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 their asymmetry. Our aim was to determine or place limits on the magnetic fields of the CSs of these nebulae, and hence to explore the role played by magnetic fields in nebular morphology and PN shaping. If magnetic fields do play a role, we expect these CSs to have fields in the range 10 − 10 G from magnetic flux conservation on the reasonable assumption that they must evolve into the high field magnetic white dwarfs. We were able to place an upper limit of ≈ 20, 000 G to the magnetic fields of the central stars of He 2-64 and MyCn 18. The spectrum of He 2-64 also shows a P-Cygni profile in He I λ5876 and λ6678, corresponding to an expanding photosphere with velocity ∼ 100 km s. The detection of helium absorption lines in the spectrum of He 2-36 confirms the existence of a hot stellar component. We did not reach the necessary line detection for magnetic field analysis in the remaining objects. Overall, our results indicate that if magnetic fields are responsible for shaping bipolar planetary nebulae, these are not required to be greater than a few tens of kilogauss. On leave from INAF-Bologna Observatory.