Detection of warm water vapour in Taurus protoplanetary discs by Herschel

TL;DR

This study reports the first detection of warm water vapour in protoplanetary discs around Taurus T Tauri stars using Herschel, revealing its potential origin and correlations with other disc and stellar parameters.

Contribution

It provides the first evidence of warm water vapour in protoplanetary discs and analyzes its likely origin within 2-3AU from the central star.

Findings

Warm water vapour detected in 24% of gas-rich discs.

Water line flux correlates with [OI] 63.18 microns and dust continuum.

Water likely originates within 2-3AU from the star.

Abstract

Line spectra of 68 Taurus T Tauri stars were obtained with the Herschel-PACS (Photodetector Array Camera & Spectrometer) instrument as part of the GASPS (Gas Evolution in Protoplanetary Systems) survey of protoplanetary discs. A careful examination of the line scans centred on the [OI] 63.18 microns fine-structure line unveiled a line at 63.32 micron in some of these spectra. We identify this line with a transition of ortho-water. It is detected confidently (i.e., >3 sigma) in eight sources, i.e., 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 Armstrong and 63.18 microns; X-ray luminosity and continuum levels at 63 microns and 850 microns. Correlations are found between the water line fluxes and the [OI] 63.18 microns 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 2-3AU from the central stars, where the gas temperature is of the order of 500-600 K.