THE ASYMMETRIC ENVELOPE OF GAMMA-CASSIOPEIAE OBSERVED WITH THE MK III OPTICAL INTERFEROMETER

We present interferometric observations of the Be star gamma Cas in the light of the Halpha emission line. North-south baselines ranging from 4.2 to 23.6 m have been used over a large range of hour angles. With the resulting coverage of the u-v plane, we have been able for the first time to measure the asymmetry of the Halpha emitting region directly. The brightness distribution on the sky can be fit by an elliptical Gaussian model with an axial ratio of 0.74 and an angular diameter (FWHM of the major axis) of 3.2 milliarcsec (mas). The radial brightness distribution is not well-constrained by our data; elliptical uniform disk or ring models also give acceptable fits. If the envelope of gamma Cas is symmetric with respect to the rotation axis, the inclination of this axis must be at least 42-degrees. The major axis of the brightness distribution and the polarization position angle are perpendicular to each other. These results support the standard interpretation of the linear polarization of Be stars, which is thought to be produced by electron scattering in a flattened envelope or disk. Competing nearly spherical models can be ruled out.