High-Resolution Spectra of Bright Central Stars of Bipolar Planetary Nebulae and the Question of Magnetic Shaping

We present ESO NTT high resolution echelle spectroscopy of the central stars(CSs) of eight southern bipolar planetary nebulae (PNe) selected for theirasymmetry. Our aim was to determine or place limits on the magnetic fields ofthe CSs of these nebulae, and hence to explore the role played by magneticfields in nebular morphology and PN shaping. If magnetic fields do play a role,we expect these CSs to have fields in the range $10^2 - 10^7$ G from magneticflux conservation on the reasonable assumption that they must evolve into thehigh field magnetic white dwarfs. We were able to place an upper limit of$\approx 20,000$ G to the magnetic fields of the central stars of He 2-64 andMyCn 18. The spectrum of He 2-64 also shows a P-Cygni profile in \ion{He}{1}$\lambda$5876 and $\lambda$6678, corresponding to an expanding photosphere withvelocity $\sim$ 100 km s$^{-1}$. The detection of helium absorption lines inthe spectrum of He 2-36 confirms the existence of a hot stellar component. Wedid not reach the necessary line detection for magnetic field analysis in theremaining objects. Overall, our results indicate that if magnetic fields areresponsible for shaping bipolar planetary nebulae, these are not required to begreater than a few tens of kilogauss.Comment: 32 pages, accepted for publication in the Astronomical Journa