22,872 research outputs found
Stellar Population Models and Individual Element Abundances I: Sensitivity of Stellar Evolution Models
Integrated light from distant galaxies is often compared to stellar
population models via the equivalent widths of spectral features--spectral
indices--whose strengths rely on the abundances of one or more elements. Such
comparisons hinge not only on the overall metal abundance but also on relative
abundances. Studies have examined the influence of individual elements on
synthetic spectra but little has been done to address similar issues in the
stellar evolution models that underlie most stellar population models. Stellar
evolution models will primarily be influenced by changes in opacities. In order
to explore this issue in detail, twelve sets of stellar evolution tracks and
isochrones have been created at constant heavy element mass fraction Z that
self-consistently account for varying heavy element mixtures. These sets
include scaled-solar, alpha-enhanced, and individual cases where the elements
C, N, O, Ne, Mg, Si, S, Ca, Ti, and Fe have been enhanced above their
scaled-solar values. The variations that arise between scaled-solar and the
other cases are examined with respect to the H-R diagram and main sequence
lifetimes.Comment: 33 pages, 13 figures, accepted to Ap
Geometric and Measure-Theoretic Shrinking Targets in Dynamical Systems
We consider both geometric and measure-theoretic shrinking targets for
ergodic maps, investigating when they are visible or invisible. Some Baire
category theorems are proved, and particular constructions are given when the
underlying map is fixed. Open questions about shrinking targets are also
described
High efficiency near diffraction-limited mid-infrared flat lenses based on metasurface reflectarrays
A limiting factor in the development of mid-infrared optics is the lack of
abundant materials that are transparent, low cost, lightweight, and easy to
machine. In this paper, we demonstrate a metasurface device that circumvents
these limitations. A flat lens based on antenna reflectarrays was designed to
achieve near diffraction-limited focusing with a high efficiency (experiment:
80%, simulation: 83%) at 45(o) incidence angle at {\lambda} = 4.6 {\mu}m. This
geometry considerably simplifies the experimental arrangement compared to the
common geometry of normal incidence which requires beam splitters. Simulations
show that the effect of comatic aberrations is small compared to parabolic
mirrors. The use of single-step photolithography allows large scale
fabrication.Comment: 9 page
The Creation and Propagation of Radiation: Fields Inside and Outside of Sources
We present a new algorithm for computing the electromagnetic fields of
currents inside and outside of finite current sources, for arbitrary time
variations in the currents. Unexpectedly, we find that our solutions for these
fields are free of the concepts of differential calculus, in that our solutions
only involve the currents and their time integrals, and do not involve the time
derivatives of the currents. As examples, we give the solutions for two
configurations of current: a planar solenoid and a rotating spherical shell
carrying a uniform charge density. For slow time variations in the currents, we
show that our general solutions reduce to the standard expressions for the
fields in classic magnetic dipole radiation. In the limit of extremely fast
turn-on of the currents, we show that for our general solutions the amount of
energy radiated is exactly equal to the magnetic energy stored in the static
fields a long time after current creation. We give three associated problem
statements which can be used in courses at the undergraduate level, and one
problem statement suitable for courses at the graduate level. These problems
are of physical interest because: (1) they show that current systems of finite
extent can radiate even during time intervals when the currents are constant;
(2) they explicitly display transit time delays across a source associated with
its finite dimensions; and (3) they allow students to see directly the origin
of the reaction forces for time-varying systemsComment: 25 pages, 5 figure
Observation of Asymmetric Transport in Structures with Active Nonlinearities
A mechanism for asymmetric transport based on the interplay between the
fundamental symmetries of parity (P) and time (T) with nonlinearity is
presented. We experimentally demonstrate and theoretically analyze the
phenomenon using a pair of coupled van der Pol oscillators, as a reference
system, one with anharmonic gain and the other with complementary anharmonic
loss; connected to two transmission lines. An increase of the gain/loss
strength or the number of PT-symmetric nonlinear dimers in a chain, can
increase both the asymmetry and transmittance intensities.Comment: 5 pages, 5 figure
Observability of pulsar beam bending by the Sgr~A* black hole
According to some models, there may be a significant population of radio
pulsars in the Galactic center. In principle, a beam from one of these pulsars
could pass close to the supermassive black hole (SMBH) at the center, be
deflected, and be detected by Earth telescopes. Such a configuration would be
an unprecedented probe of the properties of spacetime in the moderate- to
strong-field regime of the SMBH. We present here background on the problem, and
approximations for the probability of detection of such beams. We conclude that
detection is marginally probable with current telescopes, but that telescopes
that will be operating in the near future, with an appropriate multiyear
observational program, will have a good chance of detecting a beam deflected by
the SMBH.Comment: 18 pages, 16 figure
Absence of Landau-Peierls Instability in the Magnetic Dual Chiral Density Wave Phase of Dense QCD
We investigate the stability of the Magnetic Dual Chiral Density Wave (MDCDW)
phase of cold and dense QCD against collective low-energy fluctuations of the
order parameter. The appearance of additional structures in the system
free-energy due to the explicit breaking of the rotational and isospin
symmetries by the external magnetic field and the field-induced asymmetry of
the lowest Landau level modes play a crucial role in the analysis. The new
structures not only affect the condensate minimum equations, but also the
spectrum of the thermal fluctuations, which lacks the transverse soft modes
that typically affect single-modulated inhomogeneous phases in the absence of a
magnetic field. Consequently, the long-range order of the MDCDW phase is
preserved at finite temperature. The lack of Landau-Peierls instabilities in
the MDCDW phase makes this inhomogeneous phase of dense quark matter
particularly relevant for the physics of neutron stars.Comment: Typos corrected, new discussions adde
The Sensitivity of the Parkes Pulsar Timing Array to Individual Sources of Gravitational Waves
We present the sensitivity of the Parkes Pulsar Timing Array to gravitational
waves emitted by individual super-massive black-hole binary systems in the
early phases of coalescing at the cores of merged galaxies. Our analysis
includes a detailed study of the effects of fitting a pulsar timing model to
non-white timing residuals. Pulsar timing is sensitive at nanoHertz frequencies
and hence complementary to LIGO and LISA. We place a sky-averaged constraint on
the merger rate of nearby () black-hole binaries in the early phases
of coalescence with a chirp mass of 10^{10}\,\rmn{M}_\odot of less than one
merger every seven years. The prospects for future gravitational-wave astronomy
of this type with the proposed Square Kilometre Array telescope are discussed.Comment: fixed error in equation (4). [13 pages, 6 figures, 1 table, published
in MNRAS
A study on Quantization Dimension in complete metric spaces
The primary objective of the present paper is to develop the theory of
quantization dimension of an invariant measure associated with an iterated
function system consisting of finite number of contractive infinitesimal
similitudes in a complete metric space. This generalizes the known results on
quantization dimension of self-similar measures in the Euclidean space to a
complete metric space. In the last part, continuity of quantization dimension
is discussed
Unidirectional Lasing Emerging from Frozen Light in Non-Reciprocal Cavities
We introduce a class of unidirectional lasing modes associated with the
frozen mode regime of non-reciprocal slow-wave structures. Such asymmetric
modes can only exist in cavities with broken time-reversal and space inversion
symmetries. Their lasing frequency coincides with a spectral stationary
inflection point of the underlying passive structure and is virtually
independent of its size. These unidirectional lasers can be indispensable
components of photonic integrated circuitry.Comment: 5 pages, 3 figure
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