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

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 H 2 O 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 H 2 O 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 H 2 O 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 H 2 O 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 H 2 O 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.

Constraining Theories of Polarized SiO Maser Transport: Multi-epoch Analysis of a π/2 Electric Vector Rotation Feature

“…Circular polarization of non-Zeeman origin would imply a significantly lower magnetic field strength than required for Zeeman circular polarization of observed SiO masers. We note that the resonant scattering mechanism proposed by Houde (2014) requires a magnetic field strength in the foreground lower-density gas that is not inconsistent with that derived from H 2 O maser observations (Vlemmings et al 2005;Leal-Ferreira et al 2013).…”

Search for surface magnetic fields in Mira stars

“…Very recently, Leal-Ferreira et al (2013) have used polarization of H 2 O maser emissions in a sample of AGB stars to present the new panorama for magnetic fieldstrength dependence along r, the radial distance to the star (see their Fig. 5).…”

Large-scale environments of binary AGB stars probed byHerschel

“…However, only one AGB star is known to show a fast rotation (V Hya; Barnbaum et al 1995), and generally slow rotation rates are found among white dwarf stars (Kawaler et al 1999); (ii) a bipolar magnetic field causes the wind to become denser at the magnetic equator (Matt et al 2000). Surface magnetic fields have been recently reported for some AGB stars (Vlemmings et al 2011;Leal-Ferreira et al 2013;Lèbre et al 2014), but there is no observational evidence for stellar magnetic fields to shape stellar AGB winds; (iii) the most common explanation is gravitational focusing of the AGB wind on the orbital plane of a companion. This scenario is supported by hydrodynamic simulations (Mastrodemos & Morris 1999;Kim & Taam 2012a) and by observations (e.g.…”