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We present new long-baseline spectro-interferometric observations of theHerbigAe star HD163296 obtained in the H and K bands with the AMBER instrumentat VLTI. The observations cover a range of spatial resolutions between 3 and 12milli-arcseconds, with a spectral resolution of ~30. With a total of 1481visibilities and 432 closure phases, they result in the best (u,v) coverageachieved on a young star so far. The circumstellar material is resolved at thesub-AU spatial scale and closure phase measurements indicate a small butsignificant deviation from point-symmetry. We discuss the results assuming thatthe near-infrared excess in HD163296 is dominated by the emission of acircumstellar disk. A successful fit to the spectral energy distribution,near-infrared visibilities and closure phases is found with a model where adominant contribution to the H and K band emissions arises from an opticallythin, smooth and point-symmetric region extending from about 0.1 to 0.45 AU. Atthe latter distance from the star, silicates condense, the disk becomesoptically thick and develops a puffed-up rim, whose skewed emission can accountfor the non-zero closure phases. We discuss the nature of the inner diskemission and tentatively rule out dense molecular gas as well as optically thinatomic or ionized gas as its possible origin. We propose instead that the inneremission traces the presence of very refractory grains in a partially clearedregion, extending at least to 0.5 AU. If so, we may be observing the disk ofHD163296 just before it reaches the transition disk phase. However, we notethat the nature of the refractory grains or even the possibility for any grainto survive at the very high temperatures we require (~2100-2300 K at 0.1 AUfrom the star) is unclear and should be investigated further.

Strong near-infrared emission in the sub-AU disk of the Herbig Ae star HD163296: evidence for refractory dust?