11,261 research outputs found
Climbing the cosmic ladder with stellar twins
Distances to stars are key to revealing a three-dimensional view of the Milky
Way, yet their determination is a major challenge in astronomy. Whilst the
brightest nearby stars benefit from direct parallax measurements, fainter stars
are subject of indirect determinations with uncertainties exceeding 30%. We
present an alternative approach to measuring distances using
spectroscopically-identified twin stars. Given a star with known parallax, the
distance to its twin is assumed to be directly related to the difference in
their apparent magnitudes. We found 175 twin pairs from the ESO public HARPS
archives and report excellent agreement with Hipparcos parallaxes within 7.5%.
Most importantly, the accuracy of our results does not degrade with increasing
stellar distance. With the ongoing collection of high-resolution stellar
spectra, our method is well-suited to complement Gaia.Comment: published online on MNRA
SAR processing on the MPP
The processing of synthetic aperture radar (SAR) signals using the massively parallel processor (MPP) is discussed. The fast Fourier transform convolution procedures employed in the algorithms are described. The MPP architecture comprises an array unit (ARU) which processes arrays of data; an array control unit which controls the operation of the ARU and performs scalar arithmetic; a program and data management unit which controls the flow of data; and a unique staging memory (SM) which buffers and permutes data. The ARU contains a 128 by 128 array of bit-serial processing elements (PE). Two-by-four surarrays of PE's are packaged in a custom VLSI HCMOS chip. The staging memory is a large multidimensional-access memory which buffers and permutes data flowing with the system. Efficient SAR processing is achieved via ARU communication paths and SM data manipulation. Real time processing capability can be realized via a multiple ARU, multiple SM configuration
Constraining the Distribution of L- & T-Dwarfs in the Galaxy
We estimate the thin disk scale height of the Galactic population of L- &
T-dwarfs based on star counts from 15 deep parallel fields from the Hubble
Space Telescope. From these observations, we have identified 28 candidate L- &
T- dwarfs based on their (i'-z') color and morphology. By comparing these star
counts to a simple Galactic model, we estimate the scale height to be 350+-50
pc that is consistent with the increase in vertical scale with decreasing
stellar mass and is independent of reddening, color-magnitude limits, and other
Galactic parameters. With this refined measure, we predict that less than 10^9
M_{sol} of the Milky Way can be in the form L- & T- dwarfs, and confirm that
high-latitude, z~6 galaxy surveys which use the i'-band dropout technique are
97-100% free of L- & T- dwarf interlopers.Comment: 4 pages, 4 figures, accepted to ApJ
Calculation of the unitary part of the Bures measure for N-level quantum systems
We use the canonical coset parameterization and provide a formula with the
unitary part of the Bures measure for non-degenerate systems in terms of the
product of even Euclidean balls. This formula is shown to be consistent with
the sampling of random states through the generation of random unitary
matrices
Unstable recurrent patterns in Kuramoto-Sivashinsky dynamics
We undertake a systematic exploration of recurrent patterns in a
1-dimensional Kuramoto-Sivashinsky system. For a small, but already rather
turbulent system, the long-time dynamics takes place on a low-dimensional
invariant manifold. A set of equilibria offers a coarse geometrical partition
of this manifold. A variational method enables us to determine numerically a
large number of unstable spatiotemporally periodic solutions. The attracting
set appears surprisingly thin - its backbone are several Smale horseshoe
repellers, well approximated by intrinsic local 1-dimensional return maps, each
with an approximate symbolic dynamics. The dynamics appears decomposable into
chaotic dynamics within such local repellers, interspersed by rapid jumps
between them.Comment: 11 pages, 11 figure
Efficient simulation of quantum evolution using dynamical coarse-graining
A novel scheme to simulate the evolution of a restricted set of observables
of a quantum system is proposed. The set comprises the spectrum-generating
algebra of the Hamiltonian. The idea is to consider a certain open-system
evolution, which can be interpreted as a process of weak measurement of the
distinguished observables performed on the evolving system of interest. Given
that the observables are "classical" and the Hamiltonian is moderately
nonlinear, the open system dynamics displays a large time-scales separation
between the dephasing of the observables and the decoherence of the evolving
state in the basis of the generalized coherent states (GCS), associated with
the spectrum-generating algebra. The time scale separation allows the unitary
dynamics of the observables to be efficiently simulated by the open-system
dynamics on the intermediate time-scale.The simulation employs unraveling of
the corresponding master equations into pure state evolutions, governed by the
stochastic nonlinear Schroedinger equantion (sNLSE). It is proved that GCS are
globally stable solutions of the sNLSE, if the Hamilonian is linear in the
algebra elements.Comment: The version submitted to Phys. Rev. A, 28 pages, 3 figures, comments
are very welcom
Aharonov-Bohm effect and broken valley-degeneracy in graphene rings
We analyze theoretically the electronic properties of Aharonov-Bohm rings
made of graphene. We show that the combined effect of the ring confinement and
applied magnetic flux offers a controllable way to lift the orbital degeneracy
originating from the two valleys, even in the absence of intervalley
scattering. The phenomenon has observable consequences on the persistent
current circulating around the closed graphene ring, as well as on the ring
conductance. We explicitly confirm this prediction analytically for a circular
ring with a smooth boundary modelled by a space-dependent mass term in the
Dirac equation. This model describes rings with zero or weak intervalley
scattering so that the valley isospin is a good quantum number. The tunable
breaking of the valley degeneracy by the flux allows for the controlled
manipulation of valley isospins. We compare our analytical model to another
type of ring with strong intervalley scattering. For the latter case, we study
a ring of hexagonal form with lattice-terminated zigzag edges numerically. We
find for the hexagonal ring that the orbital degeneracy can still be controlled
via the flux, similar to the ring with the mass confinement.Comment: 7 pages, 7 figures, replaced with considerably extended new versio
Dynamical Evolution of Globular Cluster Systems formed in Galaxy Mergers: Deep HST/ACS Imaging of Old and Intermediate-Age Globular Clusters in NGC 3610
(ABRIDGED) The ACS camera on board the Hubble Space Telescope has been used
to obtain deep images of the giant elliptical galaxy NGC 3610, a
well-established dissipative galaxy merger remnant. These observations
supersede previous WFPC2 images which revealed the presence of a population of
metal-rich globular clusters (GCs) of intermediate age (~1.5-4 Gyr). We detect
a total of 580 GC candidates, 46% more than from the previous WFPC2 images. The
new photometry strengthens the significance of the previously found bimodality
of the color distribution of GCs. Peak colors in V-I are 0.93 +/-0.01 and 1.09
+/- 0.01 for the blue and red subpopulations, respectively. The luminosity
function (LF) of the inner 50% of the metal-rich (`red') population of GCs
differs markedly from that of the outer 50%. In particular, the LF of the inner
50% of the red GCs shows a flattening consistent with a turnover that is about
1.0 mag fainter than the turnover of the blue GC LF. This is consistent with
predictions of recent models of GC disruption for the age range mentioned above
and for metallicities that are consistent with the peak color of the red GCs as
predicted by population synthesis models. We determine the specific frequency
of GCs in NGC 3610 and find a present-day value of S_N = 1.4 +/- 0.6. We
estimate that this value will increase to S_N = 3.8 +/- 1.7 at an age of 10
Gyr, which is consistent with typical S_N values for `normal' ellipticals. Our
findings constitute further evidence in support of the notion that metal-rich
GC populations formed during major mergers involving gas-rich galaxies can
evolve dynamically (through disruption processes) into the red, metal-rich GC
populations that are ubiquitous in `normal' giant ellipticals.Comment: 15 pages, 14 figures, 4 tables. Accepted for publication in The
Astronomical Journal. Figure 6 somewhat degraded to adhere to astro-ph rule
- …