Abstract

Context. Despite their carbon-rich photospheres, silicate carbon stars show 10 mu 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 similar to 10 mu m and increase gradually longward of similar to 10 mu m, reflecting the optically thin silicate emission feature emanating from sub-micron-sized amorphous silicate grains. The differential phases obtained at baselines of similar to 44 - 46 m show significant non-zero values (similar to-70 degrees) in the central part of the silicate emission feature between similar to 9 and 11 mu 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 (similar to 70 mas across at 10 mu m) with a bright region offset from the unresolved star by similar to 20 mas at a position angle of similar to 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, which were discovered in the violet spectrum of BM Gem.