Strong near-infrared emission in the sub-AU disk of?the?Herbig?Ae?star HD?163296: evidence of refractory dust?

摘要

We present new long-baseline spectro-interferometric observations ofthe HerbigAe star HD163296 (MWC275)obtained in the H and Kbandswith the AMBER instrument at the VLTI. The observations cover a rangeof spatial resolutions between 3 and 12milliarcseconds,with a spectral resolution of 30. With a total of 1481visibilities and 432 closure phases, they represent the mostcomprehensive (u,v) coverageachieved so far for a young star. The circumstellar material isresolved at the sub-AU spatial scale and closure phase measurementsindicate a small but significant deviation from point-symmetry. Wediscuss the results assuming that the near-infrared excess inHD163296 is dominated by the emission of a circumstellardisk. A successful fit to the spectral energy distribution,near-infrared visibilities and closure phases is found with a model inwhich a dominant contribution to the H and Kbandemission originates in an optically thin, smooth andpoint-symmetric region extending from about 0.1 to 0.45AU. Ata distance of 0.45AU from the star, silicates condense, thedisk becomes optically thick and develops a puffed-up rim, whose skewedemission can account for the non-zero closure phases. We discuss thesource of the inner disk emission and tentatively exclude densemolecular gas as well as optically thin atomic or ionized gas as itspossible origin. We propose instead that the smooth inner emission isproduced by very refractory grains in a partially cleared region,extending to at least 0.5AU.If so, we may be observing the disk of HD163296 just beforeit reaches the transition disk phase. However, we note that the natureof the refractory grains or, in fact, even the possibility of any grainsurviving at the very high temperatures we require (2100-2300K at0.1AU from the star) is unclear and should be investigatedfurther. Key words: protoplanetary disks - methods:observational - techniques: interferometric - stars: pre-main sequence