Detecting the cold water reservoir in a protoplanetary disk

Date

2011-05-31

Author

Hogerheijde, Michiel
Yıldız, Umut

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Water plays a pivotal role in planet-forming disks around newly formed stars. Frozen out onto dust grains in the cold and dense disk interior, water ice likely aids coagulation of grains and enables planet formation. The ice mantles are also a locus of complex chemistry, and provide a potentially rich source of water to be delivered to early Earth-like planets. The observational evidence for the presence of water in protoplanetary disks has so-far been limited to mid-infrared emission lines of hot water in the inner regions of several disks (<3 AU) and a few rare occasions of the water-ice feature at 3 μm in absorption or in scattered light. The infrared water emission lines indicate that the water gas-phase abundance drops sharply outside a few AU, possibly because the water freezes out efficiently onto cold dust grains outside the `snow line'. We present the first detections of the rotational ground-state emission lines of both spin-isomers of water, obtained with the Heterodyne Instrument for the Far-Infrared (HIFI) on-board the Herschel Space Observatory toward the disk around the young, low-mass star TW Hya. These velocity-resolved spectra show that cold water vapor is present throughout the entire radial extent of the disk. Efficient photodesorption of water-ice molecules by stellar ultraviolet radiation is a likely explanation for the presence of water vapor beyond a few AU, and our detections proof that a water-ice reservoir extends throughout the disk. The strength of the emission lines is lower than expected based on recent laboratory measurements of photodesorption and a fiducial model for the TW Hya disk. This suggests that grains carrying as much as 80--90% of the water ice content have settled toward the disk mid-plane, and are outside the reach of the stellar ultraviolet radiation. From our calculations and the detection of both ortho- and para-water, we derive the `spin temperature' of the cold water vapor in the TW Hya disk, which we compare to interstellar clouds and Solar System comets.

URI

https://ui.adsabs.harvard.edu/abs/2011IAUS..280E..16H/abstract
https://hdl.handle.net/11511/115746

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Department of Physics, Conference / Seminar