  Infrared and (sub-)mm observations of disks around T Tauri and Herbig Ae/Be
stars point to a chemical differentiation between both types of disks, with a
lower detection rate of molecules in disks around hotter stars. To investigate
the potential underlying causes we perform a comparative study of the chemistry
of T Tauri and Herbig Ae/Be disks, using a model that pays special attention to
photochemistry. The warmer disk temperatures and higher ultraviolet flux of
Herbig stars compared to T Tauri stars induce some differences in the disk
chemistry. In the hot inner regions, H2O, and simple organic molecules like
C2H2, HCN, and CH4 are predicted to be very abundant in T Tauri disks and even
more in Herbig Ae/Be disks, in contrast with infrared observations that find a
much lower detection rate of water and simple organics toward disks around
hotter stars. In the outer regions, the model indicates that the molecules
typically observed in disks, like HCN, CN, C2H, H2CO, CS, SO, and HCO+, do not
have drastic abundance differences between T Tauri and Herbig Ae disks. Some
species produced under the action of photochemistry, like C2H and CN, are
predicted to have slightly lower abundances around Herbig Ae stars due to a
narrowing of the photochemically active layer. Observations indeed suggest that
these radicals are somewhat less abundant in Herbig Ae disks, although in any
case the inferred abundance differences are small, of a factor of a few at
most. A clear chemical differentiation between both types of disks concerns
ices, which are expected to be more abundant in Herbig Ae disks. The global
chemical behavior of T Tauri and Herbig Ae/Be disks is quite similar. The main
differences are driven by the warmer temperatures of the latter, which result
in a larger reservoir or water and simple organics in the inner regions and a
lower mass of ices in the outer disk.
