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

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 <i>Herschel<i/>/HIFI instrument. No strong lines are detected down to 3<i>σ<i/> levels in 0.5 km s<sup>-1<sup/> channels of 4.2 mK for the 1<sub>10<sub/>–1<sub>01<sub/> line and 12.6 mK for the 1<sub>11<sub/>–0<sub>00<sub/> line. We report a very tentative detection, however, of the 1<sub>10<sub/>–1<sub>01<sub/> line in the wide band spectrometer, with a strength of <i>T<i/><sub>mb<sub/> = 2.7 mK, a width of 5.6 km s<sup>-1<sup/> and an integrated intensity of 16.0 mK km s<sup>-1<sup/>. The latter constitutes a 6<i>σ<i/> 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.