N. Amiri, W. H. T. Vlemmings, A. J. Kemball, H. J. van Langevelde
SiO maser emission occurs in the extended atmosphere of evolved stars and can
be studied at high angular resolution. Very long baseline interferometry (VLBI)
observations of Mira variables indicate that SiO masers are significantly
linearly polarized with linear polarization fraction up to 100%. However, no
information is available at high angular resolution for SiO masers in higher
mass loss OH/IR stars. We extend the VLBI SiO maser studies to OH/IR stars. The
observations enable us to understand the SiO pumping mechanisms in higher mass
loss evolved objects and compare those with Mira variables. Additionally,
polarimetric observations of SiO masers help us to understand the magnetic
field strength and morphology and to distinguish between conflicting
polarization theories. The 43 GHz SiO maser observations of the OH/IR star OH
44.8-2.3 were performed with the VLBA in full polarization spectral line mode.
Auxiliary EVLA observations were performed to allow for the absolute
calibration of the polarization angle. The Zeeman splitting was measured by
cross correlating the right and left circular polarization spectra as well as
the S-curve fitting. Additionally, we analyzed the 1612 MHz OH maser
observations of OH 44.8-2.3 from the VLA archive. The SiO masers of OH 44.8-2.2
form a ring located at ~5.4 AU around the star. The masers appear to be highly
linearly polarized with fractional linear polarization up to 100%. The linear
polarization vectors are consistent with a dipole field morphology in this
star. We report a tentative detection of circular polarization of ~0.7% for the
brightest maser feature. The magnetic field measured for this feature
corresponds to 1.5\pm0.3 G. Additionally, the distribution of the 1612 MHz OH
maser emission could indicate an elongated morphology.
View original:
http://arxiv.org/abs/1110.4071
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