  The dust sublimation walls of disks around T Tauri stars represent a directly
observable cross-section through the disk atmosphere and midplane. Their
emission properties can probe the grain size distribution and composition of
the innermost regions of the disk, where terrestrial planets form. Here we
calculate the inner dust sublimation wall properties for four classical T Tauri
stars with a narrow range of spectral types and inclination angles and a wide
range of mass accretion rates to determine the extent to which the walls are
radially curved. Best-fits to the near- and mid-IR excesses are found for
curved, 2-layer walls in which the lower layer contains larger, hotter,
amorphous pyroxene grains with Mg/(Mg+Fe)=0.6 and the upper layer contains
submicron, cooler, mixed amorphous olivine and forsterite grains. As the mass
accretion rates decrease from 10^(-8) to 10^(-10) Msol/yr, the maximum grain
size in the lower layer decreases from 3 to 0.5 microns. We attribute this to a
decrease in fragmentation and turbulent support for micron-sized grains with
decreasing viscous heating. The atmosphere of these disks is depleted of dust
with dust-gas mass ratios 1x10^(-4) of the ISM value, while the midplane is
enhanced to 8 times the ISM value. For all accretion rates, the wall
contributes at least half of the flux in the optically thin 10 micron silicate
feature. Finally, we find evidence for an iron gradient in the disk, suggestive
of that found in our solar system.
