H alpha spectropolarimetry of B[e] and Herbig Be stars

We present the results of medium-resolution (Delta v approximate to 60 km s (-1)) spectropolarimetric observations across H alpha of a sample of B[e] a nd Herbig Be objects. A change in linear polarization across Hoc is detecte d in a large fraction of the objects, with characteristics ranging from sim ple depolarization in a couple of Herbig Be stars, to more complex behaviou r in the probable post-main-sequence B[e] stars. H alpha in the spectra of HD 37806 and 50138 each consist of a double-peaked polarized line and a sup erposed unpolarized single emission peak, suggesting two distinct line-form ing regions. Multiple observations of HD 45677 allow for the separation of electron and dust scattering effects for the first time: the difference bet ween derived intrinsic polarization angles of the two components indicates that the dust scattering region is clumpy. Two unexpected results are the n on-detections of H alpha polarization changes in omega Ori, where depolariz ation has previously been detected, and in MWC 297, which exhibits source e longation at radio wavelengths. In omega Ori time variability is probably r esponsible such that the electron scattering disc of this star was much wea kened at the time of observation. Two hypotheses are advanced that might ex plain the MWC 297 result. The general finding are that roughly half of the observed Herbig Be stars s how polarization changes across H alpha, implying immediately that their io nized envelopes are not spherically symmetric. This pattern, if confirmed b y observations of a larger sample, could indicate that the non-detection ra te is simply a consequence of sampling randomly oriented circumstellar disc s able to scatter starlight within a few stellar radii. The deduced alignme nt of the disc of HD 53367 with the local interstellar magnetic field sugge sts an orderly star formation process in which the star 'remembers' the lar ger scale magnetic field direction. The stars classified as B[e] stars all show startling polarization changes across H alpha. The details in each cas e are different, but the widely accepted concept of dense H alpha-emitting equatorial discs around these objects is supported.