2,606 research outputs found
Focal plane wavefront sensor achromatization : The multireference self-coherent camera
High contrast imaging and spectroscopy provide unique constraints for
exoplanet formation models as well as for planetary atmosphere models. But this
can be challenging because of the planet-to-star small angular separation and
high flux ratio. Recently, optimized instruments like SPHERE and GPI were
installed on 8m-class telescopes. These will probe young gazeous exoplanets at
large separations (~1au) but, because of uncalibrated aberrations that induce
speckles in the coronagraphic images, they are not able to detect older and
fainter planets. There are always aberrations that are slowly evolving in time.
They create quasi-static speckles that cannot be calibrated a posteriori with
sufficient accuracy. An active correction of these speckles is thus needed to
reach very high contrast levels (>1e7). This requires a focal plane wavefront
sensor. Our team proposed the SCC, the performance of which was demonstrated in
the laboratory. As for all focal plane wavefront sensors, these are sensitive
to chromatism and we propose an upgrade that mitigates the chromatism effects.
First, we recall the principle of the SCC and we explain its limitations in
polychromatic light. Then, we present and numerically study two upgrades to
mitigate chromatism effects: the optical path difference method and the
multireference self-coherent camera. Finally, we present laboratory tests of
the latter solution.
We demonstrate in the laboratory that the MRSCC camera can be used as a focal
plane wavefront sensor in polychromatic light using an 80 nm bandwidth at 640
nm. We reach a performance that is close to the chromatic limitations of our
bench: contrast of 4.5e-8 between 5 and 17 lambda/D.
The performance of the MRSCC is promising for future high-contrast imaging
instruments that aim to actively minimize the speckle intensity so as to detect
and spectrally characterize faint old or light gaseous planets.Comment: 14 pages, 20 figure
Laboratory validation of the dual-zone phase mask coronagraph in broadband light at the high-contrast imaging THD-testbed
Specific high contrast imaging instruments are mandatory to characterize
circumstellar disks and exoplanets around nearby stars. Coronagraphs are
commonly used in these facilities to reject the diffracted light of an observed
star and enable the direct imaging and spectroscopy of its circumstellar
environment. One important property of the coronagraph is to be able to work in
broadband light.
Among several proposed coronagraphs, the dual-zone phase mask coronagraph is
a promising solution for starlight rejection in broadband light. In this paper,
we perform the first validation of this concept in laboratory.
First, we recall the principle of the dual-zone phase mask coronagraph. Then,
we describe the high-contrast imaging THD testbed, the manufacturing of the
components and the quality-control procedures. Finally, we study the
sensitivity of our coronagraph to low-order aberrations (inner working angle
and defocus) and estimate its contrast performance. Our experimental broadband
light results are compared with numerical simulations to check agreement with
the performance predictions.
With the manufactured prototype and using a dark hole technique based on the
self-coherent camera, we obtain contrast levels down to between 5
and 17 in monochromatic light (640 nm). We also reach contrast
levels of between 7 and 17 in broadband
( nm, nm and %), which demonstrates the excellent chromatic performance of the dual-zone
phase mask coronagraph.
The performance reached by the dual-zone phase mask coronagraph is promising
for future high-contrast imaging instruments that aim at detecting and
spectrally characterizing old or light gaseous planets.Comment: 9 pages, 16 figure
Hip 63510C, Hip 73786B, and nine new isolated high proper motion T dwarf candidates from UKIDSS DR6 and SDSS DR7
Aims: Completing the poorly known substellar census of the solar
neighbourhood, especially with respect to the coolest brown dwarfs, will lead
to a better understanding of failed star formation processes and binary
statistics with different environmental conditions. Methods: Using UKIDSS data
and their cross-correlation with the SDSS, we searched for high proper motion
mid- to late-T dwarf candidates with extremely blue near-infrared (J-K<0) and
very red optical-to-near-infrared (z-J>+2.5) colours. Results: With 11 newly
found T dwarf candidates, the proper motions of which range between 100 and 800
mas/yr, we increased the number of UKIDSS T dwarf discoveries by 30%.
Large proper motions were also measured for six of eight previously known
T4.5-T9 dwarfs detected in our survey. All new candidates can be classified as
T5-T9 dwarfs based on their colours. Two of these objects were found to be
common proper motion companions of Hipparcos stars with accurate parallaxes.
The latter allow us to determine absolute magnitudes from which we classify Hip
63510C as T7 and Hip 73786B as T6.5 dwarfs with an uncertainty of 1
spectral subtype. The projected physical separation from their low-mass (M0.5
and K5) primaries is in both cases about 1200 AU. One of the Hipparcos stars
has already a known very low-mass star or brown dwarf companion on a close
astrometric orbit (Hip 63510B = Gl 494B). With distances of only 11.7 and 18.6
pc, deduced from their primaries respectively for Hip 63510C and Hip 73786B,
various follow-up observations can easily be carried out to study these cool
brown dwarfs in more detail and to compare their properties with those of the
already well-investigated primaries.Comment: improved and extended version accepted by Astronomy and Astrophysics
(text and references added, revised accuracies of proper motions and spectral
types, new Table 7, extended Fig.3
Mindfulness related changes in grey matter: a systematic review and meta‐analysis
International audienceAbstract Knowing target regions undergoing strfuncti changes caused by behavioural interventions is paramount in evaluating the effectiveness of such practices. Here, using a systematic review approach, we identified 25 peer-reviewed magnetic resonance imaging (MRI) studies demonstrating grey matter changes related to mindfulness meditation. An activation likelihood estimation (ALE) analysis (n = 16) revealed the right anterior ventral insula as the only significant region with consistent effect across studies, whilst an additional functional connectivity analysis indicates that both left and right insulae, and the anterior cingulate gyrus with adjacent paracingulate gyri should also be considered in future studies. Statistical meta-analyses suggest medium to strong effect sizes from Cohen’s d ~ 0.8 in the right insula to ~ 1 using maxima across the whole brain. The systematic review revealed design issues with selection, information, attrition and confirmation biases, in addition to weak statistical power. In conclusion, our analyses show that mindfulness meditation practice does induce grey matter changes but also that improvements in methodology are needed to establish mindfulness as a therapeutic intervention
Direct imaging constraints on planet populations detected by microlensing
Results from gravitational microlensing suggested the existence of a large
population of free-floating planetary mass objects. The main conclusion from
this work was partly based on constraints from a direct imaging survey. This
survey determined upper limits for the frequency of stars that harbor giant
exoplanets at large orbital separations. Aims. We want to verify to what extent
upper limits from direct imaging do indeed constrain the microlensing results.
We examine the current derivation of the upper limits used in the microlensing
study and re-analyze the data from the corresponding imaging survey. We focus
on the mass and semi-major axis ranges that are most relevant in context of the
microlensing results. We also consider new results from a recent M-dwarf
imaging survey as these objects are typically the host stars for planets
detected by microlensing. We find that the upper limits currently applied in
context of the microlensing results are probably underestimated. This means
that a larger fraction of stars than assumed may harbor gas giant planets at
larger orbital separations. Also, the way the upper limit is currently used to
estimate the fraction of free-floating objects is not strictly correct. If the
planetary surface density of giant planets around M-dwarfs is described as
df_Planet ~ a^beta da, we find that beta ~ 0.5 - 0.6 is consistent with results
from different observational studies probing semi-major axes between ~0.03 - 30
AU. Having a higher upper limit on the fraction of stars that may have gas
giant planets at orbital separations probed by the microlensing data implies
that more of the planets detected in the microlensing study are potentially
bound to stars rather than free-floating. The current observational data are
consistent with a rising planetary surface density for giant exoplanets around
M-dwarfs out to ~30 AU.Comment: Accepted for publication in A&A as Research Note, 3 page
An ARXPS and ERXPS study of quaternary ammonium and phosphonium ionic liquids: utilising a high energy Ag Lα’ X-ray source
Introduction
Ionic liquid (IL) surface science has experienced rapid expansion in recent years. As such, a multitude of ultra-high vacuum (UHV) techniques have been used to probe the IL/vacuum interface including laboratory and synchrotron X-ray photoelectron spectroscopy (XPS), metastable impact electron spectroscopy (MIES), low energy ion scattering (LEIS), Rutherford backscattering (RBS) and neutral impact collision ion scattering spectroscopy (NICISS). To date, the vast majority of these studies have focused upon cyclic nitrogen-containing cations, particularly the 1-alkyl-3-methylimidazolium family, whereas acyclic cations including tetraalkylammonium and –phosphonium have been overlooked despite their potential use in a wide range of existing applications including heterogeneous catalysis, gas capture/separation, and nanoparticle formation
Discovery of a Low-Mass Companion to the F7V star HD 984
We report the discovery of a low-mass companion to the nearby (d = 47 pc) F7V
star HD 984. The companion is detected 0.19" away from its host star in the L'
band with the Apodizing Phase Plate on NaCo/VLT and was recovered by L'-band
non-coronagraphic imaging data taken a few days later. We confirm the companion
is co-moving with the star with SINFONI integral field spectrograph H+K data.
We present the first published data obtained with SINFONI in pupil-tracking
mode. HD 984 has been argued to be a kinematic member of the 30 Myr-old Columba
group, and its HR diagram position is not altogether inconsistent with being a
ZAMS star of this age. By consolidating different age indicators, including
isochronal age, coronal X-ray emission, and stellar rotation, we independently
estimate a main sequence age of 11585 Myr (95% CL) which does not rely on
this kinematic association. The mass of directly imaged companions are usually
inferred from theoretical evolutionary tracks, which are highly dependent on
the age of the star. Based on the age extrema, we demonstrate that with our
photometric data alone, the companion's mass is highly uncertain: between 33
and 96 M (0.03-0.09 M) using the COND evolutionary
models. We compare the companion's SINFONI spectrum with field dwarf spectra to
break this degeneracy. Based on the slope and shape of the spectrum in the
H-band, we conclude that the companion is an M dwarf. The age of the
system is not further constrained by the companion, as M dwarfs are poorly fit
on low-mass evolutionary tracks. This discovery emphasizes the importance of
obtaining a spectrum to spectral type companions around F-stars.Comment: Accepted for publication in MNRAS, 10 pages, 5 figure
Corinth terraces re-visited: Improved paleoshoreline determination using Pleiades-DEMs
International audienceThe newest generation of satellites have greatly improved the capabilities of optical imagery over the last decade. Ground resolution has increased by one order of magnitude (to sub-metric pixel images), and improved sensors allow images to be located with an absolute accuracy of within a few meters. Better-resolved images facilitate refined tectonic studies of faults, basins, terraces, and other geomorphic features as it provides the opportunity to extract detailed topographic information. We have developed high-resolution digital elevation models (DEMs) in eight locations in Greece from tri-stereo satellite images acquired by the new Pleiades platform of CNES. With 0.5m resolution, these DEMs are state-of-the-art in comparison to previous DEMs made from satellite imagery. In this study we explore the potential of one of these DEMs, in the eastern Gulf of Corinth, for the analysis of a flight of marine terraces
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