1995A&A...304..531M

Long-slit spectra of high spectral and spatial resolution of the compact planetary nebula Hu 2-1 are presented. The data allow us to deduce in great detail the spatio-kinematical properties of the different components identified in the nebula and to reconstruct their geometry. The main structure of Hu 2-1 is a bipolar shell consisting of a compact (radius ≃0.9"), slowly expanding (≃15km/s) bright equatorial toroid and two faint bipolar lobes expanding at ≃30km/s. Three compact condensations have been detected in the nebula. Two of them form a pair of highly collimated, high velocity bipolar condensations separated by ≃6" on the sky. They move at higher velocity and present a higher collimation than the bipolar lobes. The bipolar condensations present noticeable differences in velocity, velocity dispersion, collimation angle and distance from the central star. The third compact condensation is not related to the main nebular axis. The deduced ionized nebular mass (<10^–2M_☉) and kinematical age (≃660yr) of these components indicate that Hu 2-1 is a very young planetary nebula. All these structures are surrounded by a faint spherical halo (radius≃12"). Position-velocity maps of the [N II]/Hα line intensity ratio are used to study the excitation conditions in the individual components. Maximum values of this ratio (1-1.5) are found in the compact condensations. A ring-like region with [N II]/Hα≃0.45 has been identified in these position-velocity maps. This region (radius≃2") is concentric with the equatorial toroid, expands at ≃20km/s and its formation seems to have preceded that of the bipolar shell. Our data suggest that the formation of the ring-like region - corresponding to anisotropic mass ejection - occurred in the very last stages of the AGB phase of the Hu 2-1 progenitor. The enhanced [N II] emission in this region could be due to overabundance of N. A mass exchanging binary could qualitatively account for the observed components in Hu 2-1. The ring-like region could represent the remnant of a common envelope or of a circumbinary disk. The highly collimated components could be related to an accretion disk around a compact secondary. The different orientation of the compact components may be attributed to precession of that disk. A complex geometry of mass accretion may lead to asymmetries in the physical conditions around the secondary so that the collimated bipolar ejections present different properties.