Thursday, June 6, 2013

1306.1006 (Julien Milli et al.)

Prospects of detecting the polarimetric signature of the Earth-mass planet alpha Centauri B b with SPHERE / ZIMPOL    [PDF]

Julien Milli, David Mouillet, Dimitri Mawet, Hans Martin Schmid, Andreas Bazzon, Julien H. Girard, Kjetil Dohlen, Ronald Roelfsema
Over the past 5 years, radial-velocity and transit techniques have revealed a new population of Earth-like planets down to a few Earth masses. Their very close orbit around their host star requires exquisite inner working angle to be detected in direct imaging and sets a challenge for visible direct imager like SPHERE / ZIMPOL. Among all known exoplanets having less than twenty-five Earth-masses we first predict the best candidate for direct imaging. Our primary objective is then to provide the best instrument setup and observing strategy to detect such a peculiar object with ZIMPOL. Secondly we aim at predicting its detectivity. Using exoplanet properties constrained by their radial velocity measurements, polarimetric models and the diffraction propagation code CAOS we build estimates of the detection sensitivity of ZIMPOL for such a planet in different observing instrument modes. We show how observing strategies can be optimized to yield the best detection performance on a specific target. In our current knowledge of exoplanetary systems, alpha Centauri B b is the most promising target having less than twenty-five Earth-masses for ZIMPOL. With a gaseous Rayleigh-scattering atmosphere and favorable inclinations, the planet could be detected in about 4 hours observing time, using the four-quadrant phase mask coronograph in the I band. However, if it displays unfavorable polarimetric and reflective properties similar to that of our Moon, it is around 50 times fainter than ZIMPOL best sensitivity. alpha Centauri B is a primary target for SPHERE. Dedicated deep observations targeting specifically the RV-detected planet can in favorable polarimetric properties of the planet lead to a detection.
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