Astronomy and Astrophysics, volume 570A, 88-88 (2014/10-1)
Weak magnetic fields in central stars of planetary nebulae?
STEFFEN M., HUBRIG S., TODT H., SCHOELLER M., HAMANN W.-R., SANDIN C. and SCHOENBERNER D.
Abstract (from CDS):
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, stellar or substellar, 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 planetary nebulae, 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 FORS2 installed on the Antu telescope of the VLT for a sample of 12 bright central stars of PNe with different morphologies, 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 100Gauss in the central star of the young elliptical planetary nebula IC418 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 250G mean longitudinal field is achieved for the A-type companion of the central star of NGC1514. 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. Since our analysis indicates only weak fields, if any, in a few targets of our sample, 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 100G may well be sufficient to contribute to the shaping of the surrounding nebulae throughout their evolution. Our current sample is too small to draw conclusions about a correlation between nebular morphology and the presence of stellar magnetic fields.
planetary nebulae: general - stars: magnetic field - stars: AGB and post-AGB - binaries: close - techniques: polarimetric
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