Optical polarization map of Eta Carinae and the nature of its outburst

Abstract

The compact nebula Eta Carinae has been mapped in optical linear polarization with a seeing-limited technique. The polarization is the result of scattering from small Mie particles, and the inner part of the polarization map shows that the illuminating source is slightly extended. That is identified as a circumstellar ring, presumably of ejected material, illuminated in turn by the central star. The position of the ring is compared with the positions of known infrared sources. The polarization information supports Thackeray’s suggestion that the outer part of the nebula is prolate and inclined at 70° to the line of sight; the scattering angles in that model can lead to the observed degrees of polarization, and the bipolar-type elongation is along the axis of the proposed circumstellar ring. It is suggested that the circumstellar ring was formed by overflow from the outer Lagrangian points of a close binary star, and that the obscuration during the historical outburst was small. In that case, the object was too faint to have been a supernova, but was brighter than cataclysmic variables known to be in binary star systems.