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

Abstract

Aims. 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. Methods. For our observations of α Cen A and B, Sirius A, Altair, Eri and τ 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.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 ) 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.

Imaging low-mass planets within the habitable zone of α Centauri

“…The nearest stellar system, α Centauri, is among the best-suited for imaging habitable-zone exoplanets (e.g., refs. 10 – 12 ). The primary components α Centauri A and B are similar in mass and temperature to the Sun, and their habitable zones are at separations of about one au (see ref.…”

Polarimetric investigation of selected cloud compositions in exoplanetary atmospheres

“…For the cloudy atmospheres, the polarization contrast is highest at short wavelengths (∼0.35 µm) with values of ∼2 × 10 −8 due to Rayleigh scattering. In addition, for NaCl clouds, the polarization contrast reaches values of 2 × 10 −8 at small phase angles and long wavelengths, making the plateau of negative polarization detectable, for instance, with SPHERE/ZIMPOL (Hunziker et al 2020). For Na 2 S and ZnS clouds, the high degree of polarization at short wavelength with a rapidly decreasing degree of polarization for increasing wavelength, as discussed in Sect.…”

“…2-5, we assumed that the exoplanet and its host star are spatially separated observationally, thus, the high fraction of unpolarized stellar radiation is not included in the net polarization. Considering an exoplanet-star system where both components cannot be sepa- rated observationally, the ratio of the polarized planetary flux and the total stellar flux, that is, the polarization contrast, is given by (Hunziker et al 2020)…”

“…To calculate the planetary temperature profile, we followed Guillot (2010) and took parameter values based on HD 209458b (𝜅 th = 10 −2 cm 2 g −1 , 𝜅 v = 4 × 10 −3 cm 2 g −1 , 𝑇 int = 500 K, Guillot 2010; 𝑟 = 1.45 R J , 𝑇 ★ = 6092 K, Boyajian et al 2015; 𝑚 = 0.64 M J , Snellen et al 2010). However, we set 𝑑 = 1 AU with an observational distance of 5 pc to be suitable for SPHERE/ZIMPOL (Hunziker et al 2020). Thus, we obtained temperatures of about 500 K to ∼1000 K in the pressure region of about 10 mbar to ∼1 bar, which means that compositions such as KCl, NaCl, Na 2 S, or ZnS potentially form cloud condensates.…”

High contrast imaging at the photon noise limit with WFS-based PSF calibration

“…One approach is to use either high spectral reolution signatures in the exoplanet light 1 or polarized light in starlight by circumstellar dust or exoplanet atmophseres. 2 The fraction of an exoplanet light containing polarization of spectral signatures is small, typically up to a few percent, so these differential imaging techniques, although generally quite reliable, are inefficient and poorly suited to detect the faintest sources. Another approach is to separate the speckles from real sources by angular differential imaging (ADI), 3 which detects the planet image thanks to the known field rotation, or spectral differential imaging (SDI), 4 which identifies speckles through their wavelength geometrical scaling.…”