Smuggling unnoticed: A 2D view of water and dust delivery to the inner regions of protoplanetary discs

Infrared spectroscopy, e.g., with JWST, provides a glimpse into the chemical inventory of the inner- most region of protoplanetary discs, where terrestrial planets eventually form. It is, however, limited by the high dust extinction, and may only characterise abundances of gaseous species in the upper layers of the disc atmosphere. In the paradigm of pebble drift, the chemical make-up of regions inside major snowlines appears deeply connected to the material drifting from the outer regions. This opens new opportunities to characterise the inventory of the inner disc using dust evolution models, though the bulk mass of delivered volatiles is not directly relatable to what may be measured through infrared spectra. In this paper, we investigate how the delivery of dust and ice after prolonged pebble drift affects the observable column density of water vapor in the inner disc. We develop a 2D approach based on dust evolution models to determine the delivery and distribution of water vapor compared to the height of the τ = 1 surface in the dust continuum. We find that the observable column density of water vapor at wavelengths probed by JWST spans many orders of magnitude over time, and ex- hibits different radial profiles depending on dust properties, drift rate, and dispersal timescale. Only a fraction of the total vapor content is observable above the τ = 1 surface. In the presence of an accumulation of dust, i.e., traffic-jam, at the snowline, the fraction of observable vapor goes down to < 0.001%, and the observable reservoir actually decreases in time despite the ongoing water delivery. As a consequence, the current water abundances observed with JWST may not only originate from the bulk delivery of volatiles, but also from the properties and distribution of dust particles.