Speckle-masking imaging polarimetry of Eta Carinae: evidence for an equatorial disk

Abstract

With our new speckle imaging polarimeter we have obtained first polarimetric images with sub-arcsecond resolution of the Luminous Blue Variable eta Carinae in the Halpha line. The polarization patterns at the 3'' scale match well earlier conventional imaging photometry and can be interpreted as Mie scattering. In centered long-exposure images we detected in polarized light a bar in the NE part of the equatorial plane of eta 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 Halpha 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.