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Silicate carbon stars show the 10 micron silicate emission, despite theircarbon-rich photospheres. They are considered to have circumbinary orcircum-companion disks, which serve as a reservoir of oxygen-rich material shedby mass loss in the past. We present N-band spectro-interferometricobservations of the silicate carbon star BM Gem using MIDI at the Very LargeTelescope Interferometer (VLTI). Our aim is to probe the spatial distributionof oxygen-rich dust with high spatial resolution. BM Gem was observed withVLTI/MIDI at 44--62 m baselines using the UT2-UT3 and UT3-UT4 baselineconfigurations. The N-band visibilities observed for BM Gem show a steepdecrease from 8 to ~10 micron and a gradual increase longward of ~10 micron,reflecting the optically thin silicate emission feature emanating fromsub-micron-sized amorphous silicate grains. The differential phases obtained atbaselines of ~44--46 m show significant non-zero values (~ -70 degrees) in thecentral part of the silicate emission feature between ~9 and 11 micron,revealing a photocenter shift and the asymmetric nature of the silicateemitting region. The observed N-band visibilities and differential phases canbe fairly explained by a simple geometrical model in which the unresolved staris surrounded by a ring with azimuthal brightness modulation. The best-fitmodel is characterized by a broad ring (~70 mas across at 10 micron) with abright region which is offset from the unresolved star by ~20 mas at a positionangle of ~280 degrees. This model can be interpreted as a system with acircum-companion disk and is consistent with the spectroscopic signatures of anaccretion disk around an unseen companion recently discovered in the violetspectrum of BM Gem.

Asymmetric silicate dust distribution toward the silicate carbon star BM Gem