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We present multi-epoch high-angular resolution observations of 22 GHz H2Omasers toward the silicate carbon star EU And to probe the spatio-kinematicdistribution of oxygen-rich material. EU And was observed at three epochs(maximum time interval of 14 months) with the Very Long Baseline Array (VLBA).Our VLBA observations of the 22 GHz H2O masers have revealed that the maserspots are distributed along a straight line across ~20 mas, with a slight hintof an S-shaped structure. The observed spectra show three prominent velocitycomponents at V_LSR = -42, -38, and -34 km s^-1, with the masers in SWredshifted and those in NE blueshifted. The maser spots located in the middleof the overall distribution correspond to the component at V_LSR = -38 km s^-1,which approximately coincides with the systemic velocity. These observationscan be interpreted as either an emerging helical jet or a disk viewed almostedge-on (a circumbinary or circum-companion disk). However, the outward motionmeasured in the VLBA images taken 14 months apart is much smaller than thatexpected from the jet scenario. Furthermore, the mid-infrared spectrum obtainedwith the Spitzer Space Telescope indicates that the 10 micron silicate emissionis optically thin and the silicate grains are of sub-micron size. This lendssupport to the presence of a circum-companion disk, because an optically thincircumbinary disk consisting of such small grains would be blown away by theintense radiation pressure of the primary (carbon-rich) star. If we assumeKeplerian rotation for the circum-companion disk, the mass of the companion isestimated to be 0.5--0.8 M_sun. We also identify CO2 emission features at13--16 micron in the Spitzer spectrum of EU And--the first unambiguousdetection of CO2 in silicate carbon stars.

Imaging the oxygen-rich disk toward the silicate carbon star EU And