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

TL;DR

This study investigates how outer dust disc structures, especially gaps and pressure bumps, influence volatile molecule emissions in the inner regions of protoplanetary discs, using JWST and ALMA observations.

Contribution

It provides new observational evidence on the relationship between outer dust structures and inner disc molecular composition, highlighting the complexity of volatile delivery mechanisms.

Findings

Presence of dust gaps does not always weaken H2O emission.

Discs with wide gaps show stronger cold H2O and weaker warm H2O emissions.

A dichotomy exists between discs with cold H2O excess and those with strong HCN and C2H2 emissions.

Abstract

ALMA has revealed that the millimetre dust structures of protoplanetary discs are extremely diverse. It has been proposed that the strength of H$_2$O 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. This work aims to assess the influence of pressure bumps on the inner disc's molecular reservoirs. Using JWST's MIRI/MRS, we compared the observational emission properties of H$_2$O, HCN, C$_2$H$_2$, and CO$_2$ 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 $M_\star \geq 0.45M_{\odot}$. We used new visibility plane fits of the ALMA data to determine the outer dust disc radius and identify substructures in the discs. We find that the presence of a dust gap does not necessarily result in weak H$_2$O emission. Furthermore, the relative lack of colder H$_2$O-emission seems to go hand in hand with elevated emission from carbon-bearing species. The discs with cavities and extremely wide gaps appear to behave as a somewhat separate group, with stronger cold H$_2$O emission and weak warm H$_2$O emission. We conclude that fully blocking radial dust drift from the outer disc seems difficult to achieve. However, there does seem to be a dichotomy between discs that show a strong cold H$_2$O excess and ones that show strong emission from HCN and C$_2$H$_2$. 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 volatiles like CO and CO$_2$ into more strongly bound ices like H$_2$O and chemical transformation of CO into less volatile species.