253 research outputs found
An experimental testbed for NEAT to demonstrate micro-pixel accuracy
NEAT is an astrometric mission proposed to ESA with the objectives of
detecting Earth-like exoplanets in the habitable zone of nearby solar-type
stars. In NEAT, one fundamental aspect is the capability to measure stellar
centroids at the precision of 5e-6 pixel. Current state-of-the-art methods for
centroid estimation have reached a precision of about 4e-5 pixel at Nyquist
sampling. Simulations showed that a precision of 2 micro-pixels can be reached,
if intra and inter pixel quantum efficiency variations are calibrated and
corrected for by a metrology system. The European part of the NEAT consortium
is designing and building a testbed in vacuum in order to achieve 5e-6 pixel
precision for the centroid estimation. The goal is to provide a proof of
concept for the precision requirement of the NEAT spacecraft. In this paper we
give the basic relations and trade-offs that come into play for the design of a
centroid testbed and its metrology system. We detail the different conditions
necessary to reach the targeted precision, present the characteristics of our
current design and describe the present status of the demonstration.Comment: SPIE proceeding
Using highly variable warfarin dosing to identify patients at risk for adverse events
<p>Abstract</p> <p>Background</p> <p>Patients who receive highly variable doses of warfarin may be at risk for poor anticoagulation control and adverse events. However, we lack a system to identify patients with the highest dose variability. Our objectives were to develop a scoring system to identify patients with high dose variability, and to validate this new measure by demonstrating that patients so identified have poor anticoagulation control and higher rates of adverse events (criterion validity).</p> <p>Methods</p> <p>We used a database of over 4, 000 patients who received oral anticoagulation in community practice between 2000-2002. We reviewed the charts of 168 patients with large warfarin dose variation and agreed on 18 risk factor definitions for high dose variability. We identified 109 patients with the highest dose variability (cases), as measured by coefficient of variation (CoV, SD/mean). We matched each case to two controls with low dose variability. Then, we examined all 327 charts, blinded to case/control status, to identify the presence or absence of the 18 risk factors for dose variability. We performed a multivariable analysis to identify independent predictors of high CoV. We also compared anticoagulation control, as measured by percent time in therapeutic range (TTR), and rates of adverse events between groups.</p> <p>Results</p> <p>CoV corresponded with other measures of anticoagulation control. TTR was 53% among cases and 79% among controls (p < 0.001). CoV also predicted adverse events. Six cases experienced a major hemorrhage versus 1 control (p < 0.001) and 3 cases had a thromboembolic event versus 0 control patients (p = 0.04). Independent predictors of high dose variability included hospitalization (OR = 21.3), decreased oral intake (OR = 12.2), use of systemic steroids (OR = 6.1), acetaminophen (OR = 4.0) and antibiotics (OR = 2.7; p < 0.05 for all).</p> <p>Conclusion</p> <p>CoV can be used to identify patients at risk for poor anticoagulation control and adverse events. This new measure has the potential to identify patients at high risk before they suffer adverse events.</p
Stellar populations -- the next ten years
The study of stellar populations is a discipline that is highly dependent on
both imaging and spectroscopy. I discuss techniques in different regimes of
resolving power: broadband imaging (R~4), intermediate band imaging (R~16, 64),
narrowband spectral imaging (R~256, 1024, 4096). In recent years, we have seen
major advances in broadband all-sky surveys that are set to continue across
optical and IR bands, with the added benefit of the time domain, higher
sensitivity, and improved photometric accuracy. Tunable filters and integral
field spectrographs are poised to make further inroads into intermediate and
narrowband imaging studies of stellar populations. Further advances will come
from AO-assisted imaging and imaging spectroscopy, although photometric
accuracy will be challenging. Integral field spectroscopy will continue to have
a major impact on future stellar population studies, extending into the near
infrared once the OH suppression problem is finally resolved. A sky rendered
dark will allow a host of new ideas to be explored, and old ideas to be
revisited.Comment: Invited review, IAUS 241, "Stellar Populations as Building Blocks of
Galaxies," eds. Vazdekis, Peletier. 12 pages, 1 table. (The sideways table
should print ok; there are 10 columns.
On the black hole content and initial mass function of 47 Tuc
The globular cluster (GC) 47 Tuc has recently been proposed to host an
intermediate-mass black hole (IMBH) or a population of stellar-mass black holes
(BHs). To shed light on its dark content, we present an application of
self-consistent multimass models with a varying mass function and content of
stellar remnants, which we fit to various observational constraints. Our
best-fitting model successfully matches the observables and correctly predicts
the radial distribution of millisecond pulsars and their gravitational
accelerations inferred from long-term timing observations. The data favours a
population of BHs with a total mass of , but the
most likely model has very few BHs. Since our models do not include a central
IMBH and accurately reproduce the observations, we conclude that there is
currently no need to invoke the presence of an IMBH in 47 Tuc. The global
present-day mass function inferred is significantly depleted in low-mass stars
(power-law slope ). Given the orbit and predicted
mass-loss history of this massive GC, the dearth of low-mass stars is difficult
to explain with a standard initial mass function (IMF) followed by long-term
preferential escape of low-mass stars driven by two-body relaxation, and
instead suggests that 47 Tuc may have formed with a bottom-light IMF. We
discuss alternative evolutionary origins for the flat mass function and ways to
reconcile this with the low BH retention fraction. Finally, by capturing the
effect of dark remnants, our method offers a new way to probe the IMF in a GC
above the current main-sequence turn-off mass, for which we find a slope of
.Comment: 16 pages, 7 figures, accepted to MNRAS after minor revisio
First experimental results of very high accuracy centroiding measurements for the neat astrometric mission
NEAT is an astrometric mission proposed to ESA with the objectives of
detecting Earth-like exoplanets in the habitable zone of nearby solar-type
stars. NEAT requires the capability to measure stellar centroids at the
precision of 5e-6 pixel. Current state-of-the-art methods for centroid
estimation have reached a precision of about 2e-5 pixel at two times Nyquist
sampling, this was shown at the JPL by the VESTA experiment. A metrology system
was used to calibrate intra and inter pixel quantum efficiency variations in
order to correct pixelation errors. The European part of the NEAT consortium is
building a testbed in vacuum in order to achieve 5e-6 pixel precision for the
centroid estimation. The goal is to provide a proof of concept for the
precision requirement of the NEAT spacecraft. In this paper we present the
metrology and the pseudo stellar sources sub-systems, we present a performance
model and an error budget of the experiment and we report the present status of
the demonstration. Finally we also present our first results: the experiment
had its first light in July 2013 and a first set of data was taken in air. The
analysis of this first set of data showed that we can already measure the pixel
positions with an accuracy of about 1e-4 pixel.Comment: SPIE conference proceeding
The VLT-FLAMES Tarantula Survey. IX. The interstellar medium seen through Diffuse Interstellar Bands and neutral sodium
The Tarantula Nebula (30 Dor) is a spectacular star-forming region in the
Large Magellanic Cloud, seen through gas in the Galactic Disc and Halo. Diffuse
Interstellar Bands offer a unique probe of the diffuse, cool-warm gas in these
regions. The aim is to use DIBs as diagnostics of the local interstellar
conditions, whilst at the same time deriving properties of the yet-unknown
carriers. Spectra of over 800 early-type stars from the VLT Flames Tarantula
Survey (VFTS) were analysed. Maps were created, separately, for the Galactic
and LMC absorption in the DIBs at 4428 and 6614 Ang and - in a smaller region
near the central cluster R136 - neutral sodium (Na I D); we also measured the
DIBs at 5780 and 5797 Ang. The maps show strong 4428 and 6614 Ang DIBs in the
quiescent cloud complex to the south of 30 Dor but weak absorption in the
harsher environments to the north (bubbles) and near the OB associations. The
Na maps show at least five kinematic components in the LMC and a shell-like
structure surrounding R136, and small-scale structure in the Milky Way. The
strengths of the 4428, 5780, 5797 and 6614 Ang DIBs are correlated, also with
Na absorption and visual extinction. The strong 4428 Ang DIB is present already
at low Na column density but the 6614, 5780 and 5797 Ang DIBs start to be
detectable at subsequently larger Na column densities. The relative strength of
the 5780 and 5797 Ang DIBs clearly confirm the Tarantula Nebula and Galactic
high-latitude gas to represent a harsh radiation environment. The resilience of
the 4428 Ang DIB suggests its carrier is large, compact and neutral. Structure
is detected in the distribution of cool-warm gas on scales between one and >100
pc in the LMC and as little as 0.01 pc in the Sun's vicinity. Stellar winds
from the central cluster R136 have created an expanding shell; some infalling
gas is also detected, reminiscent of a galactic "fountain".Comment: Accepted for publication in Astronomy and Astrophysic
Globular cluster number density profiles using Gaia DR2
Using data from Gaia DR2, we study the radial number density profiles of the
Galactic globular cluster sample. Proper motions are used for accurate
membership selection, especially crucial in the cluster outskirts. Due to the
severe crowding in the centres, the Gaia data is supplemented by literature
data from HST and surface brightness measurements, where available. This
results in 81 clusters with a complete density profile covering the full tidal
radius (and beyond) for each cluster. We model the density profiles using a set
of single-mass models ranging from King and Wilson models to generalised
lowered isothermal limepy models and the recently introduced spes models, which
allow for the inclusion of potential escapers. We find that both King and
Wilson models are too simple to fully reproduce the density profiles, with King
(Wilson) models on average underestimating(overestimating) the radial extent of
the clusters. The truncation radii derived from the limepy models are similar
to estimates for the Jacobi radii based on the cluster masses and their orbits.
We show clear correlations between structural and environmental parameters, as
a function of Galactocentric radius and integrated luminosity. Notably, the
recovered fraction of potential escapers correlates with cluster pericentre
radius, luminosity and cluster concentration. The ratio of half mass over
Jacobi radius also correlates with both truncation parameter and PE fraction,
showing the effect of Roche lobe filling.Comment: 23 pages, 23 figures. Accepted in MNRA
A detector interferometric calibration experiment for high precision astrometry
Context: Exoplanet science has made staggering progress in the last two
decades, due to the relentless exploration of new detection methods and
refinement of existing ones. Yet astrometry offers a unique and untapped
potential of discovery of habitable-zone low-mass planets around all the
solar-like stars of the solar neighborhood. To fulfill this goal, astrometry
must be paired with high precision calibration of the detector.
Aims: We present a way to calibrate a detector for high accuracy astrometry.
An experimental testbed combining an astrometric simulator and an
interferometric calibration system is used to validate both the hardware needed
for the calibration and the signal processing methods. The objective is an
accuracy of 5e-6 pixel on the location of a Nyquist sampled polychromatic point
spread function.
Methods: The interferometric calibration system produced modulated Young
fringes on the detector. The Young fringes were parametrized as products of
time and space dependent functions, based on various pixel parameters. The
minimization of func- tion parameters was done iteratively, until convergence
was obtained, revealing the pixel information needed for the calibration of
astrometric measurements.
Results: The calibration system yielded the pixel positions to an accuracy
estimated at 4e-4 pixel. After including the pixel position information, an
astrometric accuracy of 6e-5 pixel was obtained, for a PSF motion over more
than five pixels. In the static mode (small jitter motion of less than 1e-3
pixel), a photon noise limited precision of 3e-5 pixel was reached
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