Modelling Herbig Ae/Be group I/II properties
          Joke Meijer, U. of Amsterdam, The Netherlands 
          Alex De Koter, U. of Amsterdam, The Netherlands 
          Carsten Dominik, U. of Amsterdam, The Netherlands 
          Kees Dullemond, MPIA, Germany
          Roy van Boekel, U. of Amsterdam, The Netherlands 
          Rens Waters, U. of Amsterdam, The Netherlands
    Detailed studies of SEDs of intermediate-mass pre-main-sequence
(Herbig Ae/Be) stars with large infrared excesses show that there are two types: those with a strong mid-IR excess and those without. It has been suggested that this reflects the spatial distribution of the dust, either the disk is flat (weak mid-IR) or it flares (strong mid-IR). Recently it has become clear that total dust mass is the most important disk property determining the disk shape. We have used 2D disk models which consistently treat radiative transfer processes to determine the other key parameters that control the geometry of the disk. We find that a growing midplane layer of large grains combined with general grain growth could explain the absence of group II sources without a 10 micron feature. These, so called, IIb sources could be classified as Ib source