Intrinsic Linear Polarization of Be Stars as a Function of v sin i

Abstract

The polarization of 48 Be stars has been measured in two bands near Hα with the aim of determining the relation between intrinsic polarization and v sin i. A technique developed by Poeckert (1975) is used to remove the effect of interstellar polarization. It is found that intrinsic polarization depends strongly on v sin i; stars with low v sin i having little or no polarization. We have calculated the i dependence of linear polarization for a disk model envelope and find that the polarization is proportional to τ_e sin²i when the disk is optically thin (τ_e is a characteristic electron scattering optical depth). A comparison of the observed relation between intrinsic polarization and v sin i, and that predicted for the disk model is illustrated. We find that an envelope with an electron density of ≤5 × 10¹¹ cm⁻³ can account for the degree of intrinsic polarization observed in all the program stars. The fact that stars of low v sin i have little intrinsic polarization is evidence for the assumption that these stars are seen pole-on and that the envelopes around these stars are axi-symmetric. No apparent difference between pole-on stars and extreme Be stars was obtained.