MINDS: Water reservoirs of compact planet-forming dust discs

A diversity of H₂O distributions

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
² Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
³ Max-Planck-Institut für Extraterrestrische Physik, Giessenbach-straße 1, 85748 Garching, Germany
⁴ Dept. of Astrophysics, University of Vienna, Türkenschanzstr. 17, 1180 Vienna, Austria
⁵ ETH Zürich, Institute for Particle Physics and Astrophysics, Wolfgang-Pauli-Str. 27, 8093 Zürich, Switzerland
⁶ Max-Planck-Institut für Astronomie (MPIA), Königstuhl 17, 69117 Heidelberg, Germany
⁷ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
⁸ INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
⁹ Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, D02 XF86 Dublin, Ireland
¹⁰ Kapteyn Astronomical Institute, Rijksuniversiteit Groningen, Postbus 800, 9700AV Groningen, The Netherlands
¹¹ Department of Astronomy, Stockholm University, AlbaNova University Center, 10691 Stockholm, Sweden
¹² Earth and Planets Laboratory, Carnegie Institution for Science, 5241 Broad Branch Road, NW, Washington, DC 20015, USA
¹³ Department of Physics and Astronomy, University of Exeter, Exeter EX4 4QL, UK
¹⁴ Niels Bohr Institute, University of Copenhagen, NBB BA2, Jagtvej 155A, 2200 Copenhagen, Denmark
¹⁵ Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale, 91405 Orsay, France

^★ Corresponding author: temmink@strw.leidenuniv.nl

Received: 21 February 2025
Accepted: 20 May 2025

Abstract

Context. 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.