Evidence for CO depletion in the inner regions of gas-rich protoplanetary disks

Aims. We investigate the physical properties and spatial distribution of Carbon Monoxide (CO) gas in the disks around the Herbig Ae/Be stars HD 97048 and HD 100546.
Methods. Using high-spectral-resolution 4.588-4.715 mu m spectra containing fundamental CO emission taken with CRIRES on the VLT, we probe the circumstellar gas and model the kinematics of the emission lines. By using spectro-astrometry on the spatially resolved targets, we constrain the physical size of the emitting regions in the disks.
Results. We resolve, spectrally and spatially, the emission of the (CO)-C-13 upsilon(1-0) vibrational band and the (CO)-C-12 upsilon = 1-0, upsilon = 2-1, upsilon = 3-2 and upsilon = 4-3 vibrational bands in both targets, as well as the (CO)-C-12 upsilon = 5-4 band in HD 100546. Modeling of the CO emission with a homogeneous disk in Keplerian motion, yields a best fit with an inner and outer radius of the CO emitting region of 11 and >= 100 AU for HD 97048. HD 100546 is not fit well with our model, but we derive a lower limit on the inner radius of 8 AU. The fact that gaseous [OI] emission was previously detected in both targets at significantly smaller radii suggests that CO may be effectively destroyed at small radii in the surface layers of these disks.