Tracing Pebble Drift History in Two Protoplanetary Disks with CO Enhancement

TL;DR

This study uses ALMA observations and modeling to analyze CO gas enhancements in two protoplanetary disks, revealing unexpected central peaks that suggest complex pebble drift and volatile trapping processes relevant to planet formation.

Contribution

It provides the first spatially resolved measurements of CO enhancement inside snowlines and proposes volatile trapping as a key mechanism, challenging existing pebble drift models.

Findings

Both disks show centrally peaked CO enhancement up to ten times ISM levels.

Estimated pebble masses drifting across snowlines are sufficient for gas giant formation.

Volatile trapping explains the observed CO enhancement better than other hypotheses.

Abstract

Pebble drift is an important mechanism for supplying the materials needed to build planets in the inner region of protoplanetary disks. Thus, constraining pebble drift's timescales and mass flux is essential to understanding planet formation history. Current pebble drift models suggest pebble fluxes can be constrained from the enhancement of gaseous volatile abundances when icy pebbles sublimate after drifting across key snowlines. In this work, we present ALMA observations of spatially resolved $^{13}$C$^{18}$O J=2-1 line emission inside the midplane CO snowline of the HD 163296 and MWC 480 protoplanetary disks. We use radiative transfer and thermochemical models to constrain the spatial distribution of CO gas column density. We find that both disks display centrally peaked CO abundance enhancement of up to ten times of ISM abundance levels. For HD 163296 and MWC 480, the inferred enhancements require 250-350 and 480-660 Earth Masses of pebbles to have drifted across their CO snowlines, respectively. These ranges fall within cumulative pebble mass flux ranges to grow gas giants in the interior to the CO snowline. The centrally peaked CO enhancement is unexpected in current pebble drift models, which predict CO enhancement peaks at the CO snowline or is uniform inside the snowline. We propose two hypotheses to explain the centrally-peaked CO enhancement, including a large CO desorption distance and CO trapped in water ice. By testing both hypotheses with the 1D gas and dust evolution code chemcomp, we find that volatile trapping (about 30\%) best reproduces the centrally peaked CO enhancement observed.