Evidence for a hot dust-free inner disk around 51 Oph

We report on the observation of CO bandhead emission around 51 Oph (∆v = 2). A high resolving power (R � 10 000) spectrum was obtained with the infrared spectrometer ISAAC mounted on VL T − ANT U. Modeling of the profile suggests that the hot (Tgas = 2000−4000 K) and dense (nH > 10 10 cm −3 ) molecular material as probed by the CO bandhead is located in the inner AU of a Keplerian disk viewed almost edge-on. Combined with the observation of cooler gas (Tgas = 500−900 K) by ISO-SWS and the lack of cold material, our data suggest that the disk around 51 Oph is essentially warm and small. We demonstrate the presence of a dust-free inner disk that extents from the inner truncation radius until the dust sublimation radius. The disk around 51 Oph may be in a rare transition state toward a small debris disk object.