Abstract
High resolution spectroscopic data of the central knots and the outer shell of the planetary nebula Abell 78 are analyzed. Chemical abundances and kinematics were derived for each region. The inner knots appear to be denser and slightly cooler than the outer shell, and they are constituted by an extremely H-deficient material. These knots are receding from the central star with velocities in the range from +40 to -60 km s-1 and present cometary structures probably caused by the stellar wind. The outer shell appears to have very low metallicity but we conclude that abundance determinations are probably affected by large temperature fluctuations in the gas, caused by additional heating mechanisms such as shocks by high speed ejecta and photo-electrons from dust grains, etc. Our kinematical analysis clearly demonstrates that H lines are not emitted in the inner knots but in the outer shell. In the outer zone we have detected a high-velocity compact structure, with velocities from -40 to 90 km s-1. This spike emits in collisionally excited lines (mainly [O III]), but it is not detected in H or He lines. Therefore, it should consist of a ``bullet'' of H-deficient material that is colliding with the outer shell.
