Imaging the oxygen-rich disk toward the silicate carbon star EU Andromedae^*

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: kohnaka@mpifr-bonn.mpg.de
² United States Naval Observatory, 3450 Massachusetts Avenue, NW, Washington, DC 20392-5420, USA e-mail: dboboltz@usno.navy.mil

Received: 8 November 2007
Accepted: 8 December 2007

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 ~20 mas, with a slight hint of an S-shaped structure. The observed spectra show three prominent velocity components at V_LSR= -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 μm 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 . We also identify CO₂ emission features at 13–16 μm  in the Spitzer spectrum of EU And–the first unambiguous detection of CO₂ in silicate carbon stars.