RefPlanets: Search for reflected light from extrasolar planets with SPHERE/ZIMPOL^★

¹ ETH Zurich, Institute for Particle Physics and Astrophysics, Wolfgang-Pauli-Strasse 27, CH-8093 Zurich, Switzerland
e-mail: silvan.hunziker@phys.ethz.ch
² NOVA Optical Infrared Instrumentation Group at ASTRON, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
³ Université Grenoble Alpes, IPAG, 38000 Grenoble, France
⁴ CNRS, IPAG, 38000 Grenoble, France
⁵ European Southern Observatory, Alonso de Cordova 3107, Casilla 19001 Vitacura, Santiago 19, Chile
⁶ Institute for Computational Science, University of Zürich, Winterthurerstrasse 190, 8057 Zürich, Switzerland
⁷ Istituto Ricerche Solari Locarno, Via Patocchi 57, 6605 Locarno Monti, Switzerland
⁸ Kiepenheuer-Institut für Sonnenphysik, Schneckstr. 6, 79104 Freiburg, Germany
⁹ Anton Pannekoek Institute for Astronomy, University of Amsterdam, PO Box 94249, 1090 GE Amsterdam, The Netherlands
¹⁰ LESIA, CNRS, Observatoire de Paris, Université Paris Diderot, UPMC, 5 place J. Janssen, 92190 Meudon, France
¹¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
¹² Laboratoire Lagrange, UMR7293, Université de Nice Sophia-Antipolis, CNRS, Observatoire de la Côte d’Azur, Boulevard de l’Observatoire, 06304 Nice, Cedex 4, France
¹³ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹⁴ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹⁵ Aix-Marseille Université, CNRS, CNES, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388, Marseille, France
¹⁶ Unidad Mixta International Franco-Chilena de Astronomia, CNRS/INSU UMI 3386 and Departemento de Astronomia, Universidad de Chile, Casilla 36-D, Santiago, Chile
¹⁷ Nucleo de Astronomia, Facultad de Ingenieria y Ciencias, Universidad Diego Portales, Av. Ejercito 441, Santiago, Chile
¹⁸ Escuela de Ingenieria Industrial, Facultad de Ingenieria y Ciencias, Universidad Diego Portales, Av. Ejercito 441, Santiago, Chile
¹⁹ European Southern Observatory, Karl Schwarzschild St, 2, 85748 Garching, Germany
²⁰ ONERA, The French Aerospace Lab BP72, 29 avenue de la Division Leclerc, 92322 Châtillon Cedex, France
²¹ Centre de Recherche Astrophysique de Lyon, CNRS/ENSL Université Lyon 1, 9 av. Ch. André, 69561 Saint-Genis-Laval, France
²² Geneva Observatory, University of Geneva, Chemin des Mailettes 51, 1290 Versoix, Switzerland
²³ STAR Institute, Université de Liège, Allée du Six Août 19c, 4000 Liège, Belgium
²⁴ Space Telescope Science Institute, 3700 St Martin Drive, Baltimore, MD, USA
²⁵ Department of Astronomy, University of Michigan, Ann Arbor, MI 48109, USA

Received: 5 September 2019
Accepted: 24 November 2019

Abstract

Aims. RefPlanets is a guaranteed time observation programme that uses the Zurich IMaging POLarimeter (ZIMPOL) of Spectro-Polarimetric High-contrast Exoplanet REsearch instrument at the Very Large Telescope to perform a blind search for exoplanets in wavelengths from 600 to 900 nm. The goals of this study are the characterisation of the unprecedented high polarimetic contrast and polarimetric precision capabilities of ZIMPOL for bright targets, the search for polarised reflected light around some of the closest bright stars to the Sun, and potentially the direct detection of an evolved cold exoplanet for the first time.

Methods. For our observations of α Cen A and B, Sirius A, Altair, ɛ Eri and τ Ceti we used the polarimetricdifferential imaging (PDI) mode of ZIMPOL which removes the speckle noise down to the photon noise limit for angular separations ≿0.6′′. We describe some of the instrumental effects that dominate the noise for smaller separations and explain how to remove these additional noise effects in post-processing. We then combine PDI with angular differential imaging as a final layer of post-processing to further improve the contrast limits of our data at these separations.

Results. For good observing conditions we achieve polarimetric contrast limits of 15.0–16.3 mag at the effective inner working angle of ~0.13′′, 16.3–18.3 mag at 0.5′′, and 18.8–20.4 mag at 1.5′′. The contrast limits closer in (≾0.6′′) display a significant dependence on observing conditions, while in the photon-noise-dominated regime (≿0.6′′) the limits mainly depend on the brightness of the star and the total integration time. We compare our results with contrast limits from other surveys and review the exoplanet detection limits obtained with different detection methods. For all our targets we achieve unprecedented contrast limits. Despite the high polarimetric contrasts we are not able to find any additional companions or extended polarised light sources in the data obtained so far.