Abstract
We have obtained high-resolution wide- and narrowband images of the bipolar proto-planetary nebula Hen 401 with the Wide Field Planetary Camera 2 on board the Hubble Space Telescope. Two very long (145), cylindrical-shaped bipolar outflow lobes are seen in reflected light, each with a length/width ratio of ≈7, probably the largest seen in a proto-planetary nebula so far. The lobes are limb-brightened, indicating that they are optically thin to scattering. The central star, resolved for the first time from the surrounding nebulosity, is girdled by an equatorial torus and a bipolar skirtlike structure, both of which are coaxial with the lobes. A faint halo around the lobes marks the presence of an asymptotic giant branch (AGB) circumstellar envelope. We find Hα emission from photoionized gas in the vicinity of the central star, and we tentatively detect two small shock-emitting blobs located along the nebular axis about ±62 from the central star. A comparison of the lobe morphology with theoretical models indicates that the highly collimated lobes of Hen 401 result from the momentum-driven shock interaction of a high-velocity bipolar jet with the circumstellar envelope of the progenitor AGB star.
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