13,434 research outputs found
Two-Dimensional Electrons in a Strong Magnetic Field with Disorder: Divergence of the Localization Length
Electrons on a square lattice with half a flux quantum per plaquette are
considered. An effective description for the current loops is given by a
two-dimensional Dirac theory with random mass. It is shown that the
conductivity and the localization length can be calculated from a product of
Dirac Green's functions with the {\it same} frequency. This implies that the
delocalization of electrons in a magnetic field is due to a critical point in a
phase with a spontaneously broken discrete symmetry. The estimation of the
localization length is performed for a generalized model with fermion
levels using a --expansion and the Schwarz inequality. An argument for the
existence of two Hall transition points is given in terms of percolation
theory.Comment: 10 pages, RevTeX, no figure
Cluster induced quenching of galaxies in the massive cluster XMMXCSJ2215.9-1738 at z~1.5 traced by enhanced metallicities inside half R200
(Abridged) We explore the massive cluster XMMXCSJ2215.9-1738 at z~1.5 with
KMOS spectroscopy of Halpha and [NII] covering a region that corresponds to
about one virial radius. Using published spectroscopic redshifts of 108
galaxies in and around the cluster we computed the location of galaxies in the
projected velocity vs. position phase-space to separate our cluster sample into
a virialized region of objects accreted longer ago (roughly inside half R200)
and a region of infalling galaxies. We measured oxygen abundances for ten
cluster galaxies with detected [NII] lines in the individual galaxy spectra and
compared the MZR of the galaxies inside half R200 with the infalling galaxies
and a field sample at similar redshifts. We find that the oxygen abundances of
individual z~1.5 star-forming cluster galaxies inside half R200 are comparable,
at the respective stellar mass, to the higher local SDSS metallicity values. We
find that the [NII]/Halpha line ratios inside half R200 are higher by 0.2 dex
and that the resultant metallicities of the galaxies in the inner part of the
cluster are higher by about 0.1 dex, at a given mass, than the metallicities of
infalling galaxies and of field galaxies at z~1.5. The enhanced metallicities
of cluster galaxies at z~1.5 inside half R200 indicate that the density of the
ICM in this massive cluster becomes high enough toward the cluster center such
that the ram pressure exceeds the restoring pressure of the hot gas reservoir
of cluster galaxies. This can remove the gas reservoir initiating quenching;
although the galaxies continue to form stars, albeit at slightly lower rates,
using the available cold gas in the disk which is not stripped.Comment: Accepted for publication in A&
BD-22 3467, a DAO-type star exciting the nebula Abell 35
Spectral analyses of hot, compact stars with NLTE (non-local thermodynamical
equilibrium) model-atmosphere techniques allow the precise determination of
photospheric parameters. The derived photospheric metal abundances are crucial
constraints for stellar evolutionary theory.
Previous spectral analyses of the exciting star of the nebula A 35, BD-22
3467, were based on He+C+N+O+Si+Fe models only. For our analysis, we use
state-of-the-art fully metal-line blanketed NLTE model atmospheres that
consider opacities of 23 elements from hydrogen to nickel. For the analysis of
high-resolution and high-S/N (signal-to-noise) FUV (far ultraviolet, FUSE) and
UV (HST/STIS) observations, we combined stellar-atmosphere models and
interstellar line-absorption models to fully reproduce the entire observed UV
spectrum.
The best agreement with the UV observation of BD-22 3467 is achieved at Teff
= 80 +/- 10 kK and log g =7.2 +/- 0.3. While Teff of previous analyses is
verified, log g is significantly lower. We re-analyzed lines of silicon and
iron (1/100 and about solar abundances, respectively) and for the first time in
this star identified argon, chromium, manganese, cobalt, and nickel and
determined abundances of 12, 70, 35, 150, and 5 times solar, respectively. Our
results partially agree with predictions of diffusion models for DA-type white
dwarfs. A combination of photospheric and interstellar line-absorption models
reproduces more than 90 % of the observed absorption features. The stellar mass
is M ~ 0.48 Msun.
BD-22 3467 may not have been massive enough to ascend the asymptotic giant
branch and may have evolved directly from the extended horizontal branch to the
white dwarf state. This would explain why it is not surrounded by a planetary
nebula. However, the star, ionizes the ambient interstellar matter, mimicking a
planetary nebula.Comment: 13 pages, 17 figure
The Global Status of the Crowdfunding Industry
publishedVersio
Integer Quantum Hall Effect for Lattice Fermions
A two-dimensional lattice model for non-interacting fermions in a magnetic
field with half a flux quantum per plaquette and levels per site is
considered. This is a model which exhibits the Integer Quantum Hall Effect
(IQHE) in the presence of disorder. It presents an alternative to the
continuous picture for the IQHE with Landau levels. The large limit can be
solved: two Hall transitions appear and there is an interpolating behavior
between the two Hall plateaux. Although this approach to the IQHE is different
from the traditional one with Landau levels because of different symmetries
(continuous for Landau levels and discrete here), some characteristic features
are reproduced. For instance, the slope of the Hall conductivity is infinite at
the transition points and the electronic states are delocalized only at the
transitions.Comment: 9 pages, Plain-Te
Multiplicity of the Galactic Senior Citizens: A high-resolution search for cool subdwarf companions
Cool subdwarfs are the oldest members of the low mass stellar population.
Mostly present in the galactic halo, subdwarfs are characterized by their low
metallicity. Measuring their binary fraction and comparing it to solar
metallicity stars could give key insights into the star formation process early
in the history of the Milky Way. However, because of their low luminosity and
relative rarity in the solar neighborhood, binarity surveys of cool subdwarfs
have suffered from small sample sizes and incompleteness. Previous surveys have
suggested that the binary fraction of red subdwarfs is much lower than for
their main sequence cousins. Using the highly efficient RoboAO system, we
present the largest yet high-resolution survey of subdwarfs, sensitive to
angular separations, down to 0.15 arcsec, and contrast ratios, up to 6
magnitude difference, invisible in past surveys. Of 344 target cool subdwarfs,
40 are in multiple systems, 16 newly discovered, for a binary fraction of 11.6
percent and 1.8 percent error. We also discovered 6 triple star systems for a
triplet fraction of 1.7 percent and 0.7 percent error. Comparisons to similar
surveys of solar metallicity dwarf stars gives a 3 sigma disparity in
luminosity between companion stars, with subdwarfs displaying a shortage of low
contrast companions.Comment: 13 pages, 10 figures, submitted to Ap
Exponents of 2-multiarrangements and multiplicity lattices
We introduce a concept of multiplicity lattices of 2-multiarrangements,
determine the combinatorics and geometry of that lattice, and give a criterion
and method to construct a basis for derivation modules effectively.Comment: 14 page
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