The kinematics and magnetic fields in water-fountain sources based on OH maser observations^⋆

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

Received: 4 February 2011
Accepted: 21 June 2011

Abstract

Context. water-fountain sources are a class of post-AGB objects that exhibit bipolar jet-like structures traced by H₂O maser emission. The circumstellar envelopes of these objects show deviations from spherical symmetry. While the H₂O masers originate in a collimated outflow in the polar region, the OH masers may be distributed in the equatorial region or a biconical outflow in the circumstellar shell. Magnetic fields could play an important role in collimating the jet and shaping the circumstellar envelope of these objects.

Aims. The aim of this project is to understand the morphology of the OH masers in three water-fountain sources, for which we obtained high-resolution interferometric data. We compare the observational parameters of the spectral profile and spatial distribution with our models in order to constrain the morphology and velocity fields of the OH maser shell. We would also like to understand the role of the magnetic field in shaping the circumstellar envelope of these stars.

Methods. We observed the OH masers of three water-fountain sources (OH 12.8-0.9, OH 37.1-0.8, and W43A) in full polarization spectral line mode using the UK Multi-Element Radio Linked Interferometer Network. Additionally, we performed reconstruction models of OH maser shells distributed uniformly in equatorial or biconical outflows and compared them with the OH maser observations.

Results. The OH masers of W43A seem to be located in the equatorial region of the circumstellar shell, while for OH 12.8-0.9 the masers are likely distributed in a biconical outflow. We measured magnetic fields of  ~360 μG and  ~29 μG on average for OH 37.1-0.8 and OH 12.8-0.9, respectively. The measured field strengths show that a large-scale magnetic field is present in the circumstellar environment of these stars.

Conclusions. Our observations show that the OH maser region of the three water-fountain sources studied in this work show signs of aspherical expansion. Reconstruction of the OH maser distribution of these objects show both biconical and equatorial distributions. Magnetic fields likely play an important role in shaping the OH maser region of water-fountain sources.