Sensitive limits on the abundance of cold water vapor in the DM Tau protoplanetary disk

TL;DR

This study used Herschel/HIFI to search for water vapor in the DM Tau protoplanetary disk, setting strict limits on water presence and suggesting most water ice is in settled grains, with a tentative detection of water emission.

Contribution

First sensitive search for water vapor lines in DM Tau disk with Herschel, providing new constraints on water distribution and disk chemistry.

Findings

No strong water lines detected at 3sigma levels.

Tentative 6sigma detection of water line in Wide Band Spectrometer.

Limits suggest water is mostly in icy grains settled in the disk midplane.

Abstract

We performed a sensitive search for the ground-state emission lines of ortho- and para-water vapor in the DM Tau protoplanetary disk using the Herschel/HIFI instrument. No strong lines are detected down to 3sigma levels in 0.5 km/s channels of 4.2 mK for the 1_{10}--1_{01} line and 12.6 mK for the 1_{11}--0_{00} line. We report a very tentative detection, however, of the 1_{10}--1_{01} line in the Wide Band Spectrometer, with a strength of T_{mb}=2.7 mK, a width of 5.6 km/s and an integrated intensity of 16.0 mK km/s. The latter constitutes a 6sigma detection. Regardless of the reality of this tentative detection, model calculations indicate that our sensitive limits on the line strengths preclude efficient desorption of water in the UV illuminated regions of the disk. We hypothesize that more than 95-99% of the water ice is locked up in coagulated grains that have settled to the midplane.