2008A&A...478..809O

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. We present multi-epoch, high-angular resolution observations of 22GHz H₂O masers toward the silicate carbon star EU And to probe the spatio-kinematic distribution 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 22GHz H₂O masers have revealed that the maser spots are distributed along a straight line across ∼20mas, with a slight hint of an S-shaped structure. The observed spectra show three prominent velocity components at V_LSR=-42, -38, and -34km/s, 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=-38km/s, 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 10µm 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 0.5-0.8M_☉. We also identify CO₂ emission features at 13-16µm in the Spitzer spectrum of EU And-the first unambiguous detection of CO₂ in silicate carbon stars.