Polarization Observations of U Cephei

Abstract

Polarimetric and photometric observations of U Cephei during the 1974-75 outburst are discussed. An optically thin envelope or disk cannot explain the observations. The polarization too small by a factor of 10, when compared with the amount of additional light during the totality of the eclipse of the B-star. This suggests that most of the additional light came from optically thick matter around the B-star, i.e. the effective radius of the photosphere of the B-star, r_B, was increased by 15-20 % during the period of high mass transfer. The polarization observations could then be explained by a simple model containing an optically thin spherical envelope of outer radius 1.1 r_B and an equatorial disk of outer radius 1.7 r_B surrounding the expanded B-star. The mean electron density is about 1.4 × 10¹² e/cm³ in the spherical envelope and 1.6 × 10¹² e/cm³ in the preceding and 0.9 × 10¹² e/cm³ in the trailing side of the disk, the thickness of which is assumed to be 0.1 r_B normal to the orbital plane. The variations in the position angle can be explained if the inclination of the orbit is about 83°. The details will be published elsewhere.