195 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
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
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.
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
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
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
Non Destructive Evaluation of Containment Walls in Nuclear Power Plants
Two functions are regularly tested on the containment walls in order to anticipate a possible accident. The first is mechanical to resist at a possible internal over-pressure and the second is to prevent leakage. The reference accident LLOCA (Large Loss of Coolant Accident) is the rupture of a pipe in the primary circuit of a nuclear plant. In this case, the pressure and temperature can reach 5 bar and 180°C in 20 seconds.
The national project âNon-destructive testing of the containment structures of nuclear plantsâ aims at studying the non-destructive techniques capable to evaluate the concrete properties and its damaging or progression of cracks. This 4-year-project is segmented into two parts. The first consists in developing and selecting the most relevant NDEs (Non Destructive Evaluations) in the laboratory to reach these goals. These evaluations are developed in conditions representing the real conditions of the stresses generated during ten-yearly visits of the plants or those related to an accident. The second part consists in applying the selected techniques to two containment structures under pressure. The first (technique) is proposed by the ONERA (National Office for Aerospace Studies and Research of France) and the second is a mock-up of a containment wall on a 1/3 scale made by EDF (Electricity of France) within the VeRCoRs program.
Communication bears on the part of the project that concerns the damaging and cracking follow-up. The tests are done in bending on 3 or 4 points in order to study the cracksâ generation, their propagation, as well as their opening and closing. The mostly ultrasonic techniques developed concern linear or non-linear acoustic: acoustic emission [1], LOCADIFF (Locating with diffuse ultrasound) [2], energy diffusion, surface waves velocity and attenuation, DAET (Dynamic Acousto-Elasticity Testing) [3]. The data contribute to providing the mapping of the parameters searched for, either in volume, in surface or globally. Image correlation is an important additional asset to validate the coherence of the data. The spatial normalization of the data allows proposing algorithms on the combination of the experimental data.
The tests results are presented and they show the capacity and the limits of the evaluation of the volume, surface or global data. A data fusion procedure is associated with these results
NectarCAM : a camera for the medium size telescopes of the Cherenkov Telescope Array
NectarCAM is a camera proposed for the medium-sized telescopes of the
Cherenkov Telescope Array (CTA) covering the central energy range of ~100 GeV
to ~30 TeV. It has a modular design and is based on the NECTAr chip, at the
heart of which is a GHz sampling Switched Capacitor Array and a 12-bit Analog
to Digital converter. The camera will be equipped with 265 7-photomultiplier
modules, covering a field of view of 8 degrees. Each module includes the
photomultiplier bases, high voltage supply, pre-amplifier, trigger, readout and
Ethernet transceiver. The recorded events last between a few nanoseconds and
tens of nanoseconds. The camera trigger will be flexible so as to minimize the
read-out dead-time of the NECTAr chips. NectarCAM is designed to sustain a data
rate of more than 4 kHz with less than 5\% dead time. The camera concept, the
design and tests of the various subcomponents and results of thermal and
electrical prototypes are presented. The design includes the mechanical
structure, cooling of the electronics, read-out, clock distribution, slow
control, data-acquisition, triggering, monitoring and services.Comment: In Proceedings of the 34th International Cosmic Ray Conference
(ICRC2015), The Hague, The Netherlands. All CTA contributions at
arXiv:1508.0589
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