Detection of water vapor in the terrestrial planet forming region of a transition disk

TL;DR

This study reports the first detection of warm water vapor in a pre-transitional protoplanetary disk with a large inner gap, revealing that such disks can retain conditions suitable for planet formation.

Contribution

It provides the first spectroscopic detection and spatial constraint of water vapor in a disk with a large inner gap, challenging assumptions about disk clearing effects.

Findings

Water vapor detected near 0.3 AU in the disk

The gas/dust ratio in the region exceeds 1000

The disk maintains conditions similar to classical protoplanetary disks

Abstract

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^{22} cm-2) 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.