Studying gas in young and debris disks using Spitzer

We present results from models that self-consistently calculate the gas chemistry, gas temperature, dust temperature, and vertical density distribution in circumstellar disks around young stars, and that predict the emergent infrared spectra. A parameter study of optically thin disks shows that fine structure lines such as [SI] 25 mu m are generally stronger than the H-2 rotational lines. Spitzer can detect lines from disks less than or similar to 100 pc with gas masses of greater than or similar to 0.01M(J) and some dust present (greater than or similar to 10(-7) M-J) to heat the gas. We model Spitzer upper limits on line fluxes from HD 105, a 30 Myr old GOV star, and provide upper limits on the gas surface density and mass (less than or similar to 1M(J)) in the planet-forming region (1-40 AU). Preliminary results for younger, optically thick disks are briefly discussed.