  Silicate carbon stars show the 10 micron silicate emission, despite their
carbon-rich photospheres. 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. 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 with high spatial resolution. BM Gem was observed with
VLTI/MIDI at 44--62 m baselines using the UT2-UT3 and UT3-UT4 baseline
configurations. The N-band visibilities observed for BM Gem show a steep
decrease from 8 to ~10 micron and a gradual increase longward of ~10 micron,
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 degrees) in the
central part of the silicate emission feature between ~9 and 11 micron,
revealing a photocenter shift and the asymmetric nature of the silicate
emitting region. The observed N-band visibilities and differential phases can
be fairly explained 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 micron) with a
bright region which is offset from the unresolved star by ~20 mas at a position
angle of ~280 degrees. 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 recently discovered in the violet
spectrum of BM Gem.
