Abstract:

Millimetre-compact dust discs are thought to have efficient radial drift of icy dust pebbles. It has been hypothesised that this drift could produce an enhanced cold (T < 400 K) H₂O reservoir in their inner discs. Mid-infrared spectral surveys, now including the James Webb Space Telescope (JWST), pave the way to explore this hypothesis. In this work, we test this theory for eight compact discs (R_dust < 60 au) with JWST-MIRI/MRS observations.

Aims. To explore the H₂O distribution in the inner discs and consider whether these discs are enhanced in cold H₂O emission, we analyse the different reservoirs that can be probed with the pure rotational lines (>10 µm) by JWST: hot (T > 800 K), intermediate (400 < T < 800 K), and cold (T < 400 K).

Methods. We probed the H₂O reservoirs with JWST-MIRI observations for a sample of eight compact discs through parametric column density profiles (power laws, jump abundances, and parabolas), multiple-component (two or three) slab models, and line flux ratios.

Results. We find that not all compact discs show strong enhancements of the cold H₂O reservoir; instead, we propose three different classes of inner disc H₂O distributions. Four of our discs (BP Tau, CY Tau, DR Tau, and RNO 90; i.e. type N or ‘normal’ discs) appear to have similar H₂O distributions to many of the large and structured discs, as indicated by the slab model fitting and the line flux ratios. These discs have a small cold reservoir, suggesting the inward drift of dust, but it is not as efficient as hypothesised before. Only two discs (FT Tau and XX Cha; type E or cold H₂O enhanced discs) do show a strong enhancement of the cold H₂O emission, in agreement with the original hypothesis. The two remaining discs (CX Tau and DN Tau; type P or H₂O-poor discs) are found to be very H₂O-poor, yet they show emission from either the hot or immediate reservoirs (depending on the fit) in addition to emission from the cold one. For the three types, we find that different parametrisation schemes are able to provide a good description of the observed H₂O spectra. Overall, a jump abundance at a free temperature is amongst the preferred profiles for all three types, suggesting that this profile can provide a good description of the observed reservoirs for most discs. The multiple-component analysis yields similar results to those of the parametric models. However, in some cases, a power law can give an entirely different distribution compared to the other parametric models. Finally, we also report the detection of other molecules in these discs, including a tentative detection of CH₄ in CY Tau.

Conclusions. Not all compact discs follow the hypothesis that their cold H₂O reservoir is enhanced following efficient radial drift. Therefore, we introduced a classification based on the observed H₂O reservoirs, which should hold for all (isolated) discs: type N, type E, and type P. Type N discs are considered to behave as many other (large and structured) discs, with all three reservoirs present; yet the cold emission is not enhanced. The type E discs show strong enhancements of the cold H₂O emission, while the type P discs are generally H₂O-poor.

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