The tetra-lobed planetary nebula NGC 1501

Abstract

Direct imagery and long-slit, spatially resolved echellograms of the high excitation planetary nebula NGC 1501 allowed us to study in detail the expansion velocity field, the physical conditions (electron temperature, electron density, ionization) and the spatial distribution of the nebular gas. An electron temperature of 11500 K and a turbulence of 18 km/s are derived by comparing the Halpha and [OIII] emission line profiles, but large, small scale fluctuations of both these quantities are present in the ionized gas. The radial density distribution shows external peaks up to 1400 cm-3; they have steep outwards profiles and extended inwards tails probably originated by Rayleigh-Taylor instability and winds interaction. The complexity of the expanding motions indicates that the main part of NGC 1501 is a thin ellipsoid of moderate ellipticity, but the presence of a pair of large lobes along both the major and the intermediate axes and of a multitude of smaller bumps spread on the whole nebular surface, makes the general 3-D structure of NGC 1501 like a boiling, tetra-lobed shell. This peculiar morphology can be qualitatively explained in terms of interaction of the slow nebular material with the intense and fast wind from the WC4/OVI central star.