Evidence for a bipolar geometry in R coronae borealis?
Abstract
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 Å. 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. © 1997. The American Astronomical Society. All rights reserved.