Tracing Pebble Drift History in Two Protoplanetary Disks with CO Enhancement

TL;DR

This study addresses how pebble drift shapes the inner regions of protoplanetary disks by constraining the cumulative pebble flux through CO enhancements interior to snowlines. It combines spatially resolved ALMA observations of $^{13}$C$^{18}$O J=2-1 in two disks with a unified radiative-transfer and thermochemical modeling framework to recover the radial CO distribution and infer drift histories. The analysis finds centrally peaked CO enhancements up to about 10 times the ISM abundance and infers substantial pebble fluxes (HD 163296: ~250–350 M$_\oplus$, MWC 480: ~480–660 M$_\oplus$) crossing the CO snowline, challenging standard drift expectations that predict peaks at or interior to the snowline. Testing mechanisms via a 1D chemcomp framework suggests volatile trapping (≈30%) best reproduces the observed central CO enhancement, offering insights into CO delivery, ice trapping, and resultant implications for planet-formation pathways.

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.

Figures (1)

• Figure 1: This figure details $^{13}$C$^{18}$O observations of both the HD 163296 (Top) and MWC 480 protoplanetary disks (bottom). The first column depicts the moment zero map, the middle shows the moment one map, and the radial profile of the integrated spectra in the final column. In the center column we include dashed lines to indicate the major and minor axes of each disk. Shown in blue are the midplane CO snowlines at 60 au for the HD 163296 disk and 100 au for the MWC 480 disk. These values were derived by Zhang_2021.