Hints for icy pebble migration feeding an oxygen-rich chemistry in the inner planet-forming region of disks

TL;DR

This study links the migration of icy pebbles in protoplanetary disks to the chemical composition of inner disk gas, revealing that pebble drift influences molecular abundances and disk chemistry.

Contribution

It provides observational evidence connecting large-scale pebble migration with inner disk chemistry, highlighting the role of icy pebbles in feeding oxygen-rich molecules.

Findings

H2O line luminosity anti-correlates with dust disk radius.

Molecular luminosities are anti-correlated with the infrared index n13-30.

Inner disk chemistry is likely fed by sublimation of migrating icy pebbles.

Abstract

We present a synergic study of protoplanetary disks to investigate links between inner disk gas molecules and the large-scale migration of solid pebbles. The sample includes 63 disks where two types of measurements are available: i) spatially-resolved disk images revealing the radial distribution of disk pebbles (mm-cm dust grains), from millimeter observations with ALMA or the SMA, and ii) infrared molecular emission spectra as observed with Spitzer. The line flux ratios of H2O with HCN, C2H2, and CO2 all anti-correlate with the dust disk radius R$_{dust}$, expanding previous results found by Najita et al. (2013) for HCN/H2O and the dust disk mass. By normalization with the dependence on accretion luminosity common to all molecules, only the H2O luminosity maintains a detectable anti-correlation with disk radius, suggesting that the strongest underlying relation is between H2O and R$_{dust}$. If R$_{dust}$ is set by large-scale pebble drift, and if molecular luminosities trace the elemental budgets of inner disk warm gas, these results can be naturally explained with scenarios where the inner disk chemistry is fed by sublimation of oxygen-rich icy pebbles migrating inward from the outer disk. Anti-correlations are also detected between all molecular luminosities and the infrared index n$_{13-30}$, which is sensitive to the presence and size of an inner disk dust cavity. Overall, these relations suggest a physical interconnection between dust and gas evolution both locally and across disk scales. We discuss fundamental predictions to test this interpretation and study the interplay between pebble drift, inner disk depletion, and the chemistry of planet-forming material.