VLBA SiO observations of bright O-rich AGB stars

¹ National Radio Astronomy Observatory (The National Radio Astronomy Observatory (NRAO) is operated by Associated Universities Inc., under cooperative agreement with the National Science Foundation.) , 520 Edgemont Road, Charlottesville, VA 22903-2475, USA e-mail: bcotton@nrao.edu
² Observatoire de Paris, LESIA, UMR 8109, 92190 Meudon, France
³ Observatoire de la Côte d'Azur, Nice, France

Received: 8 May 2007
Accepted: 22 October 2007

Abstract

We present the results of observations of the SiO masers at 7 mm wavelength in several oxygen rich Asymptotic Giant Branch (AGB) stars. These observations were coordinated with observations using the ESO VLTI/MIDI interferometer at 10 μm wavelength. These stars are o Ceti (Mira), U Herculis (U Her), OH26.5+0.6, R Leonis (R Leo), and W Hydrae (W Hya). Both total intensity and linear polarizations are imaged. Linear radial features seen in o Ceti may be associated with an X-ray flare seen a year earlier by [CITE]. Similar linear features are also seen in R Leo, U Her and OH26.5+0.6. These features typically show systematic radial velocity variations and a tendency for the linear polarization to be perpendicular to the direction of elongation, suggesting that magnetic fields are dynamically important. A wide radial extent of the maser rings observed in U Herculis and OH26.5+0.6 is interpreted as meaning that the outer boundaries of the masing regions in these stars are not truncated by the condensation of SiO onto dust grains.