Magnetic fields around evolved stars: further observations of H₂O maser polarization ^⋆

¹ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
e-mail: ferreira@astro.uni-bonn.de
² Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden
³ Department of Astronomy, University of Illinois at Urbana-Champaign, 1002 West Green Street, Urbana, IL 61801, USA
⁴ National Center for Supercomputing Applications, University of Illinois at Urbana-Champaign, 605 East Springfield Avenue, Champaign, IL 61820, USA
⁵ Center for Astrophysics and Space Astronomy, Department of Astrophysical and Planetary Sciences, University of Colorado, 389 UCB, Boulder, CO 80309-0389, USA

Received: 1 February 2013
Accepted: 20 April 2013

Abstract

Context. A low- or intermediate-mass star is believed to maintain a spherical shape throughout the evolution from the main sequence to the asymptotic giant branch (AGB) phase. However, many post-AGB objects and planetary nebulae exhibit non-spherical symmetry. Several candidates have been suggested as factors that can play a role in this change of morphology, but the problem is still not well understood. Magnetic fields are one of these possible agents.

Aims. We aim to detect the magnetic field and infer its properties around four AGB stars using H₂O maser observations. The sample we observed consists of the following sources: the semi-regular variable RT Vir, and the Mira variables AP Lyn, IK Tau, and IRC+60370.

Methods. We observed the 6_1,6 −5_2,3 H₂O maser rotational transition in full-polarization mode to determine its linear and circular polarization. Based on the Zeeman effect, one can infer the properties of the magnetic field from the maser polarization analysis.

Results. We detected a total of 238 maser features in three of the four observed sources. No masers were found toward AP Lyn. The observed masers are all located between 2.4 and 53.0 AU from the stars. Linear and circular polarization was found in 18 and 11 maser features, respectively.

Conclusions. We more than doubled the number of AGB stars in which a magnetic field has been detected from H₂O maser polarization. Our results confirm the presence of fields around IK Tau, RT Vir, and IRC+60370. The strength of the field along the line of sight is found to be between 47 and 331 mG in the H₂O maser region. Extrapolating this result to the surface of the stars, assuming a toroidal field (∝ r^-1), we find magnetic fields of 0.3−6.9 G on the stellar surfaces. If, instead of a toroidal field, we assume a poloidal field (∝ r^-2), then the extrapolated magnetic field strength on the stellar surfaces are in the range between 2.2 and ~115 G. Finally, if a dipole field (∝ r^-3) is assumed, the field strength on the surface of the star is found to be between 15.8 and ~1945 G. The magnetic energy of our sources is higher than the thermal and kinetic energy in the H₂O maser region of this class of objects. This leads us to conclude that, indeed, magnetic fields probably play an important role in shaping the outflows of evolved stars.