  (Abridged) Near- to mid-IR observations of protoplanetary disks show that the
inner regions (<10AU) are rich in small organic volatiles (e.g., C2H2 and HCN).
Trends in the data suggest that disks around cooler stars (~3000K) are
potentially more carbon- and molecule-rich than their hotter counterparts. Our
aims are to explore the composition of the planet-forming region of disks
around stars from M dwarf to Herbig Ae and compare with the observed trends.
Models of the disk physical structure are coupled with a gas-grain chemical
network to map the abundances in the planet-forming zone. N2 self shielding,
X-ray-induced chemistry, and initial abundances, are investigated. The
composition in the 'observable' atmosphere is compared with that in the
midplane where the planet-building reservoir resides. M dwarf disk atmospheres
are relatively more molecule rich than those for T Tauri or Herbig Ae disks.
The weak far-UV flux helps retain this complexity which is enhanced by
X-ray-induced ion-molecule chemistry. N2 self shielding has only a small effect
and does not explain the higher C2H2/HCN ratios observed towards cooler stars.
The models underproduce the OH/H2O column density ratios constrained in Herbig
Ae disks, despite reproducing the absolute value for OH: H2O self shielding
only increases this discrepency. The disk midplane content is sensitive to the
initial main elemental reservoirs. The gas in the inner disk is generally more
carbon rich than the midplane ices and is most significant for disks around
cooler stars. The atmospheric C/O ratio appears larger than it actually is when
calculated using observable tracers only because gas-phase O2 is predicted to
be a significant oxygen reservoir. The models suggest that the gas in the inner
regions of disks around cooler stars is more carbon rich; however, calculations
of the molecular emission are necessary to confirm the observed trends.
