H$_2$O emission as tracer of pebble drift: insights from coupling transport and thermochemical models

RESUMO

(Abridged) The composition of the inner regions of protoplanetary disks is known to change with time due to the delivery of icy grains. Cold H$_2$O emission seen with JWST-MIRI is often hypothesized to be a tracer of this pebble drift. However, it is unclear to what extent processes such as photodissociation or the co-delivery of dust may impede such detections. We aim to obtain an improved, 2D view of transport in disks, to better understand how these processes can be traced by H$_2$O emission as seen with JWST-MIRI. We combine the 1D transport code DiscEvolution with the 2D thermochemical code DALI to create several grids of models in which the gas-phase abundances, dust properties, or both are varied according to the transport model. We consider scenarios with and without a traffic jam inside the H$_2$O snowline. The transport of both gas and dust leads to significant temperature changes within the disk, which strongly influence line fluxes and ratios. When a traffic jam is present, the delivery of H$_2$O can proceed unnoticed due to the co-delivery of dust. In addition, the relative strength of cold H$_2$O lines is not found to be sensitive to this delivery of gas-phase H$_2$O. Instead, the cold H$_2$O lines can be greatly enhanced by the delivery of only dust to the inner disk when a traffic jam is present, and one can create a spectrum with strong cold H$_2$O emission solely through the delivery of dust rather than H$_2$O. Recent work has used the 1500/6000 K H$_2$O line ratio as a proxy to determine the pebble mass flux crossing the H$_2$O snowline, but we find that this line ratio is rather sensitive to the temperature and dust distribution of the disk, and is often influenced not only by the cold H$_2$O mass, but also by the hot H$_2$O mass. This introduces complexities and trends as a function of time that do not match the true evolution of the pebble flux.