Imaging the oxygen-rich disk toward the silicate carbon star EU Andromedae
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 M⊙. 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.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- February 2008
- DOI:
- 10.1051/0004-6361:20079030
- arXiv:
- arXiv:0712.2395
- Bibcode:
- 2008A&A...478..809O
- Keywords:
-
- radio lines: stars;
- techniques: interferometric;
- stars: circumstellar matter;
- stars: carbon;
- stars: AGB and post-AGB;
- stars: individual: EU And;
- Astrophysics
- E-Print:
- 6 pages, 4 figures, accepted for publication in A&