Imaging exoplanets with coronagraphic instruments

Abstract

Exoplanetary science is a very active field of astronomy nowadays, with questions still opened such as how planetary systems form and evolve (occurrence, process), why such a diversity of exoplanets is observed (mass, radius, orbital parameters, temperature, composition), and what are the interactions between planets, circumstellar disk and their host star. Several complementary methods are used for the detection of exoplanets. Among these, imaging aims at the direct detection of the light reflected, scattered or emitted by exoplanets and circumstellar disks. This allows their spectral and polarimetric characterization. Such imaging remains challenging because of the large luminosity ratio (10 4 -10 10 ) and the small angular separation (fraction of an arcsecond) between the star and its environment. Over the past two decades, numerous techniques, including coronagraphy, have been developed to make exoplanet imaging a reality.This paper gives a broad overview of the subsystems that make up a coronagraphic instrument for imaging exoplanetary systems. It is especially intended for non-specialists or newcomers in the field. We explain the principle of coronagraphy and propose a formalism to understand their behavior. We discuss the impact of wavefront aberrations on the performance of coronagraphs and how they induce stellar speckles in the scientific image. Finally, we present instrumental and signal processing techniques used for on-sky minimization or a posteriori calibration of these speckles in order to improve the performance of coronagraphs.Résumé. L'exoplanétologie est un domaine très actif de l'astronomie moderne avec des questions encore ouvertes : comment les systèmes planétaires se forment-ils et évoluent-ils ; pourquoi une telle diversité d'exoplanètes est-elle observée (masse, rayon, paramètres orbitaux, température, composition) ; quelles sont les interactions entre les planètes, les disques circumstellaires et leur étoile hôte ? Plusieurs méthodes complémentaires sont utilisées pour la détection d'exoplanètes. Parmi celles-ci, l'imagerie permet la détection directe de la lumière réfléchie, diffusée ou émise par les exoplanètes et les disques circumstellaires. Ceci permet une caractérisation spectrale et polarimétrique. Obtenir une image d'exoplanète n'est cependant pas simple en raison du grand rapport de luminosité (10 4 -10 10 ) et de la faible séparation angulaire (fraction de seconde d'angle) entre l'étoile et son environnement. Depuis deux décennies, de nombreuses techniques, dont la coronographie, ont été développées pour faire de l'imagerie des exoplanètes une réalité.

Observations of circumstellar disks in scattered light with SPHERE at the VLT

“…In that respect, an extreme adaptive optics (AO) system like SPHERE is able to routinely achieve Strehl ratio larger than 80% in the near IR (J, H and Ks bands) for bright stars (R<10) and can even provide a Strehl of 50% at visible wavelengths for R<9 [44], allowing to deliver exquisite image quality with angular resolution of 15 to 50 mas for wavelengths in the range 0.6-2.0 µm. Such a low wavefront error level is necessary for a coronagraph to provide a large rejection of the starlight (see [45] this proceeding for a review of coronagraphic techniques). The SPHERE coronagraph [46] has a diameter of about 0.2 ′′ which means that no off-axis object can be recovered below the Inner Working Angle of ∼ 0.1 ′′ .…”

Interactions of exoplanets with their environment

“…Nevertheless, they are not necessarily the most representative of the population of planets in our galaxy [2]. The detection techniques of exoplanets will be presented in other reviews of this special issue [3,4]. Here, we focus on a few specificities of the population of exoplanets as we know it today, in comparison to the solar system planets.…”