Linking Outer Disk Pebble Dynamics and Gaps to Inner Disk Water Enrichment

TL;DR

This study models how gaps in outer protoplanetary disks influence the inward drift of icy pebbles and the resulting water vapor enrichment in the inner disk, linking disk structures to chemical composition.

Contribution

It introduces a volatile-inclusive disk evolution model to identify how outer disk gaps affect inner disk water enrichment, highlighting the importance of gap location and pebble size.

Findings

Inner disk water vapor enrichment is highly sensitive to gap location.

Pebbles of 1-10 mm size have a 'sweet spot' at 7-15 au minimizing vapor enrichment.

Inner water vapor abundance can serve as a proxy for pebble drift efficiency.

Abstract

Millimeter continuum imaging of protoplanetary disks reveals the distribution of solid particles and the presence of substructures (gaps and rings) beyond 5-10 au, while infrared (IR) spectra provide access to abundances of gaseous species at smaller disk radii. Building on recent observational findings of an anti-correlation between the inner disk water luminosity and outer dust disk radius, we aim here at investigating the dynamics of icy solids that drift from the outer disk and sublimate their ice inside the snow line, enriching the water vapor that is observed in the IR. We use a volatile-inclusive disk evolution model to explore a range of conditions (gap location, particle size, disk mass, and alpha-viscosity) under which gaps in the outer disk efficiently block the inward drift of icy solids. We find that inner-disk vapor enrichment is highly sensitive to the location of a disk gap, yielding for each particle size a radial "sweet spot" that reduces the inner-disk vapor enrichment to a minimum. For pebbles of 1-10 mm in size, which carry the most mass, this sweet spot is at 7-15 au, suggesting that inner gaps may have a key role in reducing ice delivery to the inner disk and may not allow the formation of Earths and super-Earths. This highlights the importance of observationally determining the presence and properties of inner gaps in disks. Finally, we argue that the inner water vapor abundance can be used as a proxy for estimating the pebble drift efficiency and mass-flux entering the inner disk.