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Spectral differential imaging (SDI) is part of the observing strategy of current and on-going high-contrast imaging instruments on ground-based telescopes. Although it improves the star light rejection, SDI attenuates the signature of off-axis companions to the star, just like angular differential imaging (ADI). However, the attenuation due to SDI has the peculiarity of being dependent on the spectral properties of the companions. To date, no study has investigated these effects. Our team is addressing this problem based on data from a direct imaging survey of 16 stars combining the phase-mask coronagraph, the SDI and the ADI modes of VLT/NaCo. The objective of the survey is to search for cool (Teff<1000-1300 K) giant planets at separations of 5-10 AU orbiting young, nearby stars (<200 Myr, <25 pc). The data analysis did not yield any detections. As for the estimation of the sensivity limits of SDI-processed images, we show that it requires a different analysis than that used in ADI-based surveys. Based on a method using the flux predictions of evolutionary models and avoiding the estimation of contrast, we determine directly the mass sensivity limits of the survey for the ADI processing alone and with the combination of SDI and ADI. We show that SDI does not systematically improve the sensitivity due to the spectral properties and self-subtraction of point sources.
We present numerical results of the science performance of the SPICES mission, which aims to characterize the spectro-polarimetric properties of cold exoplanets and circumstellar disks in the visible. We focus on the instrument ability to retrieve the spectral signatures of molecular species, clouds and surface of super-Earths in the habitable zone of solar-type stars. Considering realistic reflected planet spectra and instrument limitation, we show that SPICES could analyse the atmosphere and surface of a few super-Earths within 5 pc of the Sun.
SPICES (Spectro-Polarimetric Imaging and Characterization of Exoplanetary Systems) was proposed in 2010 for a five-year M-class mission in the context of ESA Cosmic Vision. Its purpose is to image and characterize long-period extrasolar planets located at several AUs (0.5-10 AU) from nearby stars (<25 pc) with masses ranging from a few Jupiter masses down to super-Earths (~2 Earth radii, ~10 M⊕), possibly habitable. In addition, circumstellar disks as faint as a few times the zodiacal light in the Solar System can be studied. SPICES is based on a 1.5-m off-axis telescope and can perform spectro-polarimetric measurements in the visible (450 - 900 nm) at a spectral resolution of about 40. This paper summarizes the top science program and the choices made to conceive the instrument. The performance is illustrated for a few emblematic cases.
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