170 research outputs found
Finite-dimensional Schwinger basis, deformed symmetries, Wigner function, and an algebraic approach to quantum phase
Schwinger's finite (D) dimensional periodic Hilbert space representations are
studied on the toroidal lattice {\ee Z}_{D} \times {\ee Z}_{D} with specific
emphasis on the deformed oscillator subalgebras and the generalized
representations of the Wigner function. These subalgebras are shown to be
admissible endowed with the non-negative norm of Hilbert space vectors. Hence,
they provide the desired canonical basis for the algebraic formulation of the
quantum phase problem. Certain equivalence classes in the space of labels are
identified within each subalgebra, and connections with area-preserving
canonical transformations are examined. The generalized representations of the
Wigner function are examined in the finite-dimensional cyclic Schwinger basis.
These representations are shown to conform to all fundamental conditions of the
generalized phase space Wigner distribution. As a specific application of the
Schwinger basis, the number-phase unitary operator pair in {\ee Z}_{D} \times
{\ee Z}_{D} is studied and, based on the admissibility of the underlying
q-oscillator subalgebra, an {\it algebraic} approach to the unitary quantum
phase operator is established. This being the focus of this work, connections
with the Susskind-Glogower- Carruthers-Nieto phase operator formalism as well
as standard action-angle Wigner function formalisms are examined in the
infinite-period limit. The concept of continuously shifted Fock basis is
introduced to facilitate the Fock space representations of the Wigner function.Comment: 19 pages, no figure
Milky Way potentials in CDM and MOND. Is the Large Magellanic Cloud on a bound orbit?
We compute the Milky Way potential in different cold dark matter (CDM) based
models, and compare these with the modified Newtonian dynamics (MOND)
framework. We calculate the axis ratio of the potential in various models, and
find that isopotentials are less spherical in MOND than in CDM potentials. As
an application of these models, we predict the escape velocity as a function of
the position in the Galaxy. This could be useful in comparing with future data
from planned or already-underway kinematic surveys (RAVE, SDSS, SEGUE, SIM,
GAIA or the hypervelocity stars survey). In addition, the predicted escape
velocity is compared with the recently measured high proper motion velocity of
the Large Magellanic Cloud (LMC). To bind the LMC to the Galaxy in a MOND
model, while still being compatible with the RAVE-measured local escape speed
at the Sun's position, we show that an external field modulus of less than
is needed.Comment: Accepted for publication in MNRAS, 13 pages, 7 figures, 3 table
A Study of the Populations of X-ray Sources in the Small Magellanic Cloud with ASCA
The Advanced Satellite for Cosmology and Astrophysics (ASCA) has made
multiple observations of the Small Magellanic Cloud (SMC). X-ray mosaic images
in the soft (0.7--2.0 keV) and hard (2.0--7.0 keV) bands are separately
constructed, and the latter provides the first hard X-ray view of the SMC. We
extract 39 sources from the two-band images with a criterion of S/N>5, and
conduct timing and spectral analyses for all of these sources. Coherent
pulsations are detected from 12 X-ray sources; five of which are new
discoveries. Most of the 12 X-ray pulsars are found to exhibit long-term flux
variabilities, hence they are likely to be X-ray binary pulsars (XBPs). On the
other hand, we classify four supernova remnants (SNRs) as thermal SNRs, because
their spectra exhibit emission lines from highly ionized atoms. We find that
XBPs and thermal SNRs in the SMC can be clearly separated by their hardness
ratio (the ratio of the count rate between the hard and soft bands). Using this
empirical grouping, we find many XBP candidates in the SMC, although no
pulsations have yet been detected from these sources. Possible implications on
the star-formation history and evolution of the SMC are presented by a
comparison of the source populations in the SMC and our Galaxy.Comment: 11 pages, 39 Figures, to be published in ApJ Supplement. Tables (body
and figures also) are available at
http://www-cr.scphys.kyoto-u.ac.jp/member/jun/job
Magellanic Cloud Structure from Near-IR Surveys II: Star Count Maps and the Intrinsic Elongation of the LMC
I construct a near-IR star count map of the LMC and demonstrate, using the
viewing angles derived in Paper I, that the LMC is intrinsically elongated. I
argue that this is due to the tidal force from the Milky Way. The near-IR data
from the 2MASS and DENIS surveys are used to create a star count map of RGB and
AGB stars, which is interpreted through ellipse fitting. The radial number
density profile is approximately exponential with a scale-length 1.3-1.5 kpc.
However, there is an excess density at large radii that may be due to the tidal
effect of the Milky Way. The position angle and ellipticity profile converge to
PA_maj = 189.3 +/- 1.4 degrees and epsilon = 0.199 +/- 0.008 for r > 5 deg. At
large radii there is a drift of the center of the star count contours towards
the near side of the plane, which can be undrestood as due to viewing
perspective. The fact that PA_maj differes from the line of nodes position
angle Theta = 122.5 +/- 8.3 (cf. Paper I) indicates that the LMC disk is not
circular, but has an intrinsic ellipticity of 0.31. The LMC is elongated in the
general direction of the Galactic center, and is elongated perpendicular to the
Magellanic Stream and the velocity vector of the LMC center of mass. This
suggests that the elongation of the LMC has been induced by the tidal force of
the Milky Way. The position angle of the line of nodes differs from the
position angle Theta_max of the line of maximum line of sight velocity
gradient: Theta_max - Theta = 20-60 degrees. This could be due to: (a)
streaming along non-circular orbits in the elongated disk; (b) uncertainties in
the transverse motion of the LMC center of mass; (c) precession and nutation of
the LMC disk as it orbits the Milky Way (expected on theoretical grounds).
[Abridged]Comment: Astronomical Journal, in press. 34 pages, LaTeX, with 7 PostScript
figures. Contains minor revisions with respect to previously posted version.
Check out http://www.stsci.edu/~marel/lmc.html for a large scale (23x21
degree) stellar number-density image of the LMC constructed from RGB and AGB
stars in the 2MASS and DENIS surveys. The paper is available with higher
resolution color figures from
http://www.stsci.edu/~marel/abstracts/abs_R32.htm
Constraints on Intervening Stellar Populations Toward the Large Magellanic Cloud
The suggestion by Zaritsky & Lin that a vertical extension of the red clump
feature in color-magnitude diagrams of the Large Magellanic Cloud (LMC) is
consistent with a significant population of foreground stars to the LMC that
could account for the observed microlensing optical depth has been challenged
by various investigators. We respond by (1) examining each of the challenges
presented and (2) presenting new photometric and spectroscopic data. We
conclude that although the CMD data do not mandate the existence of a
foreground population, they are entirely consistent with a foreground
population associated with the LMC that contributes significantly (~ 50%) to
the observed microlensing optical depth. From our new data, we conclude that <~
40% of the VRC stars are young, massive red clump stars because (1) synthetic
color-magnitude diagrams created using the star formation history derived
indepdently from HST data suggest that < 50% of the VRC stars are young,
massive red clump stars, (2) the angular distribution of the VRC stars is more
uniform than that of the young (age < 1 Gyr) main sequence stars, and (3) the
velocity dispersion of the VRC stars in the region of the LMC examined by ZL is
inconsistent with the expectation for a young disk population. Each of these
arguments is predicated on assumptions and the conclusions are uncertain.
Therefore, an exact determination of the contribution to the microlensing
optical depth by the various hypothesized foreground populations, and the
subsequent conclusions regarding the existence of halo MACHOs, requires a
detailed knowledge of many complex astrophysical issues, such as the IMF, star
formation history, and post-main sequence stellar evolution. (abridged)Comment: Scheduled for publication in AJ in May 199
A New Look at the Kinematics of Neutral Hydrogen in the Small Magellanic Cloud
We have used the latest HI observations of the Small Magellanic Cloud (SMC),
obtained with the Australia Telescope Compact Array and the Parkes telescope,
to re-examine the kinematics of this dwarf, irregular galaxy. A large velocity
gradient is found in the HI velocity field with a significant symmetry in
iso-velocity contours, suggestive of a differential rotation. A comparison of
HI data with the predictions from tidal models for the SMC evolution suggests
that the central region of the SMC corresponds to the central, disk- or
bar-like, component left from the rotationally supported SMC disk prior to its
last two encounters with the Large Magellanic Cloud. In this scenario, the
velocity gradient is expected as a left-over from the original, pre-encounter,
angular momentum. We have derived the HI rotation curve and the mass model for
the SMC. This rotation curve rapidly rises to about 60 km/s up to the turnover
radius of ~3 kpc. A stellar mass-to-light ratio of about unity is required to
match the observed rotation curve, suggesting that a dark matter halo is not
needed to explain the dynamics of the SMC. A set of derived kinematic
parameters agrees well with the assumptions used in tidal theoretical models
that led to a good reproduction of observational properties of the Magellanic
System. The dynamical mass of the SMC, derived from the rotation curve, is
2.4x10^9 Msolar.Comment: To appear in ApJ, March 20 2004, 11 figure
Magellanic Cloud Structure from Near-IR Surveys I: The Viewing Angles of the LMC
We present a detailed study of the viewing angles of the LMC disk plane. We
find that our viewing direction differs considerably from the commonly accepted
values, which has important implications for the structure of the LMC. The
discussion is based on an analysis of spatial variations in the apparent
magnitude of features in the near-IR color-magnitude diagrams extracted from
the DENIS and 2MASS surveys. Sinusoidal brightness variations with a
peak-to-peak amplitude of approximately 0.25 mag are detected as function of
position angle, for both AGB and RGB stars. This is naturally interpreted as
the result of distance variations, due to one side of the LMC plane being
closer to us than the opposite side. The best fitting geometric model of an
inclined plane yields an inclination angle i = 34.7 +/- 6.2 degrees and
line-of-nodes position angle Theta = 122.5 +/- 8.3 degrees. There is tentative
evidence that the LMC disk plane may be warped. Traditional methods to estimate
the position angle of the line of nodes have used either the major axis
position angle Theta_maj of the spatial distribution of tracers on the sky, or
the position angle Theta_max of the line of maximum gradient in the velocity
field, given that for a circular disk Theta_maj = Theta_max = Theta. The
present study does not rely on the assumption of circular symmetry, and is
considerably more accurate than previous studies of its kind. We find that the
actual position angle of the line of nodes differs considerably from both
Theta_maj and Theta_max, for which measurements have fallen in the range
140-190 degrees. This indicates that the intrinsic shape of the LMC disk is not
circular, but elliptical, as discussed further in Paper II. [Abridged]Comment: Astronomical Journal, in press. 44 pages, LaTeX, with 8 PostScript
figures. Contains minor revisions with respect to previously posted version.
Check out http://www.stsci.edu/~marel/lmc.html for a large scale (23x21
degree) stellar number-density image of the LMC constructed from RGB and AGB
stars in the 2MASS and DENIS surveys. The paper is available with higher
resolution figures from http://www.stsci.edu/~marel/abstracts/abs_R31.htm
A consistent quantum model for continuous photodetection processes
We are modifying some aspects of the continuous photodetection theory,
proposed by Srinivas and Davies [Optica Acta 28, 981 (1981)], which describes
the non-unitary evolution of a quantum field state subjected to a continuous
photocount measurement. In order to remedy inconsistencies that appear in their
approach, we redefine the `annihilation' and `creation' operators that enter in
the photocount superoperators. We show that this new approach not only still
satisfies all the requirements for a consistent photocount theory according to
Srinivas and Davies precepts, but also avoids some weird result appearing when
previous definitions are used.Comment: 12 pages, 4 figure
Adult infiltrating gliomas with WHO 2016 integrated diagnosis: additional prognostic roles of ATRX and TERT
The âintegrated diagnosisâ for infiltrating gliomas in the 2016 revised World Health Organization (WHO) classification of tumors of the central nervous system requires assessment of the tumor for IDH mutations and 1p/19q codeletion. Since TERT promoter mutations and ATRX alterations have been shown to be associated with prognosis, we analyzed whether these tumor markers provide additional prognostic information within each of the five WHO 2016 categories. We used data for 1206 patients from the UCSF Adult Glioma Study, the Mayo Clinic and The Cancer Genome Atlas (TCGA) with infiltrative glioma, grades II-IV for whom tumor status for IDH, 1p/19q codeletion, ATRX, and TERT had been determined. All cases were assigned to one of 5 groups following the WHO 2016 diagnostic criteria based on their morphologic features, and IDH and 1p/19q codeletion status. These groups are: (1) Oligodendroglioma, IDH-mutant and 1p/19q-codeleted; (2) Astrocytoma, IDH-mutant; (3) Glioblastoma, IDH-mutant; (4) Glioblastoma, IDH-wildtype; and (5) Astrocytoma, IDH-wildtype. Within each group, we used univariate and multivariate Cox proportional hazards models to assess associations of overall survival with patient age at diagnosis, grade, and ATRX alteration status and/or TERT promoter mutation status. Among Group 1 IDH-mutant 1p/19q-codeleted oligodendrogliomas, the TERT-WT group had significantly worse overall survival than the TERT-MUT group (HR: 2.72, 95% CI 1.05â7.04, p = 0.04). In both Group 2, IDH-mutant astrocytomas and Group 3, IDH-mutant glioblastomas, neither TERT mutations nor ATRX alterations were significantly associated with survival. Among Group 4, IDH-wildtype glioblastomas, ATRX alterations were associated with favorable outcomes (HR: 0.36, 95% CI 0.17â0.81, p = 0.01). Among Group 5, IDH-wildtype astrocytomas, the TERT-WT group had significantly better overall survival than the TERT-MUT group (HR: 0.48, 95% CI 0.27â0.87), p = 0.02). Thus, we present evidence that in certain WHO 2016 diagnostic groups, testing for TERT promoter mutations or ATRX alterations may provide additional useful prognostic information
The Magellanic Stream, High-Velocity Clouds and the Sculptor Group
The Magellanic Stream is a 100\deg x 10\deg filament of gas which lies within the Galactic halo and contains ~ 2 x 10^8 \Msun of neutral hydrogen. We present data from the HI Parkes All Sky Survey (HIPASS) in the first complete survey of the entire Magellanic Stream and its surroundings. We also present a summary of the reprocessing techniques used to recover large-scale structure in the Stream. The Stream properties revealed include: bifurcation along the main Stream filament; dense, isolated clouds which follow the entire length of the Stream; head-tail structures; and a complex filamentary web at the head where gas is being freshly stripped away from the Small Magellanic Cloud and the Bridge. Debris which appears to be of Magellanic origin extends out to 20\deg from the main Stream filaments. The large number of elongated Stream clouds suggests the presence of shearing motions within the Stream, arising from tidal forces or interaction with the tenuous Galactic halo. Clouds along the sightline to the less distant half of the Sculptor Group, show anomalous properties. We argue that these clouds represent halo material, and are not distant Sculptor Group clouds. This result has significant implications for the hypothesis that there might exist distant, massive HVCs within the Local Group. (abridged
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