Weak magnetic fields in central stars of planetary nebulae?
M. Steffen, S. Hubrig, H. Todt, M. Schöller, W.-R. Hamann, C. Sandin, D. Schönberner
Abstract:
It is not yet clear whether magnetic fields play an essential role in
shaping planetary nebulae (PNe), or whether stellar rotation alone
and/or a close binary companion can account for the variety of the
observed nebular morphologies. In a quest for empirical evidence
verifying or disproving the role of magnetic fields in shaping PNe, we
follow up on previous attempts to measure the magnetic field in a
representative sample of PN central stars. We obtained low-resolution
polarimetric spectra with FORS 2 at VLT for a sample of twelve bright
central stars of PNe with different morphology, including two round
nebulae, seven elliptical nebulae, and three bipolar nebulae. Two
targets are Wolf-Rayet type central stars. For the majority of the
observed central stars, we do not find any significant evidence for the
existence of surface magnetic fields. However, our measurements may
indicate the presence of weak mean longitudinal magnetic fields of the
order of 100 Gauss in the central star of the young elliptical planetary
nebula IC 418, as well as in the Wolf-Rayet type central star of the
bipolar nebula Hen2-113 and the weak emission line central star of the
elliptical nebula Hen2-131. A clear detection of a 250 G mean
longitudinal field is achieved for the A-type companion of the central
star of NGC 1514. Some of the central stars show a moderate
night-to-night spectrum variability, which may be the signature of a
variable stellar wind and/or rotational modulation due to magnetic
features. We conclude that strong magnetic fields of the order of kG are
not widespread among PNe central stars. Nevertheless, simple estimates
based on a theoretical model of magnetized wind bubbles suggest that
even weak magnetic fields below the current detection limit of the order
of 100 G may well be sufficient to contribute to the shaping of PNe
throughout their evolution.
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