1999MNRAS.305..166O

We present the results of medium-resolution (Δ v ~ 60 km.s^–1) spectropolarimetric observations across H α of a sample of B[e] and Herbig Be objects. A change in linear polarization across H α is detected in a large fraction of the objects, with characteristics ranging from simple depolarization in a couple of Herbig Be stars, to more complex behaviour in the probable post-main-sequence B[e] stars. H α in the spectra of HD 37806 and 50138 each consist of a double-peaked polarized line and a superposed unpolarized single emission peak, suggesting two distinct line-forming regions. Multiple observations of HD 45677 allow for the separation of electron and dust scattering effects for the first time: the difference between derived intrinsic polarization angles of the two components indicates that the dust scattering region is clumpy. Two unexpected results are the non-detections of H α polarization changes in ο Ori, where depolarization has previously been detected, and in MWC 297, which exhibits source elongation at radio wavelengths. In ο Ori time variability is probably responsible such that the electron scattering disc of this star was much weakened at the time of observation. Two hypotheses are advanced that might explain the MWC 297 result.

The general findings are that roughly half of the observed Herbig Be stars show polarization changes across Hα, implying immediately that their ionized envelopes are not spherically symmetric. This pattern, if confirmed by observations of a larger sample, could indicate that the non-detection rate is simply a consequence of sampling randomly oriented circumstellar discs able to scatter starlight within a few stellar radii. The deduced alignment of the disc of HD 53367 with the local interstellar magnetic field suggests an orderly star formation process in which the star `remembers' the larger scale magnetic field direction. The stars classified as B[e] stars all show startling polarization changes across Hα. The details in each case are different, but the widely accepted concept of dense Hα-emitting equatorial discs around these objects is supported.