The magnetic field of the evolved star W43A

¹ Sterrewacht Leiden, Leiden University, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands e-mail: amiri@strw.leidenuniv.nl
² Joint Institute for VLBI in Europe (JIVE), Postbus 2, 7990 AA Dwingeloo, The Netherlands
³ Argelander Institute for Astronomy, University of Bonn, Auf dem Hügel 71, 53121 Bonn, Germany

Received: 27 August 2009
Accepted: 13 October 2009

Abstract

Context. The majority of the observed planetary nebulae exhibit elliptical or bipolar structures. Recent observations have shown that asymmetries already start during the last stages of the AGB phase. Theoretical modeling has indicated that magnetically collimated jets may be responsible for the formation of the non-spherical planetary nebulae. Direct measurement of the magnetic field of evolved stars is possible using polarization observations of different maser species occurring in the circumstellar envelopes around these stars.

Aims. The aim of this project is to measure the Zeeman splitting caused by the magnetic field in the OH and H₂O maser regions occurring in the circumstellar envelope and bipolar outflow of the evolved star W43A. We compare the magnetic field obtained in the OH maser region with the one measured in the H₂O maser jet.

Methods. We used the UK Multi-Element Radio Linked Interferometer Network (MERLIN) to observe the polarization of the OH masers in the circumstellar envelope of W43A. Likewise, we used the Green Bank Telescope (GBT) observations to measure the magnetic field strength obtained previously in the H₂O maser jet.

Results. We report a measured magnetic field of approximately 100 G in the OH maser region of the circumstellar envelope around W43A. The GBT observations reveal a magnetic field strength of ~30 mG changing sign across the H₂O masers at the tip of the red-shifted lobe of the bipolar outflow. We also find that the OH maser shell shows no sign of non-spherical expansion and that it probably has an expansion velocity that is typical for the shells of regular OH/IR stars.

Conclusions. The GBT observations confirm that the magnetic field collimates the H₂O maser jet, while the OH maser observations show that a strong large-scale magnetic field is present in the envelope surrounding the W43A central star. The magnetic field in the OH maser envelope is consistent with the one extrapolated from the H₂O measurements, confirming that magnetic fields play an important role in the entire circumstellar environment of W43A.