A high resolution spectroscopic study of the extraordinary planetary nebula LMC-N66
M. Pena1, W.-R. Hamann2, M. T. Ruiz3, A. Peimbert1, M. Peimbert1
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. 2003) we analyzed the changing
stellar spectra and found evidence that the central star is most likely
a binary system where a white dwarf 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 kinematic 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
material at an average heliocentric radial velocity of
Vrad = 248 plusminus 30 km s-1 and similarly bright
"receding" material at Vrad = 331 plusminus 25 km s-1. A
systemic velocity of about 300 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 km s-1
relative to the system). The morphology and kinematics of
LMC-N66 can be explained as 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.
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