Data-reduction techniques for high-contrast imaging polarimetry

Applications to ExPo

Sterrekundig Instituut, Universiteit Utrecht PO Box 80000 3508 TA Utrecht The Netherlands
e-mail: H.CanovasCabrera@uu.nl

Received: 18 March 2011
Accepted: 10 May 2011

Abstract

Context. Imaging polarimetry is a powerful tool for detecting and characterizing exoplanets and circumstellar environments. Polarimetry allows a separation of the light coming from an unpolarized source such as a star and the polarized source such as a planet or a protoplanetary disk. Future facilities like SPHERE at the VLT or EPICS at the E-ELT will incorporate imaging polarimetry to detect exoplanets. The Extreme Polarimeter (ExPo) is a dual-beam imaging polarimeter that can currently reach contrast ratios of 10⁵, enough to characterize circumstellar environments.

Aims. We present the data-reduction steps for a dual-beam imaging polarimeter that can reach contrast ratios of 10⁵.

Methods. The data obtained with ExPo at the William Herschel Telescope (WHT) are analyzed. Instrumental artifacts and noise sources are discussed for an unpolarized star and for a protoplanetary disk (AB Aurigae).

Results. The combination of fast modulation and dual-beam techniques allows us to minimize instrumental artifacts. A proper data processing and alignment of the images is fundamental when dealing with high contrasts. Imaging polarimetry proves to be a powerful method to resolve circumstellar environments even without a coronagraph mask or an adaptive optics system.