Evidence for a Bipolar Geometry in R Coronae Borealis?

The R Coronae Borealis (RCB) stars produce dust at irregular intervals, but the distribution of the circumstellar material is not known. We report spectropolarimetry of R Coronae Borealis (R CrB) itself, obtained in a deep decline with the Wisconsin-Indiana-Yale-NOAO telescope. The continuum is polarized ~1% in the red, rising to ~2% in the blue. There are significant polarization variations across the emission lines. The polarization of the Na I D lines indicates that light from the emission-line region is scattered but at a different angle from the stellar continuum. The position angle of the continuum polarization is almost constant from 1 μm to 7000 A but then changes rapidly, rotating by ~60° between 7000 and 4000 A. This behavior is strikingly similar to that produced in post-asymptotic giant branch (AGB) stars having an obscuring torus and bipolar dust lobes. These new data strengthen the earlier suggestion that there is a preferred direction to the dust ejections in R CrB. Dust ejections seem to occur predominantly along two roughly orthogonal directions consistent with a bipolar geometry. If confirmed, this finding will reinforce the relationship between the RCB stars and other post-AGB stars.