54,929 research outputs found
Method for reducing snap in magnetic amplifiers
Method of reducing snap in magnetic amplifiers uses a degenerative feedback circuit consisting of a resistor and a separate winding on a magnetic core. The feedback circuit extends amplifier range by allowing it to be used at lower values of output current
Perturbations of Spatially Closed Bianchi III Spacetimes
Motivated by the recent interest in dynamical properties of topologically
nontrivial spacetimes, we study linear perturbations of spatially closed
Bianchi III vacuum spacetimes, whose spatial topology is the direct product of
a higher genus surface and the circle. We first develop necessary mode
functions, vectors, and tensors, and then perform separations of (perturbation)
variables. The perturbation equations decouple in a way that is similar to but
a generalization of those of the Regge--Wheeler spherically symmetric case. We
further achieve a decoupling of each set of perturbation equations into
gauge-dependent and independent parts, by which we obtain wave equations for
the gauge-invariant variables. We then discuss choices of gauge and stability
properties. Details of the compactification of Bianchi III manifolds and
spacetimes are presented in an appendix. In the other appendices we study
scalar field and electromagnetic equations on the same background to compare
asymptotic properties.Comment: 61 pages, 1 figure, final version with minor corrections, to appear
in Class. Quant. Gravi
Speed Limits in General Relativity
Some standard results on the initial value problem of general relativity in
matter are reviewed. These results are applied first to show that in a well
defined sense, finite perturbations in the gravitational field travel no faster
than light, and second to show that it is impossible to construct a warp drive
as considered by Alcubierre (1994) in the absence of exotic matter.Comment: 7 pages; AMS-LaTeX; accepted for publication by Classical and Quantum
Gravit
Thermal Conductivity of Single Wall Carbon Nanotubes: Diameter and Annealing Dependence
The thermal conductivity, k(T), of bulk single-wall carbon nanotubes (SWNT's)
displays a linear temperature dependence at low T that has been attributed to
1D quantization of phonons. To explore this issue further, we have measured the
k(T) of samples with varying average tube diameters. We observe linear k(T) up
to higher temperatures in samples with smaller diameters, in agreement with a
quantization picture. In addition, we have examined the effect of annealing on
k(T). We observe an enhancement in k(T) for annealed samples which we attribute
to healing of defects and removal of impurities. These measurements demonstrate
how the thermal properties of an SWNT material can be controlled by
manipulating its intrinsic nanoscale properties.Comment: Proc. of the XV. Int. Winterschool on Electronic Properties of Novel
Materials, Kirchberg/Tirol, Austria, 200
Inflowing gas onto a compact obscured nucleus in Arp 299A: Herschel spectroscopic studies of H2O and OH
Aims. We probe the physical conditions in the core of Arp 299A and try to put
constraints to the nature of its nuclear power source. Methods. We used
Herschel Space Observatory far-infrared and submillimeter observations of H2O
and OH rotational lines in Arp 299A to create a multi-component model of the
galaxy. In doing this, we employed a spherically symmetric radiative transfer
code. Results. Nine H2O lines in absorption and eight in emission as well as
four OH doublets in absorption and one in emission, are detected in Arp 299A.
No lines of the 18O isotopologues, which have been seen in compact obscured
nuclei of other galaxies, are detected. The absorption in the ground state OH
doublet at 119 {\mu}m is found redshifted by ~175 km/s compared to other OH and
H2O lines, suggesting a low excitation inflow. We find that at least two
components are required in order to account for the excited molecular line
spectrum. The inner component has a radius of 20-25 pc, a very high infrared
surface brightness (> 3e13 Lsun/kpc^2), warm dust (Td > 90 K), and a large H2
column density (NH2 > 1e24 cm^-2). The outer component is larger (50-100 pc)
with slightly cooler dust (70-90 K). In addition, a much more extended
inflowing component is required to also account for the OH doublet at 119
{\mu}m. Conclusions. The Compton-thick nature of the core makes it difficult to
determine the nature of the buried power source, but the high surface
brightness indicates that it is either an active galactic nucleus and/or a
dense nuclear starburst. The high OH/H2O ratio in the nucleus indicates that
ion-neutral chemistry induced by X-rays or cosmic-rays is important. Finally we
find a lower limit to the 16O/18O ratio of 400 in the nuclear region, possibly
indicating that the nuclear starburst is in an early evolutionary stage, or
that it is fed through a molecular inflow of, at most, solar metallicity.Comment: 14 pages, 13 figures, Accepted for publication in Astronomy and
Astrophysic
Modeling the H2O submillimeter emission in extragalactic sources
Recent observational studies have shown that H2O emission at (rest)
submillimeter wavelengths is ubiquitous in infrared galaxies, both in the local
and in the early Universe, suggestive of far-infrared pumping of H2O by dust in
warm regions. In this work, models are presented that show that (i) the
highest-lying H2O lines (E_{upper}>400 K) are formed in very warm (T_{dust}>~90
K) regions and require high H2O columns (N_{H2O}>~3x10^{17} cm^{-2}), while
lower lying lines can be efficiently excited with T_{dust}~45-75 K and
N_{H2O}~(0.5-2)x10^{17} cm^{-2}; (ii) significant collisional excitation of the
lowest lying (E_{upper}<200 K) levels, which enhances the overall
L_{H2O}-L_{IR} ratios, is identified in sources where the ground-state para-H2O
1_{11}-0_{00} line is detected in emission; (iii) the H2O-to-infrared (8-1000
um) luminosity ratio is expected to decrease with increasing T_{dust} for all
lines with E_{upper}<~300 K, as has recently been reported in a sample of
LIRGs, but increases with T_{dust} for the highest lying H2O lines
(E_{upper}>400 K); (iv) we find theoretical upper limits for L_{H2O}/L_{IR} in
warm environments, owing to H2O line saturation; (v) individual models are
presented for two very different prototypical galaxies, the Seyfert 2 galaxy
NGC 1068 and the nearest ultraluminous infrared galaxy Arp 220, showing that
the excited submillimeter H2O emission is dominated by far-infrared pumping in
both cases; (vi) the L_{H2O}-L_{IR} correlation previously reported in
observational studies indicates depletion or exhaustion time scales,
t_{dep}=Sigma_{gas}/Sigma_{SFR}, of <~12 Myr for star-forming sources where
lines up to E_{upper}=300 K are detected, in agreement with the values
previously found for (U)LIRGs from HCN millimeter emission...Comment: 13 pages, 13 figure
Emissivity measurements of reflective surfaces at near-millimeter wavelengths
We have developed an instrument for directly measuring the emissivity of reflective surfaces at near-millimeter wavelengths. The thermal emission of a test sample is compared with that of a reference surface, allowing the emissivity of the sample to be determined without heating. The emissivity of the reference surface is determined by one’s heating the reference surface and measuring the increase in emission. The instrument has an absolute accuracy of Δe = 5 x 10^-4 and can reproducibly measure a difference in emissivity as small as Δe = 10^-4 between flat reflective samples. We have used the instrument to measure the emissivity of metal films evaporated on glass and carbon fiber-reinforced plastic composite surfaces. We measure an emissivity of (2.15 ± 0.4) x 10^-3 for gold evaporated on glass and (2.65 ± 0.5) x 10^-3 for aluminum evaporated on carbon fiber-reinforced plastic composite
The nonrelativistic limit of Dirac-Fock codes: the role of Brillouin configurations
We solve a long standing problem with relativistic calculations done with the
widely used Multi-Configuration Dirac-Fock Method (MCDF). We show, using
Relativistic Many-Body Perturbation Theory (RMBPT), how even for relatively
high-, relaxation or correlation causes the non-relativistic limit of states
of different total angular momentum but identical orbital angular momentum to
have different energies. We show that only large scale calculations that
include all single excitations, even those obeying the Brillouin's theorem have
the correct limit. We reproduce very accurately recent high-precision
measurements in F-like Ar, and turn then into precise test of QED. We obtain
the correct non-relativistic limit not only for fine structure but also for
level energies and show that RMBPT calculations are not immune to this problem.Comment: AUgust 9th, 2004 Second version Nov. 18th, 200
- …