A high resolution spectroscopic study of the extraordinary planetary nebula LMC-N66

Abstract

The planetary nebula N66 in the Large Magellanic Cloud is an extraordinary object, as it is the only confirmed PN where the central star is a Wolf-Rayet star of the nitrogen sequence, i.e. of type [WN]. Moreover, the star showed a dramatic brightness outburst in 1993–1994. In a previous paper (Hamann et al. [CITE]) we analyzed the changing stellar spectra and found evidence that the central star is most likely a binary system where a white dwarf presently accretes matter from a non-degenerate companion at a high rate. Thus the object is a candidate for a future type Ia supernova in our cosmic neighborhood. In the present paper we analyze the morphology and kinematics of the nebula, using images and high-resolution spectra obtained with the Hubble Space Telescope (HST) and the Very Large Telescope (ESO-VLT). The object presents a complex multipolar structure, dominated by very bright lobes located at both sides of the central star and separated by a narrow waist. In addition there is a pair of very extended and twisted loops, also pointing in opposite directions; their symmetry axis and collimation angle differs from those of the bright lobes. High resolution spectroscopy reveals two main velocity components, “approaching” material at an average heliocentric radial velocity of $V_{\rm rad} = 248 \pm 30\,{\rm km\,s}^{-1}$ and similarly bright “receding” material at $V_{\rm rad} = 331 \pm 25\,{\rm km\,s}^{-1}$. A systemic velocity of about $300\,{\rm km\,s}^{-1}$ is derived. Opposite lobes and loops possess opposite velocities. Furthermore there are knots and filaments of complex structure and kinematics. Close to the central star, nebular gas is found, receding at very high velocity ($125\,{\rm km\,s}^{-1}$ relative to the system). The morphology and kinematics of LMC-N66 can be explained as the result of episodic bipolar ejections with changing axis. The bipolar structures could have been produced by collimated streams ejected from a precessing central source. We suggest that the precession could have been produced by an external torque, possibly due to a binary companion. Young, fast-moving nebular knots close to the star appear slightly He- and N-richer than the main body of the nebula, but are still hydrogen-rich in contrast to the helium-dominated atmosphere of the [WN]-type central star. In the binary scenario, this nebular matter must have been accreted from the non-degenerate companion and re-ejected before it was fully burnt.