EMISSION LINES FROM THE GAS DISK AROUND TW HYDRA AND THE ORIGIN OF THE INNER HOLE

We compare line emission calculated from theoretical disk models with optical to submillimeter wavelength observational data of the gas disk surrounding TW Hya and infer the spatial distribution of mass in the gas disk. The model disk that best matches observations has a gas mass ranging from ∼10-4 to 10-5 Msun for 0.06 AU < r < 3.5 AU and ∼0.06 Msun for 3.5 AU < r < 200 AU. We find that the inner dust hole (r < 3.5 AU) in the disk must be depleted of gas by ∼1-2 orders of magnitude compared with the extrapolated surface density distribution of the outer disk. Grain growth alone is therefore not a viable explanation for the dust hole. CO vibrational emission arises within r ∼ 0.5 AU from thermal excitation of gas. [O I] 6300 A and 5577 A forbidden lines and OH mid-infrared emission are mainly due to prompt emission following UV photodissociation of OH and water at r ∼< 0.1 AU and at r ∼ 4 AU. [Ne II] emission is consistent with an origin in X-ray heated neutral gas at r ∼< 10 AU, and may not require the presence of a significant extreme-ultraviolet (hν > 13.6 eV) flux from TW Hya. H2 pure rotational line emission comes primarily from r ∼ 1 to 30 AU. [O I] 63 μm, HCO+, and CO pure rotational lines all arise from the outer disk at r ∼ 30-120 AU. We discuss planet formation and photoevaporation as causes for the decrease in surface density of gas and dust inside 4 AU. If a planet is present, our results suggest a planet mass ∼4-7 MJ situated at ∼3 AU. Using our photoevaporation models and the best surface density profile match to observations, we estimate a current photoevaporative mass loss rate of 4 x 10-9 Msun yr-1 and a remaining disk lifetime of ∼5 million years.