Program: GS-2008A-C-7

The morphology of transitional disks (i.e. disks with optically thin inner regions and optically thick outer regions) is often thought to be due to the dynamical sculpting of an embedded planet. As intriguing as this possibility is, it is important to note that there are several other physical effects that can give rise to transitional disks without the presence of a massive planet such as planetesimal formation and photoevaporation. In this proposal we summarize four physical scenarios that can give rise to transitional disks and describe their distinct gas phase signatures. We propose to trace these distinct gas-phase signatures using high-resolution fundamental ro-vibrational spectra of CO near 4.7$\micron$. Thus we propose to observe seven well-characterized transitional disks around Herbig Ae/Be stars using Phoenix, in order to determine whether the transitional morphology of these systems is due to an embedded planet or some other physics.