The magnetic field of the evolved star W43A

TL;DR

This study measures magnetic fields in the evolved star W43A, demonstrating that magnetic fields influence jet collimation and the structure of the circumstellar environment, supporting theories of magnetically shaped planetary nebulae.

Contribution

It provides the first direct measurements of magnetic fields in both OH and H2O maser regions of W43A, confirming their role in jet collimation and nebula shaping.

Findings

Magnetic field of ~100 micro-gauss in OH maser region.

Magnetic field of ~30 mG in H2O maser region, changing sign across the jet.

OH maser shell shows typical expansion velocity, not non-spherical.

Abstract

The majority of the observed planetary nebulae exhibit elliptical or bipolar structures. Theoretical modeling has indicated that magnetically collimated jets may be responsible for the formation of the non-spherical planetary nebulae. 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 report a measured magnetic field of approximately 100 micro-gauss in the OH maser region of the circumstellar envelope around W43A. The GBT observations reveal a magnetic field strength B|| 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. 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.