MINDS: The influence of outer dust disc structure on the volatile delivery to the inner disc

Abstract

Context. The Atacama Large Millimeter/submillimeter Array (ALMA) has revealed that the millimetre dust structures of protoplanetary discs are extremely diverse, ranging from small and compact dust discs to large discs with multiple rings and gaps. It has been proposed that the strength of H₂O emission in the inner disc particularly depends on the influx of icy pebbles from the outer disc, a process that would correlate with the outer dust disc radius, and that could be prevented by pressure bumps. Additionally, the dust disc structure should also influence the emission of other gas species in the inner disc. Since terrestrial planets likely form in the inner disc regions, understanding their composition is of interest. Aims. This work aims to assess the influence of pressure bumps on the inner disc's molecular reservoirs. The presence of a dust gap, and potentially giant planet formation farther out in the disc, may influence the composition of the inner disc, and thus the building blocks of terrestrial planets. Methods. Using the improved sensitivity and spectral resolution of the Mid-InfraRed Instrument's (MIRI) Medium Resolution Spectrometer (MRS) on the James Webb Space Telescope (JWST) compared to Spitzer, we compared the observational emission properties of H₂O, HCN, C₂H₂, and CO₂ with the outer dust disc structure from ALMA observations, in eight discs with confirmed gaps in ALMA observations, and two discs with gaps of tens of astronomical units in width, around stars with M_⋆ ≥ 0.45 M_⊙ . We used new visibility plane fits of the ALMA data to determine the outer dust disc radius and identify substructures in the discs. Results. We find that the presence of a dust gap does not necessarily result in weak H₂O emission. Furthermore, the relative lack of colder H₂O-emission seems to go hand in hand with elevated emission from carbon-bearing species. Of the discs that show significant substructure within the CO and CH₄ snowlines, most show detectable emission from the carbon-bearing species. The discs with cavities and extremely wide gaps appear to behave as a somewhat separate group, with stronger cold H₂O emission and weak warm H₂O emission. Conclusions. We conclude that fully blocking radial dust drift from the outer disc seems difficult to achieve, even for discs with very wide gaps or cavities, which can still show significant cold H₂O emission. However, there does seem to be a dichotomy between discs that show a strong cold H₂O excess and ones that show strong emission from HCN and C₂H₂. Better constraints on the influence of the outer dust disc structure and inner disc composition require more information on substructure formation timescales and disc ages, along with the importance of trapping of (hyper)volatiles like CO and CO₂ into more strongly bound ices like H₂O and chemical transformation of CO into less volatile species.