A reconsideration of disk properties in Herbig Ae stars

This paper presents state-of-the-art spectral energy distributions (SEDs) o f four Herbig Ae stars, based in part on new data in the mid and far-infrar ed and at millimeter wavelengths. The SEDs are discussed in the context of circumstellar disk models. We show that models of irradiated disks provide a good fit to the observations over the whole range of wavelengths. We offe r a possible solution to the long-standing puzzle caused by the excess emis sion of Herbig Ae stars, where a large fraction of the stellar luminosity i s re-radiated between similar to1.25 and 7 mum, with a peak at about 3 mum. We suggest that this general behaviour can be caused by dust evaporation i n disks where the gas component is optically thin to the stellar radiation, as expected if the accretion rate is very low. The creation of a puffed-up inner wall of optically thick dust at the dust sublimation radius can acco unt for the near-infrared characteristics of the SEDs. It can also naturall y explain the H and K band interferometric observations of AB Aur (Millan-G abet et al. 2001), which reveal a ring of emission of radius similar to0.3 AU. Finally, irradiated disk models can easily explain the observed intensi ty of the 10 mum silicate features and their variation from star to star.