RefPlanets: Search for reflected light from extra-solar planets with SPHERE/ZIMPOL

Abstract

RefPlanets is a guaranteed time observation (GTO) programme that uses the Zurich IMaging POLarimeter (ZIMPOL) of SPHERE/VLT for a blind search for exoplanets in wavelengths from 600-900 nm. The goals of this study are the characterization of the unprecedented high polarimetic contrast and polarimetric precision capabilities of ZIMPOL for bright targets, the search for polarized 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. For our observations of Alpha Cen A and B, Sirius A, Altair, Eps Eri and Tau Ceti we used the polarimetric differential 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 (ADI) as a final layer of post-processing to further improve the contrast limits of our data at these separations. For good observing conditions we achieve polarimetric contrast limits of 15.0-16.3 mag at the effective inner working angle of about 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") depend significantly on the 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 polarized light sources in the data that has been taken so far.

23 pages, 17 figures, 2 table, Accepted for publication in A&A

References in corpus (19)

• Direct Imaging of Multiple Planets Orbiting the Star HR 8799
• The Gemini Planet Imager: First Light
• Fundamental limitations of high contrast imaging set by small sample statistics
• Imaging the Surface of Altair
• Hints for a Turnover at the Snow Line in the Giant Planet Occurrence Rate
• Discovery of a warm, dusty giant planet around HIP65426
• Spectropolarimetric signatures of Earth-like extrasolar planets
• Extrasolar planets and brown dwarfs around A-F type stars VI. High precision RV survey of early type dwarfs with HARPS
• Color difference makes a difference: four planet candidates around tau Ceti
• Direct detection of exoplanet host star companion gamma Cep B and revised masses for both stars and the sub-stellar object
• A grid of polarization models for Rayleigh scattering planetary atmospheres
• The Northern Arc of $\epsilon$ Eridani's Debris Ring as Seen by ALMA
• Close stellar conjunctions of alpha Centauri A and B until 2050 - An mK = 7.8 star may enter the Einstein ring of alpha Cen A in 2028
• The Debris Disk of Solar Analogue $\tau$ Ceti: Herschel Observations and Dynamical Simulations of the Proposed Multiplanet System
• A New Standard for Assessing the Performance of High Contrast Imaging Systems
• Planet Detectability in the Alpha Centauri System
• A high precision polarimeter for small telescopes
• Predicting exoplanet observability in time, contrast, separation and polarization, in scattered light
• HST observations of the limb polarization of Titan