Deriving the Geometry of Be Star Circumstellar Envelopes from Continuum Spectropolarimetry. I. The Case of ζ Tauri

We present a general scheme for determining the circumstellar geometry of Be stars from modeling of optical continuum spectropolarimetry. Our modeling scheme utilizes a Monte Carlo radiation transfer code that determines the polarization due to multiple photon scattering, including the effects of continuous hydrogen absorption and emission from a disklike circumstellar envelope. We show that by matching the polarization level on either side of the Balmer jump, one can determine the opening angle of the disk. In general, there are only two solutions, either a geometrically thin or a geometrically thick disk.

Applying our analysis techniques to observations of the Be star ζ Tauri, we find that the two solutions for the disk have half-opening angles of either 25 or 52°. The geometricaly thick disk can be excluded based on recent interferometric imaging of Be star disks, including ζ Tau. The thin disk solution is consistent with either a Keplerian or a wind-compressed disk; both models predict small disk opening angles at small radii. In addition to matching the continuum polarization, our thin disk model reproduces the continuum spectral energy distribution from the UV through IRAS wavelengths, providing convincing evidence that Be circumstellar disks are geometrically thin.