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 3 \sigma levels in 0.5 km s ^ { -1 } 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 ^ { -1 } and an integrated intensity of 16.0 mK km s ^ { -1 } .
The latter constitutes a 6 \sigma 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 .