Context. This paper discusses the sensitivity of water lines to chemical processes and radiative transfer for the protoplanetary disk around TW Hya. The study focuses on the Herschel spectral range in the context of new line detections with the PACS
instrument from the Gas in Protoplanetary Systems project (GASPS). Aims. The paper presents an overview of the chemistry in the main water reservoirs in the disk around TW Hya. It discusses the limitations in the interpretation of observed water line fluxes. Methods. ... (abbreviated) Results. We report new line detections of p-H2O (3_22-2_11) at 89.99 micron and CO J=18-17 at 144.78 micron for the disk around TW Hya. Disk modeling shows that the far-IR fine structure lines ([OI], [CII]) and molecular submm lines are very robust to uncertainties in the chemistry, while the water line fluxes can change by factors of a few. The water lines are optically thick, sub-thermally excited and can couple to the background continuum radiation field. The low-excitation water lines are also sensitive to uncertainties in the collision rates, e.g. with neutral hydrogen. The gas temperature plays an important role for the [OI] fine structure line fluxes, the water line fluxes originating from the inner disk as well as the high excitation CO, CH+ and OH lines. Conclusions. Due to their sensitivity on chemical input data and radiative transfer, water lines have to be used cautiously for understanding details of the disk structure. Water lines covering a wide range of excitation energies provide access to the various gas phase water reservoirs (inside and outside the snow line) in protoplanetary disks and thus provide important information on where gas-phase water is potentially located. Experimental and/or theoretical collision rates for H2O with atomic hydrogen are needed to diminish uncertainties from water line radiative transfer.
Circumstellar disc evolution is paramount for the understanding of planet formation. The GASPS program aims at determining the circumstellar gas and solid mass around ~250 pre-main-sequence Herbig Ae and TTauri stars. We aim to understand the origin
and nature of the circumstellar matter orbiting 51 Oph, a young (<1 Myr) luminous B9.5 star. We obtained continuum and line observations with the PACS instrument on board the Herschel Space Observatory and continuum data at 1.2 mm with the IRAM 30m telescope. The SED and line fluxes were modelled using the physico-chemo radiative transfer code ProDiMo. We detected a strong emission by OI at 63 microns using the Herschel Space Observatory. The [OI] emission at 145 microns, the [CII] emission at 158 microns, the high-J CO emissions, and the warm water emissions were not detected. Continuum emission was detected at 1.2 mm. The continuum from the near- to the far-infrared and the [OI] emission are well explained by the emission from a compact hydrostatic disc model with a gas mass of 5E-6 MSun, 100 times that of the solid mass. However, this model fails to match the continuum millimeter flux, which hints at a cold outer disc with a mass in solids of 1E-6 MSun or free-free emission from a photoevaporative disc wind. This outer disc can either be devoid of gas and/or is to cold to emit in the [OI] line. A very flat extended disc model (Rout=400 AU) with a fixed vertical structure and dust settling matches all photometric points and most of the [O I] flux. The observations can be explained by an extended flat disc where dust grains have settled. However, a flat gas disc cannot be reproduced by hydrostatic disc models. The low mass of the 51 Oph inner disc in gas and dust may be explained either by the fast dissipation of an initial massive disc or by a very small initial disc mass.
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 o
f 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.
