K4-55 - A BIPOLAR PLANETARY-NEBULA OBSERVED NEAR POLE-ON

A comprehensive study of the planetary nebula K4-55 is presented. In a ddition to CCD narrowband images, spatially resolved low-dispersion an d high-resolution spectra were obtained. This has allowed the differen t morphological features in the images to be connected with the kinema tics and with the physical information, leading to the establishment o f the complex nebular structure. K4-55 is a new multiple-shell planeta ry nebula displaying three different parts: a faint halo that extends up to 2' from the main nebula; an outer part revealed by the kinematic al study as a pole-on bipolar structure; and an inner, bright ring. Th e bipolar structure is expanding at similar to 100 km s(-1) at the pol es, while the halo has V-exp less than or similar to 8.6 km s(-1). The inner ring exhibits intriguing kinematics, indicating a likely presen ce of rotation. Abundance variations are found throughout the nebula, with a N/O abundance ratio enhancement in the [N Pi]-bright central ri ng, whereas the extended halo presents a much smaller ratio. The N/O r atio reported is among the highest ever found in planetary nebulae. Th e outer halo is consistent with the asymptotic giant branch wind, wher eas the inner nebula was formed during a later mass-loss phase.