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

Carbon nanotubes–Fe–Alumina nanocomposites. Part II: microstructure and mechanical properties of the hot-Pressed composites

Carbon nanotubes-Fe-Al2O3 massive composites have been prepared by hot-pressing the corresponding composite powders, in which the carbon nanotubes are arranged in bundles smaller than 100 nm in diameter and several tens of micrometers long, forming a web-like network around the Fe-Al2O3 grains. In the powders, the quantity and the quality of the carbon nanotubes both depend on the Fe content (2, 5, 10, 15 and 20 wt%) and on the reduction temperature (900 or 1000°C) used for the preparation. Bundles of carbon nanotubes are present in the hot-pressed materials but with a decrease in quantity in comparison to the powders. This phenomenon appear to be less pronounced for the powders containing higher-quality carbon, i. e. a higher proportion of nanotubes with respect to the total carbon content. The presence of carbon as nanotubes and others species (Fe carbides, thick and short tubes, graphene layers) in the powders modifies the microstructure of the hot-pressed specimens in comparison to that of similar carbon-free nanocomposites : the densifications are lower, the matrix grains and the intergranular metal particles are smaller. The fracture strength of most carbon nanotubes-Fe-Al2O3 composites is only marginally higher than that of Al2O3 and are generally markedly lower than those of the carbon-free Fe-Al2O3 composites. The fracture toughness values are lower than or similar to that of Al2O3. However, SEM observations of composite fractures indicate that the nanotubes bundles, which are very flexible, could dissipate some fracture energy

Carbon nanotubes–Fe–alumina nanocomposites. Part I: influence of the Fe content on the synthesis of powders

Oxides based on a-alumina and containing various amounts of Fe (2, 5, 10, 15 and 20 cat.%) were prepared by decomposition and calcination of the corresponding mixed-oxalates. Selective reduction of the oxides in a H2-CH4 atmosphere produces nanometric Fe particles which are active for the in-situ nucleation and growth of carbon nanotubes. These form bundles smaller than 100 nm in diameter and several tens of micrometers long. However, the carbon nanotubes-Fe-Al2O3 nanocomposite powders may also contain Fe carbide nanoparticles as well as undesirable thick, short carbon tubes and thick graphene layers covering the Fe/Fe carbide nanoparticles. The influence of the Fe content and the reduction temperature on the composition and micro/nanostructure of the nanocomposite powders have been investigated with the aim of improving both the quantity of nanotubes and the quality of carbon, i. e. a smaller average tube diameter and/or more carbon in tubular form. A higher quantity of carbon nanotubes is obtained using a-Al1.8Fe0.2O3 as starting compound, i. e. the maximum Fe concentration (10 cat.%) allowing to retain the monophase solid solution. A further increase in Fe content provokes a phase partitioning and the formation of a Fe2O3-rich phase which upon reduction produces too large Fe particles. The best carbon quality is obtained with only 5 cat.% Fe (a-Al1.9Fe0.1O3), probably because the surface Fe nanoparticles formed upon reduction are a bit smaller than those formed from a-Al1.8Fe0.2O3, thereby allowing the formation of carbon nanotubes of a smaller diameter. For a given Fe content (≤ 10 cat.%), increasing the reduction temperature favours the quantity of nanotubes because of a higher CH4 sursaturation level in the gas atmosphere, but also provokes a decrease in carbon quality

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 $2\,10^{-8}$ between 5 and 17$\,\lambda_0/D$ in monochromatic light (640 nm). We also reach contrast levels of $4\,10^{-8}$ between 7 and 17$\lambda_0/D$ in broadband ($\lambda_0=675$ nm, $\Delta\lambda=250$ nm and $\Delta\lambda / \lambda_0 = 40$ %), 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

Orbital motion of the young brown dwarf companion TWA 5 B

With more adaptive optics images available, we aim at detecting orbital motion for the first time in the system TWA 5 A+B. We measured separation and position angle between TWA 5 A and B in each high-resolution image available and followed their change in time, because B should orbit around A. The astrometric measurement precision is about one milli arc sec. With ten year difference in epoch, we can clearly detect orbital motion of B around A, a decrease in separation by ~ 0.0054 arc sec per year and a decrease in position angle by ~ 0.26 degrees per year. TWA 5 B is a brown dwarf with ~ 25 Jupiter masses (Neuh\"auser et al. 2000), but having large error bars (4 to 145 Jupiter masses, Neuh\"auser et al. 2009). Given its large projected separation from the primary star, ~ 86 AU, and its young age ~ 10 Myrs), it has probably formed star-like, and would then be a brown dwarf companion. Given the relatively large changes in separation and position angle between TWA 5 A and B, we can conclude that they orbit around each other on an eccentric orbit. Some evidence is found for a curvature in the orbital motion of B around A - most consistent with an elliptic (e=0.45) orbit. Residuals around the best-fit ellipse are detected and show a small-amplitude (~ 18 mas) periodic sinusoid with ~ 5.7 yr period, i.e., fully consistent with the orbit of the inner close pair TWA 5 Aa+b. Measuring these residuals caused by the photocenter wobble - even in unresolved images - can yield the total mass of the inner pair, so can test theoretical pre-main sequence models.Comment: 6 pages, 4 figures, accepted for publication in A&A; corrected typo in amplitude below Fig.

Astrometric and photometric monitoring of GQ Lup and its sub-stellar companion

Neuhaeuser et al. (2005) presented direct imaging evidence for a sub-stellar companion to the young T Tauri star GQ Lup. Common proper motion was highly significant, but no orbital motion was detected. Faint luminosity, low gravity, and a late-M/early-L spectral type indicated that the companion is either a planet or a brown dwarf. We have monitored GQ Lup and its companion in order to detect orbital and parallactic motion and variability in its brightness. We also search for closer and fainter companions. We have taken six more images with the VLT Adaptive Optics instrument NACO from May 2005 to Feb 2007, always with the same calibration binary from Hipparcos for both astrometric and photometric calibration. By adding up all the images taken so far, we search for additional companions. The position of GQ Lup A and its companion compared to a nearby non-moving background object varies as expected for parallactic motion by about one pixel (2 \pi with parallax \pi). We could not find evidence for variability of the GQ Lup companion in the K-band (standard deviation being \pm 0.08 mag), which may be due to large error bars. No additional companions are found with deep imaging. There is now exceedingly high significance for common proper motion of GQ Lup A and its companion. In addition, we see for the first time an indication for orbital motion (about 2 to 3 mas/yr decrease in separation, but no significant change in the position angle), consistent with a near edge-on or highly eccentric orbit. We measured the parallax for GQ Lup A to be \pi = 6.4 \pm 1.9 mas (i.e. 156 \pm 50 pc) and for the GQ Lup companion to be 7.2 \pm 2.1 mas (i.e. 139 \pm 45 pc), both consistent with being in the Lupus I cloud and bound to each other.Comment: A&A in pres

Joint evolution of multiple social traits: a kin selection analysis

General models of the evolution of cooperation, altruism and other social behaviours have focused almost entirely on single traits, whereas it is clear that social traits commonly interact. We develop a general kin-selection framework for the evolution of social behaviours in multiple dimensions. We show that whenever there are interactions among social traits new behaviours can emerge that are not predicted by one-dimensional analyses. For example, a prohibitively costly cooperative trait can ultimately be favoured owing to initial evolution in other (cheaper) social traits that in turn change the cost-benefit ratio of the original trait. To understand these behaviours, we use a two-dimensional stability criterion that can be viewed as an extension of Hamilton's rule. Our principal example is the social dilemma posed by, first, the construction and, second, the exploitation of a shared public good. We find that, contrary to the separate one-dimensional analyses, evolutionary feedback between the two traits can cause an increase in the equilibrium level of selfish exploitation with increasing relatedness, while both social (production plus exploitation) and asocial (neither) strategies can be locally stable. Our results demonstrate the importance of emergent stability properties of multidimensional social dilemmas, as one-dimensional stability in all component dimensions can conceal multidimensional instability

Adaptive optics in high-contrast imaging

The development of adaptive optics (AO) played a major role in modern astronomy over the last three decades. By compensating for the atmospheric turbulence, these systems enable to reach the diffraction limit on large telescopes. In this review, we will focus on high contrast applications of adaptive optics, namely, imaging the close vicinity of bright stellar objects and revealing regions otherwise hidden within the turbulent halo of the atmosphere to look for objects with a contrast ratio lower than 10^-4 with respect to the central star. Such high-contrast AO-corrected observations have led to fundamental results in our current understanding of planetary formation and evolution as well as stellar evolution. AO systems equipped three generations of instruments, from the first pioneering experiments in the nineties, to the first wave of instruments on 8m-class telescopes in the years 2000, and finally to the extreme AO systems that have recently started operations. Along with high-contrast techniques, AO enables to reveal the circumstellar environment: massive protoplanetary disks featuring spiral arms, gaps or other asymmetries hinting at on-going planet formation, young giant planets shining in thermal emission, or tenuous debris disks and micron-sized dust leftover from collisions in massive asteroid-belt analogs. After introducing the science case and technical requirements, we will review the architecture of standard and extreme AO systems, before presenting a few selected science highlights obtained with recent AO instruments.Comment: 24 pages, 14 figure

Uniform regularity for the Navier-Stokes equation with Navier boundary condition

We prove that there exists an interval of time which is uniform in the vanishing viscosity limit and for which the Navier-Stokes equation with Navier boundary condition has a strong solution. This solution is uniformly bounded in a conormal Sobolev space and has only one normal derivative bounded in $L^\infty$. This allows to get the vanishing viscosity limit to the incompressible Euler system from a strong compactness argument