A highly collimated bipolar outflow in a proto-planetary nebula: Hubble Space Telescope imaging of Hen 401

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 b oard the Hubble Space Telescope. Two very long (14." 5), cylindrical-shaped bipolar outflow lobes are seen in reflected light, each with a length/widt h ratio of approximate to 7, probably the largest seen in a proto-planetary nebula so far. The lobes are limb-brightened, indicating that they are opt ically thin to scattering. The central star, resolved for the first time fr om the surrounding nebulosity, is girdled by an equatorial torus and a bipo lar 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 H alpha emission from photoionized gas in the vicinity of the central star, and we tentatively detect two small shoc k-emitting blobs located along the nebular axis about +/-6." 2 from the cen tral star. A comparison of the lobe morphology with theoretical models indi cates that the highly collimated lobes of Hen 401 result from the momentum- driven shock interaction of a high-velocity bipolar jet with the circumstel lar envelope of the progenitor AGB star.