RESOLUTION OF THE CIRCUMSTELLAR GAS AROUND THE BE STAR PSI PERSEI

Be STARS are rapidly rotating, non-supergiant B stars showing hydrogen-line emission and characterized by an excess of continuum emission from near-infrared to radio wavelengths. The latter is thought to arise from thermal bremsstrahlung in hot circumstellar gas1, for which both spherical and disk-like geometries have been proposed2,3 to explain observations at different wavelengths. The first Be star to be detected at radio wavelengths was the B5 giant psi Persei4-6, which is the brightest radio-emitting Be star. Using the Very Large Array, we have resolved the circumstellar envelope of psi Per at 15 GHz, and find that the radio emission comes from a non-spherical distribution of thermally radiating gas. The radio-emitting region bas a major axis of 111 +/- 16 mas (17 AU at a distance of 155 pc), and is unresolved, with a 3sigma upper limit of 68 mas, along its minor axis. Our observations confirm the proposal, due to Struve in 1931 (ref. 5), of equatorially enhanced circumstellar plasma distributions as the source of Be star emission.