Detection of warm water vapour in Taurus protoplanetary discs by Herschel

Line spectra of 68 Taurus T Tauri stars were obtained with the Herschel-PACS (Photodetector Array Camera and Spectrometer) instrument as part of the GASPS (GAS evolution in Protoplanetary Systems) survey of protoplanetary discs. A careful examination of the linescans centred on the [OI] 63.18 mu m fine-structure line unveiled a line at 63.32 mu m in some of these spectra. We identify this line with the 8(18) -> 7(07) transition of ortho-water. It is detected confidently (i.e., > 3 sigma) in eight sources, i.e., similar to 24% of the sub-sample with gas-rich discs. Several statistical tests were used to search for correlations with other disc and stellar parameters such as line fluxes of [OI] 6300 angstrom and 63.18 mu m; X-ray luminosity and continuum levels at 63 mu m and 850 mu m. Correlations are found between the water line fluxes and the [OI] 63.18 mu m line luminosity, the dust continuum, and possibly with the stellar X-ray luminosity. This is the first time that this line of warm water vapour has been detected in protoplanetary discs. We discuss its origins, in particular whether it comes from the inner disc and/or disc surface or from shocks in outflows and jets. Our analysis favours a disc origin, with the observed water vapour line produced within 2-3 AU from the central stars, where the gas temperature is of the order of 500-600 K.