High-velocity regions in planetary nebulae

Abstract

The internal velocity fields of planetary nebulae are studied with a resolution of 5 km s −1 .W e analyse deep echelle spectra from three nebulae in the Galactic bulge, the Sagittarius Dwarf and the Small Magellanic Cloud (SMC). No effects of metallicity are seen, except possibly a slower onset of the fast wind from the central star. Robust evidence is found for the existence of a highvelocity shock at the inner edges of the nebulae. Such a shock is predicted in hydrodynamical models but had not previously been observed. The shock gas is accelerated by the fast wind from the central star. A similar shock at the outer edges traces the expansion of the ionized shell into the ambient asymptotic giant branch (AGB) wind. Evidence for localized regions of high velocity is also found from lines of intermediate excitation, for two of the nebulae. We explore several possible interpretations: (i) an embedded shock at intermediate radii, as predicted by hydrodynamic models at the position of the outer edge of the swept-up inner shell; (ii) deviations from spherical symmetry, where in some directions the intermediate-excitation lines extend into the region of the outer shock; (iii) an intermediate swept-up shell, as seen in some Galactic planetary nebulae. The remaining nebula, with a low-mass Wolf‐Rayet central star ([WC] star), shows strong turbulence. This may trace a superposition of many embedded shock-lets. We suggest a relation to the time-variable [WC] wind, giving a planetary nebula subjected to a multitude of sound waves.