An attempt to detect polarization effects in the envelope of gamma Cassiopeiae with theGI2T interferometer

Abstract

We report on the results of an observing run of γ Cassiopeiae using the optical long baseline interferometer Grand Interféromètre à 2 Télescopes (GI2T) in the southern France equipped with film-sheet polarizers. Four interferometric baselines ranging from 20 to 40 m were obtained on this star during 1994 on September 5th and 6th, with α Cephei as a calibrator. We aimed at detecting possible variations of the angular diameter of γ Cas as a function of polarization directions parallel and orthogonal to the north-south baseline of the GI2T. We do not detect any variation of the visibility as a function of Hour Angle on α Cephei, neither on γ Cas. We find an upper limit of 1.3 on the ratio of the angular diameters of the envelope of γ Cas seen through polarizers parallel and perpendicular to the North-South direction of the GI2T's baseline. We discuss how this limit could constrain density laws in the wind of this Be star. We clearly resolve γ Cas for increasing baselines in the red continuum whatever the polarization. Assuming the continuum flux originating from a photosphere surrounded by an elliptical gaussian envelope we estimate the extent of the latter as () mas. This diameter agrees well with the Hα emitting envelope as obtained by other interferometers.