Speckle noise reduction techniques for high-dynamic range imaging

High-dynamic range imaging from space in the visible, aiming in particular at the detection of terrestrial exoplanets, necessitates not only the use of a coronagraph, but also of adaptive optics to correct optical defects in real time. Indeed, these defects scatter light and give birth to speckles in the image plane. Speckles can be cancelled by driving a deformable mirror to measure and compensate wavefront aberrations. In a first approach, targeted speckle nulling, speckles are cancelled iteratively by starting with the brightest ones. This first method has demonstrated a contrast better than 1e9 in laboratory. In a second approach, zonal speckle nulling, the total energy of speckles is minimized in a given zone of the image plane. This second method has the advantage to tackle simultaneously all speckles from the targeted zone, but it still needs better experimental demonstration.Comment: 7 pages, 3 figures, in Optical techniques for direct imaging of exoplanets (a special issue of Comptes Rendus de Physique

A catalog of bright calibrator stars for 200-meter baseline near-infrared stellar interferometry

9 pagesWe present in this paper a catalog of reference stars suitable for calibrating infrared interferometric observations. In the K band, visibilities can be calibrated with a precision of 1 % on baselines up to 200 meters for the whole sky, and up to 300 meters for some part of the sky. This work, extending to longer baselines a previous catalog compiled by Bordé et al. (2002), is particularly well adapted to hectometric-class interferometers such as the Very Large Telescope Interferometer (VLTI, Glindemann et al. 2003) or the CHARA array (ten Brummelaar et al. 2003) when one is observing well-resolved, high-surface brightness objects (

CoRoT: harvest of the exoplanet program

One of the objectives of the CoRoT mission is the search for transiting extrasolar planets using high-precision photometry, and the accurate characterization of their fundamental parameters. The CoRoT satellite consecutively observes crowded stellar fields since February 2007, in high-cadence precise photometry; periodic eclipses are detected and analysed in the stellar light curves. Then complementary observations using ground-based facilities allows establishing the nature of the transiting body and its mass. CoRoT has acquired more than 163,000 light curves and detected about 500 planet candidates. A fraction of them (5%) are confirmed planets whose masses are independently measured. Main highlights of the CoRoT discoveries are: i) the variety of internal structures in close-in giant planets, ii) the characterisation of the first known transiting rocky planet, CoRoT-7 b, iii) multiple constraints on the formation, evolution, role of tides in planetary systems.Comment: Icarus, in press, special issue on Exoplanet

Measuring stellar granulation during planet transits

Context. Stellar activity and convection-related surface structures might cause bias in planet detection and characterization that use these transits. Surface convection simulations help to quantify the granulation signal. Aims. We used realistic three-dimensional (3D) radiative hydrodynamical (RHD) simulations from the Stagger grid and synthetic images computed with the radiative transfer code Optim3D to model the transits of three prototype planets: a hot Jupiter, a hot Neptune, and a terrestrial planet. Methods. We computed intensity maps from RHD simulations of the Sun and a K-dwarf star at different wavelength bands from optical to far-infrared that cover the range of several ground-and space-based telescopes which observe exoplanet transits. We modeled the transit using synthetic stellar-disk images obtained with a spherical-tile imaging method and emulated the temporal variation of the granulation intensity generating random images covering a granulation time-series of 13.3 h. We measured the contribution of the stellar granulation on the light curves during the planet transit. Results. We identified two types of granulation noise that act simultaneously during the planet transit: (i) the intrinsic change in the granulation pattern with timescale (e.g., 10 min for solar-type stars assumed in this work) is smaller than the usual planet transit (~hours as in our prototype cases); and (ii) the fact that the transiting planet occults isolated regions of the photosphere that differ in local surface brightness as a result of convective-related surface structures. First, we showed that our modeling approach returns granulation timescale fluctuations that are comparable with what has been observed for the Sun. Then, our statistical approach shows that the granulation pattern of solar and K-dwarf-type stars have a non-negligible effect of the light curve depth during the transit, and, consequentially on the determination of the planet transit parameters such as the planet radius (up to 0.90% and ~0.47% for terrestrial and gaseous planets, respectively). We also showed that larger (or smaller) orbital inclination angles with respect to values corresponding to transit at the stellar center display a shallower transit depth and longer ingress and egress times, but also granulation fluctuations that are correlated to the center-to-limb variation: they increase (or decrease) the value of the inclination, which amplifies the fluctuations. The granulation noise appears to be correlated among the different wavelength ranges either in the visible or in the infrared regions. Conclusions. The prospects for planet detection and characterization with transiting methods are excellent with access to large amounts of data for stars. The granulation has to be considered as an intrinsic uncertainty (as a result of stellar variability) on the precise measurements of exoplanet transits of planets. The full characterization of the granulation is essential for determining the degree of uncertainty on the planet parameters. In this context, the use of 3D RHD simulations is important to measure the convection-related fluctuations. This can be achieved by performing precise and continuous observations of stellar photometry and radial velocity, as we explained with RHD simulations, before, after, and during the transit period

Recherche des exoplanètes, mesure de leurs propriétés physiques et orbitales

Exoplanetology is a young science that genuinely took off in 1995 when a giant planet was discovered in close orbit to the solar star 51 Pegasi. My first researches in this field started in 1999 and were concerned with the space mission Corot that has lead since 2006 to the discovery of more than thirty diverse exoplanets. I have been fully engaged in the Corot adventure, starting with estimating the number of detectable planets from 1999 to 2003, continuing with the search for and characterization of partial eclipse signals, or transits, when the data became available in 2007, and finishing with the measure of orbital and physical properties of detected exoplanets, most notably Corot-7b in 2009, the first rocky exoplanet with measured mass and radius, and Corot-8b in 2010, a dense small-Saturn. Currently, I work on computing the probability of the planetary nature of all transit signals detected by Corot. My other works deal with two high angular resolution techniques for the direct detection of exoplanets: long-baseline infrared interferometry and visible coronagraphy. Interferometry-wise, I compiled two catalogs of calibrator stars suitable for high-precision instrumental calibrations, I measured the properties of a single-mode fiber prototype in the mid-infrared, and I observed dwarf, giant, and double stars. My major result is the detection of the faint companion to Theta Draconis. Finally, coronagraphy-wise, I devised in 2006 a speckle-nulling approach by way of a deformable mirror, to be used in the framework of Nasa's Terrestrial Planet Finder Coronagraph project. In case Esa selects the Echo project in February 2014, my future works might be dealing with the transmission spectrometry of short-period exoplanet atmospheres.L'exoplanétologie est une science jeune qui n'a véritablement démarré qu'en 1995 avec la découverte d'une exoplanète géante en orbite rapprochée autour de l'étoile solaire 51 Pegasi. Mes premières recherches dans de domaine datent de 1999 et concernent la mission spatiale Corot qui a conduit depuis 2006 à la détection de plus d'une trentaine de nouvelles exoplanètes très diverses. J'ai pleinement participé à l'aventure de Corot, en commençant par l'estimation du nombre de planètes détectables de 1999 à 2003, en poursuivant par la recherche et la caractérisation des signaux d'éclipses partielles, ou transits, lorsque les données furent disponibles à partir de 2007, pour finir par la mesure des propriétés physiques et orbitales des exoplanètes détectées, notamment Corot-7b en 2009, la première exoplanète rocheuse au rayon et à la masse mesurés, et Corot-8b en 2010, un mini-saturne dense. Je travaille actuellement au calcul de la probabilité de la nature planétaire de tous les signaux de transits détectés par la mission. Mes autres travaux de recherche concernent deux techniques de haute résolution angulaire pour la détection directe d'exoplanètes : l'interférométrie à longue base dans l'infrarouge et la coronographie dans le visible. En interférométrie, j'ai contribué à la précision de l'étalonnage des instruments en compilant deux catalogues d'étoiles-étalons, j'ai mesuré les propriétés d'un prototype de fibre monomode dans l'infrarouge moyen, et j'ai observé des étoiles naines, géantes et doubles. Mon résultat majeur est la détection directe du compagnon faible de l'étoile Theta Draconis. Enfin, en coronographie, j'ai développé en 2006 une méthode de correction de tavelures à l'aide d'un miroir déformable dans le cadre du projet Terrestrial Planet Finder Coronagraph de la Nasa. En cas de sélection du projet Echo par l'Esa en février 2014, mes recherches futures pourraient concerner la spectrométrie par transmission de l'atmosphère d'exoplanètes à courte période

Détection et caractérisation de planètes extrasolaires par photométrie visible et interférométrie infrarouge à très haute précision

Membres du jury : Pierre Encrenaz (président), Vincent Coudé du Foresto, Pierre Léna, Pierre Barge (rapporteur), Didier Queloz (rapporteur), Alain Léger (examinateur)Thanks to radial velocity spectroscopy, over a hundred giant exoplanets around solar type stars have been discovered to date (2003). The limits of this technique motivate the use of other methods to obtain a full characterization of the known planets, to detect terrestrial size planets, and to expand our statistical knowledge over a larger population of objects. This thesis deals with two of these complementary techniques, namely transit photometry and long baseline infrared interferometry. Transit photometry has been studied in the framework of the space mission COROT: we have used a match-filter algorithm to determine (i) the detection efficiency of the instrument as a function of different planetary orbital distributions, and (ii) the histograms of detections vs. the spectral type and the magnitude of the host star. Moreover, we have studied the use of COROT's photometric channels to diagnose false detections, and to reduce stellar variability noise. With a joint processing of spectroscopic and long baseline interferometric data, a full characterization of double systems can be achieved. We have applied this processing to real observations of spectroscopic binaries, then we have studied the feasibility of the observation of 51 Pegasi-like double systems. Furthermore, we have contributed to the effort to improve precision in infrared interferometry by compiling a catalog of calibrator stars, and by characterizing a prototype 10 µm single-mode fiber, a critical component for space missions dedicated to terrestrial planets finding like Darwin/TPF.Grâce à la spectroscopie des vitesses radiales, on connaît en 2003 plus d'une centaines d'exoplanètes géantes autour d'étoiles de type solaire. Les limites de cette technique poussent à utiliser d'autres méthodes pour caractériser complètement les planètes connues, détecter des planètes de taille terrestre, et accéder à des informations statistiques sur une grande population d'objets. Cette thèse porte sur deux de ces techniques complémentaires : d'une part la photométrie des transits et d'autre part l'interférométrie infrarouge à très longue base. La photométrie des transits a été étudiée dans le cadre de la mission spatiale COROT : on a mis en oeuvre un algorithme de détection par filtrage adapté afin de déterminer (i) l'efficacité de détection de l'instrument pour différentes distributions orbitales des planètes, et (ii) la répartition des détections en fonction du type spectral et de la magnitude de l'étoile hôte. En outre, on a étudié l'utilisation des canaux photométriques de COROT pour discriminer les fausses détections et pour réduire la part du bruit de variabilité stellaire. Un traitement global des mesures par interférométrie à très longue base et par spectroscopie permet la caractérisation complète des systèmes doubles. On a appliqué ce traitement à des observations réelles d'étoiles binaires spectroscopiques, puis on a étudié la faisabilité d'observations de couples du type de 51 Pegasi. Par ailleurs, on a contribué à l'effort vers la très haute précision en interférométrie infrarouge par la réalisation d'un catalogue d'étoiles-étalons, et par la caractérisation instrumentale d'un prototype de fibre monomode à 10 µm, un composant essentiel pour les missions spatiales dédiées aux exoplanètes telluriques comme Darwin/TPF

BART: A Probabilistic and automated tool for the vetting of transits

With the present and future surveys for exoplanet detection by transit photometry (K2, TESS and PLATO), one of the biggest challenge of the next decade for the exoplanet community will be to maximise the scientific return from this huge amount of data. These missions will give us hundreds of thousands of light-curves, amongst which approximatively 10 % will exhibit transit signals. Without any vetting, around 90 % of them will be false positive, in the sense that they will not correspond to an exoplanet transiting the target star. So a key step will be to focus the follow-up ressources (in terms of telescope time, computer time and human ressources) one the planetary transit.In order address this problematic, we developed a software called BART which stands for Bayesian Analysis for the Ranking of Transits. When most of the existing ranking tools rely on a set of individual criteria, our tool performs a fully coherent analysis combining the most used criteria and taking into account their correlations.It is automated and relies on bayesian model comparison, bayesian parameter inference and Monte Carlo Markov Chain exploration technique. It produces a ranked list of transits where those which are the most likely to be due to planets (around the target stars) have a higher rank allowing to focus the follow-up on targets with the highest scientific merit.But it also gives the most probable scenario for the transits and provides an estimate of the parameters for each possible scenarios. This is a priceless information since it allows to estimate the precision required for a given follow-up observation. Thus it will allow to optimize the follow-up strategy and direct the observations.This tool has been originally developed and tested with CoRoT and I will present the results obtained on the CoRoT transit list. I will also show the results recently obtained with K2 data

A probabilistic and automated tool for the vetting of transit candidates

We developed, based on the CoRoT experience, an automated tool called BART for Bayesian Analysis for the Ranking of Transit in order to perform a homogeneous and automated ranking of planetary candidates. We applied it to the candidates detected in the campaign 1 of K2

Exoplanet detection capability of the COROT space mission

8 pages, 7 figuresCOROT will be the first high precision photometric satellite to be launched with the aim of detecting exoplanets by the transit method. In this paper, we present the simulations we have carried out in order to assess the detection capability of COROT. Using the model of stellar population synthesis of the Galaxy developed at Besancon Observatory (Robin & Creze 1986) and a simple cross-correlation technique (Borde et al. 2001), we find that COROT has the capacity to detect numerous exoplanets, not only Jupiter and Uranus-class ones, but also hot terrestrial planets, if they exist. We show that small exoplanets should be mainly gathered around 14-15th magnitude K2-M2 dwarfs and giant exoplanets around 15-16th magnitude F7-G2 dwarfs. We study the effect of crowding and the impact of a high stellar variability noise that both reduce the detection capability of the instrument