Photoelectric polarization maps of two bipolar reflection nebulae

A rapidlt chopping polarimeter which uses a large-aperture Pockels cell and 200-element digicon detector has been constructed for mapping in two dimensions. This instrument has been used to obtain accurate, high-resolution maps of the optical polarization and surface brightness in the bipolar nebulae CRL 2688 and M1-92. Both nebulae reveal radially symmetric polarization patterns characteristic of reflection. The degree of polarization and comparison of the nebular surface brightness with the luminosity of the central star argue that, except for the NW lobe of M 1-92, the nebulae are optically thin to scattering. In addition, the very high polarization in CRL 2688 requires small scattering grains (a9 or approx. =0.1 μm). It follows that the stellar spectrum is substantially blued upon reflection and, by comparison of observed and intrinsic stellar colors, that a color excess E(B-V) approx. =1.7 is produced by circumstellar and interstellar extinction. We conclude that the visual extinction along the optical path of a scattered photon is approx.5 mag.The small grains, optical thinness, and symmetric geometry indicate that scattering by or transmission through a medium of aligned grains, rather than multiple scattering by randomly oriented stellar magnetic field at an angle to the axis of the nebula, could provide the necessary skew geometry. The grain masses of the scattering nebulae are found to be approx.10-4 M/sub sun/, comparable to those deduced from the IR emission of some planetary nebulae. We note the similarity in appearance between many planetaries and bipolar reflection nebulae and suggest that this structure may be explained by mass loss driven by nonradial pulsations