2,717 research outputs found
Equilibration in long-range quantum spin systems from a BBGKY perspective
The time evolution of -spin reduced density operators is studied for a
class of Heisenberg-type quantum spin models with long-range interactions. In
the framework of the quantum Bogoliubov-Born-Green-Kirkwood-Yvon (BBGKY)
hierarchy, we introduce an unconventional representation, different from the
usual cluster expansion, which casts the hierarchy into the form of a
second-order recursion. This structure suggests a scaling of the expansion
coefficients and the corresponding time scales in powers of with the
system size , implying a separation of time scales in the large system
limit. For special parameter values and initial conditions, we can show
analytically that closing the BBGKY hierarchy by neglecting -spin
correlations does never lead to equilibration, but gives rise to quasi-periodic
time evolution with at most independent frequencies. Moreover, for the
same special parameter values and in the large- limit, we solve the complete
recursion relation (the full BBGKY hierarchy), observing a superexponential
decay to equilibrium in rescaled time .Comment: 3 figure
Molecules in the Circumstellar Disk Orbiting BP Piscium
BP Psc is a puzzling late-type, emission-line field star with large infrared
excess. The star is encircled and enshrouded by a nearly edge-on, dust
circumstellar disk, and displays an extensive jet system similar to those
associated with pre-main sequence (pre-MS) stars. We conducted a mm-wave
molecular line survey of BP Psc with the 30 m telescope of the Institut de
Radio Astronomie Millimetrique (IRAM). We detected lines of 12CO and 13CO and,
possibly, very weak emission from HCO+ and CN; HCN, H2CO, and SiO are not
detected. The CO line profiles of BP Psc are well fit by a model invoking a
disk in Keplerian rotation. The mimumum disk gas mass, inferred from the 12CO
line intensity and 13CO/12CO line ratio, is ~0.1 Jupiter masses. The weakness
of HCO+ and CN (relative to 13CO) stands in sharp contrast to the strong HCO+
and CN emission that characterizes most low-mass, pre-main sequence stars that
have been the subjects of molecular emission-line surveys, and is suggestive of
a very low level of X-ray-induced molecular ionization within the BP Psc disk.
These results lend some support to the notion that BP Psc is an evolved star
whose circumstellar disk has its origins in a catastrophic interaction with a
close companion.Comment: 6 pages, 4 figures; to appear in Astronomy & Astrophysic
On Visibility in the Afshar Two-Slit Experiment
A modified version of Young's experiment by Shahriar Afshar indirectly
reveals the presence of a fully articulated interference pattern prior to the
post-selection of a particle in a "which-slit" basis. While this experiment
does not constitute a violation of Bohr's Complementarity Principle as claimed
by Afshar, both he and many of his critics incorrectly assume that a commonly
used relationship between visibility parameter V and "which-way" parameter K
has crucial relevance to his experiment. It is argued here that this
relationship does not apply to this experimental situation and that it is wrong
to make any use of it in support of claims for or against the bearing of this
experiment on Complementarity.Comment: Final version; to appear in Foundations of Physic
Cognitive and Perceptual Functions of the Visual Thalamus
The thalamus is classically viewed as passively relaying information to the cortex. However, there is growing evidence that the thalamus actively regulates information transmission to the cortex and between cortical areas using a variety of mechanisms, including the modulation of response magnitude, firing mode, and synchrony of neurons according to behavioral demands. We discuss how the visual thalamus contributes to attention, awareness, and visually guided actions, to present a general role for the thalamus in perception and cognition
The Multiple Young Stellar Objects of HBC 515: An X-ray and Millimeter-wave Imaging Study in (Pre-main Sequence) Diversity
We present Chandra X-ray Observatory and Submillimeter Array (SMA) imaging of
HBC 515, a system consisting of multiple young stellar objects (YSOs). The five
members of HBC 515 represent a remarkably diverse array of YSOs, ranging from
the low-mass Class I/II protostar HBC 515B, through Class II and transition
disk objects (HBC 515D and C, respectively), to the "diskless", intermediate-
mass, pre-main sequence binary HBC 515A. Our Chandra/ACIS imaging establishes
that all five components are X-ray sources, with HBC 515A - a
subarcsecond-separation binary that is partially resolved by Chandra - being
the dominant X-ray source. We detect an X-ray flare associated with HBC 515B.
In the SMA imaging, HBC 515B is detected as a strong 1.3 mm continuum emission
source; a second, weaker mm continuum source is coincident with the position of
the transition disk object HBC 515C. These results strongly support the
protostellar nature of HBC 515B, and firmly establish HBC 515A as a member of
the rare class of relatively massive, X-ray luminous "weak-lined T Tauri stars"
that are binaries and have shed their disks at very early stages of pre-MS
evolution. The coexistence of two such disparate objects within a single,
presumably coeval multiple YSO system highlights the influence of pre- MS star
mass, binarity, and X-ray luminosity in regulating the lifetimes of
circumstellar, planet-forming disks and the timescales of star-disk
interactions.Comment: Accepted for publication in A&A; 11 pages, 5 figure
Nearby Young, Active, Late-type Dwarfs in Gaia's First Data Release
The Galex Nearby Young Star Survey (GALNYSS) has yielded a sample of
2000 UV-selected objects that are candidate nearby (150 pc), young (age 10--100 Myr), late-type stars.
Here, we evaluate the distances and ages of the subsample of (19) GALNYSS stars
with Gaia Data Release 1 (DR1) parallax distances pc. The overall
youth of these 19 mid-K to early-M stars is readily apparent from their
positions relative to the loci of main sequence stars and giants in Gaia-based
color-magnitude and color-color diagrams constructed for all Galex- and
WISE-detected stars with parallax measurements included in DR1. The isochronal
ages of all 19 stars lie in the range 10--100 Myr. Comparison with
Li-based age estimates indicates a handful of these stars may be young
main-sequence binaries rather than pre-main sequence stars. Nine of the 19
objects have not previously been considered as nearby, young stars, and all but
one of these are found at declinations north of 30. The Gaia DR1
results presented here indicate that the GALNYSS sample includes several
hundred nearby, young stars, a substantial fraction of which have not been
previously recognized as having ages 100 Myr.Comment: 30 pages, 4 tables, 7 figures; to appear in The Astrophysical
Journal; 1st replacement to correct typos/omissions in Table 3 and
acknowledgments; 2nd replacement to incorporate corrections to ApJ proof
Measurements of quasi-particle tunneling in the nu = 5/2 fractional quantum Hall state
Some models of the 5/2 fractional quantum Hall state predict that the
quasi-particles, which carry the charge, have non-Abelian statistics: exchange
of two quasi-particles changes the wave function more dramatically than just
the usual change of phase factor. Such non-Abelian statistics would make the
system less sensitive to decoherence, making it a candidate for implementation
of topological quantum computation. We measure quasi-particle tunneling as a
function of temperature and DC bias between counter-propagating edge states.
Fits to theory give e*, the quasi-particle effective charge, close to the
expected value of e/4 and g, the strength of the interaction between
quasi-particles, close to 3/8. Fits corresponding to the various proposed wave
functions, along with qualitative features of the data, strongly favor the
Abelian 331 state
Effective Sample Size: Quick Estimation of the Effect of Related Samples in Genetic Case-Control Association Analyses
Correlated samples have been frequently avoided in case-control
genetic association
 studies in part because the methods for handling them are either not
easily implemented or not widely known. We
advocate one method for case-control association analysis of correlated
samples -- the effective sample size method -- as a simple and
accessible approach that does not require specialized computer programs.
The effective sample size method captures the variance inflation
of allele frequency estimation exactly, and can be used to modify the
chi-square test statistic, p-value, and 95% confidence interval of
odds-ratio simply by replacing the apparent number of allele counts with the
effective ones. For genotype frequency estimation, although a single
effective sample size is unable to completely characterize the variance inflation,
an averaged one can satisfactorily approximate the simulated result.
The effective sample size method is applied to the rheumatoid arthritis
siblings data collected from the North American Rheumatoid Arthritis Consortium (NARAC)
to establish a significant association with the interferon-induced
helicasel gene (IFIH1) previously being identified as a type 1 diabetes
susceptibility locus. Connections between the effective sample size
method and other methods, such as generalized estimation equation,
variance of eigenvalues for correlation matrices, and genomic controls,
are also discussed.

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