4,827 research outputs found
Pupil remapping for high contrast astronomy: results from an optical testbed
The direct imaging and characterization of Earth-like planets is among the
most sought-after prizes in contemporary astrophysics, however current optical
instrumentation delivers insufficient dynamic range to overcome the vast
contrast differential between the planet and its host star. New opportunities
are offered by coherent single mode fibers, whose technological development has
been motivated by the needs of the telecom industry in the near infrared. This
paper presents a new vision for an instrument using coherent waveguides to
remap the pupil geometry of the telescope. It would (i) inject the full pupil
of the telescope into an array of single mode fibers, (ii) rearrange the pupil
so fringes can be accurately measured, and (iii) permit image reconstruction so
that atmospheric blurring can be totally removed. Here we present a laboratory
experiment whose goal was to validate the theoretical concepts underpinning our
proposed method. We successfully confirmed that we can retrieve the image of a
simulated astrophysical object (in this case a binary star) though a pupil
remapping instrument using single mode fibers.Comment: Accepted in Optics Expres
Near-IR imaging of T Cha: evidence for scattered-light disk structures at solar system scales
T Chamaeleontis is a young star surrounded by a transitional disk, and a
plausible candidate for ongoing planet formation. Recently, a substellar
companion candidate was reported within the disk gap of this star. However, its
existence remains controversial, with the counter-hypothesis that light from a
high inclination disk may also be consistent with the observed data. The aim of
this work is to investigate the origin of the observed closure phase signal to
determine if it is best explained by a compact companion. We observed T Cha in
the L and K s filters with sparse aperture masking, with 7 datasets covering a
period of 3 years. A consistent closure phase signal is recovered in all L and
K s datasets. Data were fit with a companion model and an inclined
circumstellar disk model based on known disk parameters: both were shown to
provide an adequate fit. However, the absence of expected relative motion for
an orbiting body over the 3-year time baseline spanned by the observations
rules out the companion model. Applying image reconstruction techniques to each
dataset reveals a stationary structure consistent with forward scattering from
the near edge of an inclined disk.Comment: 6 pages, 3 figures, accepted for publication in MNRAS Letter
Quelle intégration des pays en développement dans le régime climatique ? Le Mécanisme pour un Développement Propre en Asie
En analysant la mise en oeuvre du Mécanisme pour un Développement Propre (MDP), cet article vise à préciser les modalités d'intégration des pays en développement dans le régime climatique afin de s'interroger sur deux aspects particuliers : la dimension régionale de ce mécanisme de flexibilité - dont la plupart des projets ont été accueillis en Asie - et la pertinence des projets MDP face au défi d'une bonne articulation entre les politiques climatiques et les stratégies de développement soutenable. Ces aspects sont analysés dans le cas des pays d'Asie Orientale, qui présentent à la fois un enjeu fort de réduction des émissions de CO2 et une diversité de situations concernant les secteurs ciblés et les politiques énergie-climat
Unveiling the near-infrared structure of the massive-young stellar object NGC 3603 IRS 9A with sparse aperture masking and spectroastrometry
Contemporary theory holds that massive stars gather mass during their initial
phases via accreting disk-like structures. However, conclusive evidence for
disks has remained elusive for the most massive young objects. This is mainly
due to significant observational challenges. Incisive studies, even targeting
individual objects, are therefore relevant to the progression of the field. NGC
3603 IRS 9A* is a young massive stellar object still surrounded by an envelope
of molecular gas. Previous mid-infrared observations with long-baseline
interferometry provided evidence for a disk of 50 mas diameter at its core.
This work aims at a comprehensive study of the physics and morphology of IRS 9A
at near-infrared wavelengths. New sparse aperture masking interferometry data
taken with NACO/VLT at Ks and Lp filters were obtained and analysed together
with archival CRIRES spectra of the H2 and BrG lines. The calibrated
visibilities recorded at Ks and Lp bands suggest the presence of a partially
resolved compact object of 30 mas at the core of IRS 9A, together with the
presence of over-resolved flux. The spectroastrometric signal of the H2 line
shows that this spectral feature proceeds from the large scale extended
emission (300 mas) of IRS 9A, while the BrG line appears to be formed at the
core of the object (20 mas). This scenario is consistent with the brightness
distribution of the source for near- and mid-infrared wavelengths at various
spatial scales. However, our model suffers from remaining inconsistencies
between SED modelling and the interferometric data. Moreover, the BrG
spectroastrometric signal indicates that the core of IRS 9A exhibits some form
of complexity such as asymmetries in the disk. Future high-resolution
observations are required to confirm the disk/envelope model and to flesh out
the details of the physical form of the inner regions of IRS 9A.Comment: Accepted to be published in Astronomy & Astrophysics, 13 pages, 14
figure
High resolution observations of the outer disk around T Cha: the view from ALMA
T Cha is a young star surrounded by a transitional disk with signatures of
planet formation. We have obtained high-resolution and high-sensitivity ALMA
observations of T Cha in the --, --, and
-- emission lines to reveal the spatial distribution of the
gaseous disk around the star. In order to study the dust within the disk we
have also obtained continuum images at 850m from the line-free channels.
We have spatially resolved the outer disk around T Cha. Using the CO(3-2)
emission we derive a radius of 230 AU. We also report the detection of
the CO(3-2) and the CS(7-8) molecular emissions, which show smaller
radii than the CO(3-2) detection. The continuum observations at 850m allow
the spatial resolution of the dusty disk, which shows two emission bumps
separated by 40AU, consistent with the presence of a dust gap in the
inner regions of the disk, and an outer radius of 80AU. Therefore, T Cha
is surrounded by a compact dusty disk and a larger and more diffuse gaseous
disk, as previously observed in other young stars. The continuum intensity
profiles are different at both sides of the disk suggesting possible dust
asymmetries. We derive an inclination of i(deg)=675, and a position angle
of PA (deg)= 1136, for both the gas and dust disks. The comparison of the
ALMA data with radiative transfer models shows that the gas and dust components
can only be simultaneously reproduced when we include a tapered edge
prescription for the surface density profile. The best model suggests that most
of the disk mass is placed within a radius of 50AU. Finally, we derive a
dynamical mass for the central object of =1.50.2M,
comparable to the one estimated with evolutionary models for an age of
10Myr.Comment: 5 pages, 5 figures, accepted for publication in A&A Letter
Active force generation contributes to the complexity of spontaneous activity and to the response to stretch of murine cardiomyocyte cultures.
Monolayer cultures of cardiac cells exhibit spontaneous electrical and contractile activity, as in a natural cardiac pacemaker. Beating variability in these preparations recapitulates the power-law behavior of heart rate variability in vivo. However, the effects of mechano-electrical feedback on beating variability are not yet fully understood. Using stretchable microelectrode arrays, we examined the effects of the contraction uncoupler blebbistatin and the non-specific stretch activated channel blocker streptomycin on beating variability and on stretch-induced changes of beat rate. Without stretch, blebbistatin decreased the spatial complexity of beating variability, while streptomycin had no effects. Both stretch and release transiently increased beat rate; blebbistatin attenuated the increase of beat rate upon stretch, while streptomycin had no effects. Active force generation contributes to the complexity of spatiotemporal patterns of beating variability and to the increase of beat rate upon mechanical deformation. Our study contributes to understanding how mechano-electric feedback influences heart rate variability. Cardiomyocyte cultures exhibit spontaneous electrical and contractile activity, as in a natural cardiac pacemaker. In such preparations, beat rate variability exhibits features similar to those of heart rate variability in vivo. Mechanical deformations and forces feedback on the electrical properties of cardiomyocytes, but it is not fully elucidated how this mechano-electrical interplay affects beating variability in such preparations. Using stretchable microelectrode arrays, we assessed the effects of the myosin inhibitor blebbistatin and the nonselective stretch-activated channel blocker streptomycin on beating variability and on the response of neonatal or foetal murine ventricular cell cultures against deformation. Spontaneous electrical activity was recorded without stretch and upon predefined deformation protocols (5% uniaxial and 2% equibiaxial strain, applied repeatedly for 1 min every 3 min). Without stretch, spontaneous activity originated from the edge of the preparations, and its site of origin switched frequently in a complex manner across the cultures. Blebbistatin did not change mean beat rate, but it decreased the spatial complexity of spontaneous activity. In contrast, streptomycin did not exert any manifest effects. During the deformation protocols, beat rate transiently increased upon stretch, but paradoxically also upon release. Blebbistatin attenuated the response to stretch, while this response was not affected by streptomycin. Therefore, our data support the notion that in a spontaneously firing network of cardiomyocytes, active force generation, rather than stretch-activated channels, is mechanistically involved in the complexity of the spatiotemporal patterns of spontaneous activity and in the stretch-induced acceleration of beating. Abstract figure legend Mechano-electric feedback modulates myocardial electrical function, including pacemaking. By growing monolayer cultures of spontaneously active murine cardiac cells on stretchable microelectrode arrays, we examined whether active contractions influence the spatiotemporal characteristics of beating variability and the effects of stretching on beat rate. Under control conditions (no stretch and no pharmacological agent), the origin of the electrical activity changed frequently. After blocking contractions with blebbistatin, the spatiotemporal pattern of electrical activity became less variable and less complex. Under control conditions (no pharmacological agent), stretching (and also releasing) the cardiomyocyte monolayers transiently increased beat rate. Blebbistatin attenuated the acceleration of beating upon stretch. In contrast, streptomycin had no detectable effects. Thus, active force generation is involved in determining beating variability in spontaneously active cardiac tissue. Possible mechanisms may include cellular processes that sense contraction and chemical messengers. Our study contributes to understanding how mechano-electric feedback influences heart rate variability. This article is protected by copyright. All rights reserved
High resolution imaging of young M-type stars of the solar neighborhood: Probing the existence of companions down to the mass of Jupiter
Context. High contrast imaging is a powerful technique to search for gas
giant planets and brown dwarfs orbiting at separation larger than several AU.
Around solar-type stars, giant planets are expected to form by core accretion
or by gravitational instability, but since core accretion is increasingly
difficult as the primary star becomes lighter, gravitational instability would
be the a probable formation scenario for yet-to-be-found distant giant planets
around a low-mass star. A systematic survey for such planets around M dwarfs
would therefore provide a direct test of the efficiency of gravitational
instability. Aims. We search for gas giant planets orbiting around late-type
stars and brown dwarfs of the solar neighborhood. Methods. We obtained deep
high resolution images of 16 targets with the adaptive optic system of VLT-NACO
in the Lp band, using direct imaging and angular differential imaging. This is
currently the largest and deepest survey for Jupiter-mass planets around
Mdwarfs. We developed and used an integrated reduction and analysis pipeline to
reduce the images and derive our 2D detection limits for each target. The
typical contrast achieved is about 9 magnitudes at 0.5" and 11 magnitudes
beyond 1". For each target we also determine the probability of detecting a
planet of a given mass at a given separation in our images. Results. We derived
accurate detection probabilities for planetary companions, taking into account
orbital projection effects, with in average more than 50% probability to detect
a 3MJup companion at 10AU and a 1.5MJup companion at 20AU, bringing strong
constraints on the existence of Jupiter-mass planets around this sample of
young M-dwarfs.Comment: Accepted for publication in A&
Nanostructuring lithium niobate substrates by focused ion beam milling
We report on two novel ways for patterning Lithium Niobate (LN) at
submicronic scale by means of focused ion beam (FIB) bombardment. The first
method consists of direct FIB milling on LiNbO3 and the second one is a
combination of FIB milling on a deposited metallic layer and subsequent RIE
(Reactive Ion Etching) etching. FIB images show in both cases homogeneous
structures with well reproduced periodicity. These methods open the way to the
fabrication of photonic crystals on LiNbO3 substrates
Characterization of integrated optics components for the second generation of VLTI instruments
Two of the three instruments proposed to ESO for the second generation
instrumentation of the VLTI would use integrated optics for beam combination.
Several design are studied, including co-axial and multi-axial recombination.
An extensive quantity of combiners are therefore under test in our
laboratories. We will present the various components, and the method used to
validate and compare the different combiners. Finally, we will discuss the
performances and their implication for both VSI and Gravity VLTI instruments.Comment: SPIE Astronomical Instrumentation 2008 in Marseille, France --
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