2,165 research outputs found
Panchromatic Imaging of a Transitional Disk: The Disk of GM Aur in Optical and FUV Scattered Light
We have imaged GM Aur with HST, detected its disk in scattered light at 1400A
and 1650A, and compared these with observations at 3300A, 5550A, 1.1 microns,
and 1.6 microns. The scattered light increases at shorter wavelengths. The
radial surface brightness profile at 3300A shows no evidence of the 24AU radius
cavity that has been previously observed in sub-mm observations. Comparison
with dust grain opacity models indicates the surface of the entire disk is
populated with sub-micron grains. We have compiled an SED from 0.1 microns to 1
mm, and used it to constrain a model of the star+disk system that includes the
sub-mm cavity using the Monte Carlo Radiative Transfer code by Barbara Whitney.
The best-fit model image indicates that the cavity should be detectable in the
F330W bandpass if the cavity has been cleared of both large and small dust
grains, but we do not detect it. The lack of an observed cavity can be
explained by the presence of sub-microns grains interior to the sub-mm cavity
wall. We suggest one explanation for this which could be due to a planet of
mass <9 Jupiter masses interior to 24 AU. A unique cylindrical structure is
detected in the FUV data from the Advanced Camera for Surveys/Solar Blind
Channel. It is aligned along the system semi-minor axis, but does not resemble
an accretion-driven jet. The structure is limb-brightened and extends 190 +/-
35 AU above the disk midplane. The inner radius of the limb-brightening is 40
+/- 10 AU, just beyond the sub-millimeter cavity wall.Comment: 40 pages, 11 figures, 4 tables, accepted to Ap
Long-Term Clinical Impact of Coronary CT Angiography in Patients With Recent Acute-Onset Chest Pain The Randomized Controlled CATCH Trial
AbstractObjectivesThe aim of the CATCH (CArdiac cT in the treatment of acute CHest pain) trial was to investigate the long-term clinical impact of a coronary computed tomographic angiography (CTA)-guided treatment strategy in patients with recent acute-onset chest pain compared to standard care.BackgroundThe prognostic implications of a coronary CTA-guided treatment strategy have not been compared in a randomized fashion to standard care in patients referred for acute-onset chest pain.MethodsPatients with acute chest pain but normal electrocardiograms and troponin values were randomized to treatment guided by either coronary CTA or standard care (bicycle exercise electrocardiogram or myocardial perfusion imaging). In the coronary CTA-guided group, a functional test was included in cases of nondiagnostic coronary CTA images or coronary stenoses of borderline severity. The primary endpoint was a composite of cardiac death, myocardial infarction (MI), hospitalization for unstable angina pectoris (UAP), late symptom-driven revascularizations, and readmission for chest pain.ResultsWe randomized 299 patients to coronary CTA-guided strategy and 301 to standard care. After inclusion, 24 patients withdrew their consent. The median (interquartile range) follow-up duration was 18.7 (range 16.8 to 20.1) months. In the coronary CTA-guided group, 30 patients (11%) had a primary endpoint versus 47 patients (16%) in the standard care group (p = 0.04; hazard ratio [HR]: 0.62 [95% confidence interval: 0.40 to 0.98]). A major adverse cardiac event (cardiac death, MI, hospitalization for UAP, and late symptom-driven revascularization) was observed in 5 patients (2 MIs, 3 UAPs) in the coronary CTA-guided group versus 14 patients (1 cardiac death, 7 MIs, 5 UAPs, 1 late symptom-driven revascularization) in the standard care group (p = 0.04; HR: 0.36 [95% CI: 0.16 to 0.95]). Differences in cardiac death and MI (8 vs. 2) were insignificant (p = 0.06).ConclusionsA coronary CTA-guided treatment strategy appears to improve clinical outcome in patients with recent acute-onset chest pain and normal electrocardiograms and troponin values compared to standard care with a functional test. (Cardiac-CT in the Treatment of Acute Chest Pain [CATCH]; NCT01534000
A large X-ray flare from the Herbig Ae star V892 Tau
We report the XMM-Newton observation of a large X-ray flare from the Herbig
Ae star V892 Tau. The apparent low mass companion of V892 Tau, V892 Tau NE, is
unresolved by XMM-Newton. Nevertheless there is compelling evidence from
combined XMM-Newton and Chandra data that the origin of the flare is the Herbig
Ae star V892 Tau. During the flare the X-ray luminosity of V892 Tau increases
by a factor of ~15, while the temperature of the plasma increases from kT ~ 1.5
keV to kT ~ 8 keV. From the scaling of the flare event, based on hydrodynamic
modeling, we conclude that a 500 G magnetic field is needed in order to confine
the plasma. Under the assumptions that a dynamo mechanism is required to
generate such a confining magnetic field and that surface convection is a
necessary ingredient for a dynamo, our findings provide indirect evidence for
the existence of a significant convection zone in the stellar envelope of
Herbig Ae stars.Comment: accepted for publication in A&A (12 pages - 6 figures
an overview of the MHONGOOSE survey: Observing nearby galaxies with MeerKAT
© Copyright owned by the author(s). MHONGOOSE is a deep survey of the neutral hydrogen distribution in a representative sample of 30 nearby disk and dwarf galaxies with H I masses from ∼ 106 to ∼ 1011 M, and luminosities from MR ∼ 12 to MR ∼ −22. The sample is selected to uniformly cover the available range in log(MHI). Our extremely deep observations, down to H I column density limits of well below 1018 cm−2 — or a few hundred times fainter than the typical H I disks in galaxies — will directly detect the effects of cold accretion from the intergalactic medium and the links with the cosmic web. These observations will be the first ever to probe the very low-column density neutral gas in galaxies at these high resolutions. Combination with data at other wavelengths, most of it already available, will enable accurate modeling of the properties and evolution of the mass components in these galaxies and link these with the effects of environment, dark matter distribution, and other fundamental properties such as halo mass and angular momentum. MHONGOOSE can already start addressing some of the SKA-1 science goals and will provide a comprehensive inventory of the processes driving the transformation and evolution of galaxies in the nearby universe at high resolution and over 5 orders of magnitude in column density. It will be a Nearby Galaxies Legacy Survey that will be unsurpassed until the advent of the SKA, and can serve as a highly visible, lasting statement of MeerKAT’s capabilities
Exploring the Partonic Structure of Hadrons through the Drell-Yan Process
The Drell-Yan process is a standard tool for probing the partonic structure
of hadrons. Since the process proceeds through a quark-antiquark annihilation,
Drell-Yan scattering possesses a unique ability to selectively probe sea
distributions. This review examines the application of Drell-Yan scattering to
elucidating the flavor asymmetry of the nucleon's sea and nuclear modifications
to the sea quark distributions in unpolarized scattering. Polarized beams and
targets add an exciting new dimension to Drell-Yan scattering. In particular,
the two initial-state hadrons give Drell-Yan sensitivity to chirally-odd
transversity distributions.Comment: 23 pages, 9 figures, to appear in J. Phys. G, resubmission corrects
typographical error
Variable selection: current practice in epidemiological studies
Selection of covariates is among the most controversial and difficult tasks in epidemiologic analysis. Correct variable selection addresses the problem of confounding in etiologic research and allows unbiased estimation of probabilities in prognostic studies. The aim of this commentary is to assess how often different variable selection techniques were applied in contemporary epidemiologic analysis. It was of particular interest to see whether modern methods such as shrinkage or penalized regression were used in recent publications. Stepwise selection methods remained the predominant method for variable selection in publications in epidemiological journals in 2008. Shrinkage methods were not used in any of the reviewed articles. Editors, reviewers and authors have insufficiently promoted the new, less controversial approaches of variable selection in the biomedical literature, whereas statisticians may not have adequately addressed the method’s feasibility
Control of star formation by supersonic turbulence
Understanding the formation of stars in galaxies is central to much of modern
astrophysics. For several decades it has been thought that stellar birth is
primarily controlled by the interplay between gravity and magnetostatic
support, modulated by ambipolar diffusion. Recently, however, both
observational and numerical work has begun to suggest that support by
supersonic turbulence rather than magnetic fields controls star formation. In
this review we outline a new theory of star formation relying on the control by
turbulence. We demonstrate that although supersonic turbulence can provide
global support, it nevertheless produces density enhancements that allow local
collapse. Inefficient, isolated star formation is a hallmark of turbulent
support, while efficient, clustered star formation occurs in its absence. The
consequences of this theory are then explored for both local star formation and
galactic scale star formation. (ABSTRACT ABBREVIATED)Comment: Invited review for "Reviews of Modern Physics", 87 pages including 28
figures, in pres
Quantum state preparation and macroscopic entanglement in gravitational-wave detectors
Long-baseline laser-interferometer gravitational-wave detectors are operating
at a factor of 10 (in amplitude) above the standard quantum limit (SQL) within
a broad frequency band. Such a low classical noise budget has already allowed
the creation of a controlled 2.7 kg macroscopic oscillator with an effective
eigenfrequency of 150 Hz and an occupation number of 200. This result, along
with the prospect for further improvements, heralds the new possibility of
experimentally probing macroscopic quantum mechanics (MQM) - quantum mechanical
behavior of objects in the realm of everyday experience - using
gravitational-wave detectors. In this paper, we provide the mathematical
foundation for the first step of a MQM experiment: the preparation of a
macroscopic test mass into a nearly minimum-Heisenberg-limited Gaussian quantum
state, which is possible if the interferometer's classical noise beats the SQL
in a broad frequency band. Our formalism, based on Wiener filtering, allows a
straightforward conversion from the classical noise budget of a laser
interferometer, in terms of noise spectra, into the strategy for quantum state
preparation, and the quality of the prepared state. Using this formalism, we
consider how Gaussian entanglement can be built among two macroscopic test
masses, and the performance of the planned Advanced LIGO interferometers in
quantum-state preparation
Searching for a Stochastic Background of Gravitational Waves with LIGO
The Laser Interferometer Gravitational-wave Observatory (LIGO) has performed
the fourth science run, S4, with significantly improved interferometer
sensitivities with respect to previous runs. Using data acquired during this
science run, we place a limit on the amplitude of a stochastic background of
gravitational waves. For a frequency independent spectrum, the new limit is
. This is currently the most sensitive
result in the frequency range 51-150 Hz, with a factor of 13 improvement over
the previous LIGO result. We discuss complementarity of the new result with
other constraints on a stochastic background of gravitational waves, and we
investigate implications of the new result for different models of this
background.Comment: 37 pages, 16 figure
- …