Abstract

Context. Silicate carbon stars are characterized by oxygen-rich circumstellar environments as revealed by prominent silicate emission, despite their carbon-rich photospheres. While the presence of a circumbinary disk or a disk around an unseen, low-luminosity companion has been suggested to explain the peculiar dust chemistry, the origin of silicate carbon stars is still a puzzle to date. Aims. We present multi-epoch, high-angular resolution observations of 22 GHz H2O masers toward the silicate carbon star EU And to probe the spatio-kinematic distribution of oxygen-rich material. Methods. EU And was observed at three epochs (maximum time interval of 14 months) with the Very Long Baseline Array (VLBA). Results. Our VLBA observations of the 22 GHz H2O masers have revealed that the maser spots are distributed along a straight line across similar to 20 mas, with a slight hint of an S-shaped structure. The observed spectra show three prominent velocity components at VLSR= -42, -38, and -34 km s(-1), with the masers in SW redshifted and those in NE blueshifted. The maser spots located in the middle of the overall distribution correspond to the component at V-LSR = -38 km s(-1), which approximately coincides with the systemic velocity. These observations can be interpreted as either an emerging helical jet or a disk viewed almost edge-on (a circumbinary or circum-companion disk). However, the outward motion measured in the VLBA images taken 14 months apart is much smaller than what is expected from the jet scenario. Furthermore, the mid-infrared spectrum obtained with the Spitzer Space Telescope indicates that the 10 mu m silicate emission is optically thin and the silicate grains are of sub-micron size. This lends support to the presence of a circum-companion disk, because an optically thin circumbinary disk consisting of such small grains would be blown away by the intense radiation pressure of the primary (carbon-rich) star. If we assume Keplerian rotation for the circum-companion disk, the mass of the companion is estimated to be 0.5-0.8 M-circle dot. We also identify CO2 emission features at 13-16 mu m in the Spitzer spectrum of EU And-the first unambiguous detection of CO2 in silicate carbon stars.