Detection of Water Vapor in the Terrestrial Planet Forming Region of a Transition Disk

We report a detection of water vapor in the protoplanetary disk around DoAr 44 with the Texas Echelon Cross Echelle Spectrograph — a visitor instrument on the Gemini north telescope. The DoAr 44 disk consists of an optically thick inner ring and outer disk, separated by a dust-cleared 36 AU gap, and has therefore been termed “pre-transitional”. To date, this is the only disk with a large inner gap known to harbor detectable quantities of warm (T ∼ 450 K) water vapor. In this work, we detect and spectrally resolve three mid-infrared pure rotational emission lines of water vapor from this source, and use the shapes of the emission lines to constrain the location of the water vapor. We find that the emission originates near 0.3 AU — the inner disk region. This characteristic region coincides with that inferred for both optically thick and thin thermal infrared dust emission, as well as rovibrational CO emission. The presence of water in the dust-depleted region implies substantial columns of hydrogen (> 10 cm) as the water vapor would otherwise be destroyed by photodissociation. Combined with the dust modeling, this column implies a gas/small-dust ratio in the optically thin dusty region of & 1000. These results demonstrate that DoAr 44 has maintained similar physical and chemical conditions to classical protoplanetary disks in its terrestrial-planet forming regions, in spite of having formed a large gap. Subject headings: stars: pre-main sequence — protoplanetary disks