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

We report on the observation of CO bandhead emission around 51 Oph (Deltav=2). A high resolving power (Rsimilar or equal to10000) spectrum was obtained with the infrared spectrometer ISAAC mounted on VLT-ANTU. Modeling of the profile suggests that the hot (T-gas=2000-4000 K) and dense (n(H)>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 (T-gas=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.