Speckle-masking imaging polarimetry of η Carinae: Evidence for an equatorial disk

With our new speckle imaging polarimeter we have obtained the first polarimetric images with sub-arcsecond resolution of the Luminous Blue Variable η Carinae in the Ha line. The polarization patterns at the 3" scale match well earlier conventional imaging photometry and can be interpreted as Mie scattering. In crosscorrelation-centered images we detected in polarized light a bar in the NE part of the equatorial plane of η Carinae. High-resolution 0.11" polarimetric speckle reconstructions reveal a compact structure elongated in the same direction which is consistent, in degree and position angle of the polarisation, with the presence of a circumstellar, equatorial disk. The degree of polarization of the previously discovered speckle objects and the Hα arm is relatively low (∼ 10%) and thus may indicate a position within the equatorial plane. We also discovered a highly polarized (20% - 40%) bipolar structure along the major axis of the Homunculus nebula which can be traced down to the sub-arcsecond scale. This is probably the inner part of a bipolar outflow into the Homunculus.