A&A 490, 173-178 (2008)
DOI: 10.1051/0004-6361:200810229

Asymmetric silicate dust distribution toward the silicate carbon star BM Geminorum

K. Ohnaka¹, H. Izumiura², Ch. Leinert³, T. Driebe¹, G. Weigelt¹, and M. Wittkowski<sup>4

1</sup>  Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
    e-mail: kohnaka@mpifr-bonn.mpg.de
²  Okayama Astrophysical Observatory, National Astronomical Observatory, Kamogata, Asakuchi, Okayama 719-0232, Japan
³  Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁴  European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany

Received 20 May 2008 / Accepted 9 July 2008

Abstract
Context. Despite their carbon-rich photospheres, silicate carbon stars show 10 m silicate emission. They are considered to have circumbinary or circum-companion disks, which serve as a reservoir of oxygen-rich material shed by mass loss in the past.
Aims. We present N-band spectro-interferometric observations of the silicate carbon star BM Gem using MIDI at the Very Large Telescope Interferometer (VLTI). Our aim is to probe the spatial distribution of oxygen-rich dust at high spatial resolution.
Methods. Using the UT2-UT3 and UT3-UT4 configurations, BM Gem was observed with VLTI/MIDI at 44–62 m baselines.
Results. The N-band visibilities observed for BM Gem decrease steeply between 8 and ~10 m and increase gradually longward of ~10 m, reflecting the optically thin silicate emission feature emanating from sub-micron-sized amorphous silicate grains. The differential phases obtained at baselines of ~44–46 m show significant non-zero values (-70°) in the central part of the silicate emission feature between ~9 and 11 m, revealing a photocenter shift and the asymmetric nature of the silicate emitting region. The observed N-band visibilities and differential phases can be described adequately by a simple geometrical model in which the unresolved star is surrounded by a ring with azimuthal brightness modulation. The best-fit model is characterized by a broad ring (~70 mas across at 10 m) with a bright region offset from the unresolved star by ~20 mas at a position angle of ~280°. This model can be interpreted as a system with a circum-companion disk and is consistent with the spectroscopic signatures of an accretion disk around an unseen companion, which were discovered in the violet spectrum of BM Gem.

Key words: infrared: stars -- techniques: interferometric -- stars: circumstellar matter -- stars: carbon -- stars: AGB and post-AGB -- stars: individual: BM Gem

© ESO 2008

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