Hα spectropolarimetry of B[e] and Herbig Be stars

We present the results of medium resolution (∆v ≈ 60 km s) spectropolarimetric observations across Hα of a sample of B[e] and Herbig Be objects. A change in linear polarization across Hα 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. Hα in the spectra of HD 37806 and HD 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 indicate that the dust-scattering region is clumpy. Two unexpected results are the non-detections of Hα polarization changes in ω Ori, where depolarization has previously been detected, and in MWC 297, which exhibits source elongation at radio wavelengths. In ω Ori time variability is probably responsible such that this star’s electron-scattering disk 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 disks able to scatter starlight within a few stellar radii. 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 disks around these objects is supported.