Imaging polarimetry of circumstellar environments with the Extreme Polarimeter

Abstract

Three successful observation campaigns have been conducted with the Extreme Polarimeter, an imaging polarimeter for the study of circumstellar environments in scattered light at visible wavelengths. A contrast ratio between the central star and the circumstellar source of 10-5 can be achieved with polarimetry, with a Lyot coronograph capable of increasing this contrast by several orders of magnitude. The instrument currently operates without an adaptive optics system. An Adaptive Optics system under development for ExPo is expected to increase the contrast further. The polarimeter uses the dual-beam exchange technique, in which the two orthogonal polarisation states are imaged simultaneously after which a polarisation modulator is used to swap the polarisation states of the two beams before the next image is taken. The imaging polarimetry technique developed with ExPo will be used in the polarimetry arm of the EPICS exoplanet characterisation instrument proposed for the E-ELT. Here we present the results from the first observation campaigns, highlighting observations of protoplanetary disks around several young stars. Systematic effects that limit the polarimetric sensitivity, and the strategies we employ to overcome them, are discussed in detail. In particular, the advantages of the dual-beam exchange polarimetry method are demonstrated.