar wind nebula shocks, have also been suggested.
Observations at >100 GeV by using Fermi/LAT and atmospheric Cherenkov
telescopes such as H.E.S.S.-II, MAGIC-II, VERITAS, and CTA will help to settle
some questions unanswered by current data.Comment: 11 pages, 7 figures, 2 tables, J. Astron. Space Sci., in pres
Macro aerodynamic devices controlled by micro systems
Micro-ElectroMechanical-Systems (MEMS) have emerged as a major enabling technology across the engineering disciplines. In this study, the possibility of applying MEMS to the aerodynamic field was explored. We have demonstrated that microtransducers can be used to control the motion of a delta wing in a wind tunnel and can even maneuver a scaled aircraft in flight tests. The main advantage of using micro actuators to replace the traditional control surface is the significant reduction of radar cross-sections. At a high angle of attack, a large portion of the suction loading on a delta wing is contributed by the leading edge separation vortices which originate from thin boundary layers at the leading edge. We used microactuators with a thickness comparable to that of the boundary layer in order to alter the separation process and thus achieved control of the global motion by minute perturbations
Fourier's Law for Quasi One--Dimensional Chaotic Quantum Systems
We derive Fourier's law for a completely coherent quasi one--dimensional
chaotic quantum system coupled locally to two heat baths at different
temperatures. We solve the master equation to first order in the temperature
difference. We show that the heat conductance can be expressed as a
thermodynamic equilibrium coefficient taken at some intermediate temperature.
We use that expression to show that for temperatures large compared to the mean
level spacing of the system, the heat conductance is inversely proportional to
the level density and, thus, inversely proportional to the length of the
system
Real-time diagnostics of gas/water assisted injection moulding using integrated ultrasonic sensors
YesAn ultrasound sensor system has been applied to the mould of both the water and gas assisted
injection moulding processes. The mould has a cavity wall mounted pressure sensor and instrumentation to
monitor the injection moulding machine. Two ultrasound sensors are used to monitor the arrival of the fluid
(gas or water) bubble tip through the detection of reflected ultrasound energy from the fluid polymer
boundary and the fluid bubble tip velocity through the polymer melt is estimated. The polymer contact with
the cavity wall is observed through the reflected ultrasound energy from that boundary. A theoretically
based estimation of the residual wall thickness is made using the ultrasound reflection from the fluid (gas or
water) polymer boundary whilst the samples are still inside the mould and a good correlation with a physical
measurement is observed
SUMS experiment flight results on STS-35
Calibrated pressure measurements for species with mass to charge ratios up to 50 amu/e(-) were obtained from the Shuttle Upper Atmosphere Mass Spectrometer (SUMS) experiment during reentry on the STS-35 mission. Data were collected from 180 km, when the signal rose above the background, to about 87 km, when the SUMS system automatically closed the gas inlet value. However, data above 115 km was contaminated from a source of gas emanating from pressure transducers connected in parallel to the mass spectrometer. At lower altitudes, the pressure transducer data is compared with the mass spectrometer total pressure with excellent agreement. The free-stream density in the rarefied flow flight regime is calculated using an orifice pressure coefficient model based upon direct simulation Monte Carlo results. This density, when compared with the 1976 U.S. standard atmosphere model, exhibits the wave-like nature seen on previous flights using accelerometry. In addition, selected spectra are presented at higher altitudes (320 km) showing the effects of the ingestion of gases from a forward fuselage fuel dump. An analysis of the spectra data from this event is presented to show that no significant permanent changes occurred which affected the data interpretation at lower altitudes. Further, the localized chemistry from the individual species during the onset of aerodynamic heating is examined to the extent possible for a closed source system, such as SUMS. Near the orifice entrance, a significant amount of CO2 was generated from chemical reactions with the carbon panels of the Orbiter and absorbed oxygen on the system tubing
- …