M. L. Leal-Ferreira, W. H. T. Vlemmings, P. J. Diamond, A. Kemball, N. Amiri, J. -F. Desmurs
Most of PNe are not spherical. The loss of spherical symmetry occurs
somewhere between the AGB and PN phase. The cause of this change of morphology
is not yet well known, but magnetic fields are one of the possible agents. Its
origin remains to be determined, and potentially requires the presence of a
massive companion to the AGB star. Therefore, further detections of the
magnetic field around evolved stars (in particular those thought to be part of
a binary system) are crucial to improve our understanding of the origin and
role of magnetism on evolved stars. One such binaries is the pre-PN OH231.8,
around which a magnetic field was detected in the OH maser region of the outer
circumstellar envelope. We aim to detect and infer the properties of the
magnetic field of this source in the water maser region.
We observed the 6_{1,6}-5_{2,3} water maser rotational transition to
determine its linear and circular polarization. These emissions are located
within the inner regions of OH231.8 (at few tens of AU). We detected 30 water
maser features, which occur in two distinct regions that are moving apart with
a velocity on the sky of 2.3 mas/year. Taking into account the inclination
angle of the source, this corresponds to an average separation velocity of 15
km/s. Based on the velocity gradient of the maser emission, the masers appear
to be dragged along the direction of the nebula jet. Linear polarization is
present in 3 of the features, and circular polarization was detected in the 2
brightest ones. We found that the strength of the magnetic field is |B_{||}|~45
mG which, when assuming a toroidal magnetic field, implies B~1.5-2.0 G on the
stellar surface. The morphology of the field is not yet determined, but the
high scatter found on the directions of the linear polarization vectors could
indicate that the masers occur near the tangent points of a toroidal field.
View original:
http://arxiv.org/abs/1201.3839
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