970 research outputs found
Infrared Imaging of Capella with the IOTA Closure Phase Interferometer
We present infrared aperture synthesis maps produced with the upgraded IOTA
interferometer. Michelson interferograms on the close binary system Capella
(Alpha Aur) were obtained in the H-band between 2002 November 12 and 16 using
the IONIC3 beam combiner. With baselines of 15m < B < 38m, we were able to
determine the relative position of the binary components with milliarcsecond
(mas) precision and to track their movement along the approx. 14 degree arc
covered by our observation run. We briefly describe the algorithms used for
visibility and closure phase estimation. Three different Hybrid Mapping and
Bispectrum Fitting techniques were implemented within one software framework
and used to reconstruct the source brightness distribution. By dividing our
data into subsets, the system could be mapped at three epochs, revealing the
motion of the stars. The precise position of the binary components was also
determined with model fits, which in addition revealed I_Aa/I_Ab=1.49 +/- 0.10
and apparent stellar uniform-disk (UD) diameters of Theta_Aa=8.9 +/- 0.6 mas
and Theta_Ab=5.8 +/- 0.8 mas.
To improve the u, v-plane coverage, we compensated this orbital motion by
applying a rotation-compensating coordinate transformation. The resulting
model-independent map with a beam size of 5.4 x 2.6 mas allows the resolution
of the stellar surfaces of the Capella giants themselves.Comment: Accepted by the Astronomical Journal (2005-03-21
Physical Orbit for Lambda Virginis and a Test of Stellar Evolution Models
Lambda Virginis (LamVir) is a well-known double-lined spectroscopic Am binary
with the interesting property that both stars are very similar in abundance but
one is sharp-lined and the other is broad-lined. We present combined
interferometric and spectroscopic studies of LamVir. The small scale of the
LamVir orbit (~20 mas) is well resolved by the Infrared Optical Telescope Array
(IOTA), allowing us to determine its elements as well as the physical
properties of the components to high accuracy. The masses of the two stars are
determined to be 1.897 Msun and 1.721 Msun, with 0.7% and 1.5% errors
respectively, and the two stars are found to have the same temperature of 8280
+/- 200 K. The accurately determined properties of LamVir allow comparisons
between observations and current stellar evolution models, and reasonable
matches are found. The best-fit stellar model gives LamVir a subsolar
metallicity of Z=0.0097, and an age of 935 Myr. The orbital and physical
parameters of LamVir also allow us to study its tidal evolution time scales and
status. Although currently atomic diffusion is considered to be the most
plausible cause of the Am phenomenon, the issue is still being actively debated
in the literature. With the present study of the properties and evolutionary
status of LamVir, this system is an ideal candidate for further detailed
abundance analyses that might shed more light on the source of the chemical
anomalies in these A stars.Comment: 43 Pages, 13 figures. Accepted for publication in Ap
The equation of state for two flavor QCD at N_t=6
We calculate the two flavor equation of state for QCD on lattices with
lattice spacing a=(6T)^{-1} and find that cutoff effects are substantially
reduced compared to an earlier study using a=(4T)^{-1}. However, it is likely
that significant cutoff effects remain. We fit the lattice data to expected
forms of the free energy density for a second order phase transition at
zero-quark-mass, which allows us to extrapolate the equation of state to m_q=0
and to extract the speed of sound. We find that the equation of state depends
weakly on the quark mass for small quark mass.Comment: 24 pages, latex, 11 postscipt figure
Intermediate Vancomycin Susceptibility in a Community-associated MRSA Clone
We describe a case of treatment failure caused by a strain of USA300 community-associated methicillin-resistant Staphylococcus aureus (MRSA) with intermediate susceptibility to vancomycin and reduced susceptibility to daptomycin. The strain was isolated from the bone of a 56-year-old man with lumbar osteomyelitis after a 6-week treatment course of vancomycin for catheter-associated septic thrombophlebitis
An examination of the precipitation delivery mechanisms for Dolleman Island, eastern Antarctic Peninsula
Copyright @ 2004 Wiley-BlackwellThe variability of size and source of significant precipitation events were studied at an Antarctic ice core drilling site: Dolleman Island (DI), located on the eastern coast of the Antarctic Peninsula. Significant precipitation events that occur at DI were temporally located in the European Centre for Medium-Range Weather Forecasting (ECMWF) reanalysis data set, ERA-40. The annual and summer precipitation totals from ERA-40 at DI both show significant increases over the reanalysis period. Three-dimensional backwards air parcel trajectories were then run for 5 d using the ECMWF ERA-15 wind fields. Cluster analyses were performed on two sets of these backwards trajectories: all days in the range 1979–1992 (the climatological time-scale) and a subset of days when a significant precipitation event occurred. The principal air mass sources and delivery mechanisms were found to be the Weddell Sea via lee cyclogenesis, the South Atlantic when there was a weak circumpolar trough (CPT) and the South Pacific when the CPT was deep. The occurrence of precipitation bearing air masses arriving via a strong CPT was found to have a significant correlation with the southern annular mode (SAM); however, the arrival of air masses from the same region over the climatological time-scale showed no such correlation. Despite the dominance in both groups of back trajectories of the westerly circulation around Antarctica, some other key patterns were identified. Most notably there was a higher frequency of lee cyclogenesis events in the significant precipitation trajectories compared to the climatological time-scale. There was also a tendency for precipitation trajectories to come from more northerly latitudes, mostly from 50–70°S. The El Niño Southern Oscillation (ENSO) was found to have a strong influence on the mechanism by which the precipitation was delivered; the frequency of occurrence of precipitation from the east (west) of DI increased during El Niño (La Niña) events
First astronomical unit scale image of the GW Ori triple. Direct detection of a new stellar companion
Young and close multiple systems are unique laboratories to probe the initial
dynamical interactions between forming stellar systems and their dust and gas
environment. Their study is a key building block to understanding the high
frequency of main-sequence multiple systems. However, the number of detected
spectroscopic young multiple systems that allow dynamical studies is limited.
GW Orionis is one such system. It is one of the brightest young T Tauri stars
and is surrounded by a massive disk. Our goal is to probe the GW Orionis
multiplicity at angular scales at which we can spatially resolve the orbit. We
used the IOTA/IONIC3 interferometer to probe the environment of GW Orionis with
an astronomical unit resolution in 2003, 2004, and 2005. By measuring squared
visibilities and closure phases with a good UV coverage we carry out the first
image reconstruction of GW Ori from infrared long-baseline interferometry. We
obtain the first infrared image of a T Tauri multiple system with astronomical
unit resolution. We show that GW Orionis is a triple system, resolve for the
first time the previously known inner pair (separation 1.4 AU) and
reveal a new more distant component (GW Ori C) with a projected separation of
8 AU with direct evidence of motion. Furthermore, the nearly equal (2:1)
H-band flux ratio of the inner components suggests that either GW Ori B is
undergoing a preferential accretion event that increases its disk luminosity or
that the estimate of the masses has to be revisited in favour of a more equal
mass-ratio system that is seen at lower inclination. Accretion disk models of
GW Ori will need to be completely reconsidered because of this outer companion
C and the unexpected brightness of companion B.Comment: 5 pages, 9 figures, accepted Astronomy and Astrophysics Letters. 201
Characterizing closure-phase measurements at IOTA
We are working towards imaging the surfaces and circumstellar envelopes of Mira stars in the near-infrared, using the IOTA interferometer and the IONIC integrated-optics 3-beam combiner. In order to study atmospheric structures of these stars, we installed 3 narrow-band filters that subdivide H-band into 3 roughly equal-width sub-bands - a central one for continuum, and 2 adjacent ones to sample Mira star's (mostly water) absorption-bands. We present here our characterization of the IOTA 3-Telescope interferometer for closure-phase measurements with broad and narrow-band filters in the H atmospheric window. This includes characterizing the stability, chromaticity, and polarization effects of the present IOTA optics with the IONIC beam-combiner, and characterizing the accuracy of our closure phase measurements
Quenched hadron spectroscopy with improved staggered quark action
We investigate light hadron spectroscopy with an improved quenched staggered
quark action. We compare the results obtained with an improved gauge plus an
improved quark action, an improved gauge plus standard quark action, and the
standard gauge plus standard quark action. Most of the improvement in the
spectroscopy results is due to the improved gauge sector. However, the improved
quark action substantially reduces violations of Lorentz invariance, as
evidenced by the meson dispersion relations.Comment: New references adde
Baryon Density Correlations in High Temperature Hadronic Matter
As part of an ongoing effort to characterize the high temperature phase of
QCD, in a numerical simulation using the staggered fermion scheme, we measure
the quark baryon density in the vicinity of a fixed test quark at high
temperature and compare it with similar measurements at low temperature and at
the crossover temperature. We find an extremely weak correlation at high
temperature, suggesting that small color singlet clusters are unimportant in
the thermal ensemble. We also find that at the total induced
quark number shows a surprisingly large component attributable to baryonic
screening. A companion simulation of a simple flux tube model produces similar
results and also suggests a plausible phenomenological scenario: As the
crossover temperature is approached from below, baryonic states proliferate.
Above the crossover temperature the mean size of color singlet clusters grows
explosively, resulting in an effective electrostatic deconfinement.Comment: 26 pp, RevTeX, 12 postscript figures, combined in a single shell
archive file. (Also available in 13 postscript files by anonymous ftp from
einstein.physics.utah.edu, /pub/milc/paper.sh.Z.
Tunneling and the Spectrum of the Potts Model
The three-dimensional, three-state Potts model is studied as a paradigm for
high temperature quantum chromodynamics. In a high statistics numerical
simulation using a Swendson-Wang algorithm, we study cubic lattices of
dimension as large as and measure correlation functions on long lattices
of dimension and . These correlations are
controlled by the spectrum of the transfer matrix. This spectrum is studied in
the vicinity of the phase transition. The analysis classifies the spectral
levels according to an underlying symmetry. Near the phase transition the
spectrum agrees nicely with a simple four-component hamiltonian model. In the
context of this model, we find that low temperature ordered-ordered interfaces
nearly always involve a disordered phase intermediate. We present a new
spectral method for determining the surface tension between phases.Comment: 26 pages plus 13 Postscript figures (Axis versions also provided),
UU-HEP-92/
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