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.
Highlights
High-contrast imaging is a key technique for the search and classification of extrasolar planets, which is one of the primary goals in modern astronomy
We investigate in this paper the achievable contrast of SPHERE/Zurich IMaging POLarimeter (ZIMPOL) for a first series of deep observations of promising targets obtained within the RefPlanets project, which is a part of the guaranteed time observation (GTO) program of the SPHERE consortium
The properties of the noise at different separations and for different total detector integration times (DITs). This detailed analysis shows the outstanding performance of ZIMPOL in terms of speckle suppression in polarimetric differential imaging (PDI) mode
Summary
High-contrast imaging is a key technique for the search and classification of extrasolar planets, which is one of the primary goals in modern astronomy. The technical requirements are very challenging and up to now only about a dozen young, giant planets have been directly imaged (e.g., Macintosh et al 2015; Bowler 2016; Schmidt et al 2016; Chauvin et al 2017; Keppler et al 2018). Baraffe et al 2003; Spiegel & Burrows 2012) They are bright in the nearinfrared (NIR) and the required contrast C = Fpl/Fstar ≈ 10−5±1 is within reach of modern extreme adaptive optics (AO) systems, like the Spectro-Polarimetric High-contrast Exoplanet REsearch. Instrument (SPHERE; Beuzit et al 2008) at the Very Large Telescope (VLT), the Gemini Planet Imager (GPI; Macintosh et al 2014), the Natural Guide Star (NGS) AO system (van Dam et al 2004) at Keck or the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO; Jovanovic et al 2015). For the young stars in the nearest star forming regions at d ≈ 150 pc the expected angular separations of planets tend to already be quite small and these objects are difficult to detect in observations
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