Asymmetric silicate dust distribution toward the silicate carbon star BM Geminorum

Ohnaka, K., Izumiura, H., Leinert, Ch., Driebe, T., Weigelt, G., Wittkowski, M.

A&A, Volume 490, Issue 1, pp.173-178 (2008)


Context: Despite their carbon-rich photospheres, silicate carbon stars show 10 µm 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 µm and increase gradually longward of ~10 µm, 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 µm, 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 µm) 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.

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