[Paper Review] Molecules with ALMA at Planet-forming Scales (MAPS). XV. Tracing protoplanetary disk structure within 20 au

Arthur D. Bosman, Edwin A. Bergin+26 more|arXiv (Cornell University)|Sep 13, 2021

Astrophysics and Star Formation Studies102 references39 citations

TL;DR

This study uses high-resolution ALMA observations from the MAPS program to trace gas and dust structure in the inner 20 au of protoplanetary disks via CO isotopologue line wings, leveraging Keplerian kinematics to achieve ~3 au effective resolution. It reveals dust pileups with gas-to-dust ratios <10 in IM Lup and GM Aur, indicating efficient grain drift and potential streaming instability, while radial surface brightness profiles show gas-rich gaps and low-gas-rings correlated with dust structures in AS 209 and HD 163296.

ABSTRACT

Constraining the distribution of gas and dust in the inner 20 au of protoplanetary disks is difficult. At the same time, this region is thought to be responsible for most planet formation, especially around the water ice line at 3-10 au. Under the assumption that the gas is in a Keplerian disk, we use the exquisite sensitivity of the Molecules with ALMA at Planet-forming Scales (MAPS) ALMA large program to construct radial surface brightness profiles with a ~3 au effective resolution for the CO isotopologue J=2-1 lines using the line velocity profile. IM Lup reveals a central depression in 13CO and C18O that is ascribed to a pileup of ~500 $M_\oplus$ of dust in the inner 20 au, leading to a gas-to-dust ratio of around <10. This pileup is consistent with efficient drift of grains ($\gtrsim$ 100 $M_\oplus$ Myr$^{-1}$) and a local gas-to-dust ratio that suggests that the streaming instability could be active. The CO isotopologue emission in the GM Aur disk is consistent with a small (~15 au), strongly depleted gas cavity within the ~40 au dust cavity. The radial surface brightness profiles for both the AS 209 and HD 163296 disks show a local minimum and maximum in the C18O emission at the location of a known dust ring (~14 au) and gap (~10 au), respectively. This indicates that the dust ring has a low gas-to-dust ratio ($>$ 10) and that the dust gap is gas-rich enough to have optically thick C18O.

Motivation & Objective

• To map the radial distribution of gas and dust in the inner 20 au of protoplanetary disks, a region critical for planet formation but observationally challenging.
• To overcome the resolution and sensitivity limitations of standard imaging by using high-resolution spectral line kinematics to infer surface brightness profiles.
• To determine the gas-to-dust ratio and radial drift efficiency in the inner disk by analyzing CO isotopologue emission profiles.
• To investigate the connection between dust structures (rings and gaps) and underlying gas morphology in multiple disks.
• To assess the potential for streaming instability and pebble accretion in regions of high dust concentration and low gas-to-dust ratios.

Proposed method

• Utilizes high spectral resolution (R > 10^6) ALMA data from the MAPS large program to extract line profiles of CO isotopologues (13CO J=2–1, C18O J=2–1).
• Applies Keplerian rotation modeling to decompose the line-of-sight velocity structure into radial surface brightness profiles.
• Constructs radial surface brightness profiles with an effective resolution of ~3 au by analyzing the wings of the CO line profiles.
• Compares observed CO emission features (peaks, dips, holes) with known dust continuum structures from ALMA imaging.
• Estimates dust mass and gas-to-dust ratios by modeling the central depression in 13CO and C18O emission as a dust pileup.
• Uses kinematic modeling to infer radial drift rates and assess conditions favorable for streaming instability (e.g., low gas-to-dust ratio, high dust mass).

Experimental results

Research questions

• RQ1 What is the radial distribution of gas and dust in the inner 20 au of protoplanetary disks, and how does it relate to planet formation zones?
• RQ2 Can CO isotopologue line wings reveal substructure (gaps, rings, cavities) that is unresolved in standard CLEANed images?
• RQ3 What is the gas-to-dust ratio in the inner disk, and does it support the operation of the streaming instability?
• RQ4 How do dust structures (e.g., rings and gaps) correlate with the underlying gas distribution in disks like AS 209 and HD 163296?
• RQ5 What causes the observed central depression in CO emission in IM Lup and GM Aur, and what does it imply about dust evolution?

Key findings

• The radial surface brightness profiles derived from CO isotopologue line wings reveal previously unresolved features such as gaps, peaks, and central depressions in the gas emission across multiple disks.
• In IM Lup, a central depression in 13CO and C18O emission at 20–30 au indicates a dust pileup of ~500 M⊕ within 20 au, implying a gas-to-dust ratio of <10 and radial drift rates >100 M⊕ Myr⁻¹.
• In GM Aur, a central hole in 12CO, 13CO, and C18O emission is consistent with a gas cavity within the known dust cavity, with 12CO remaining optically thin within 15 au.
• In AS 209 and HD 163296, the innermost dust ring correlates with a dip in C18O surface brightness, while the gap inward of the ring shows enhanced C18O emission, indicating a low gas-to-dust ratio in the ring and optically thick gas in the gap.
• The CO isotopologue emission in MWC 480 shows a strong, unexplained variation in 13CO and C18O surface brightness at 10 au, suggesting a complex local structure not yet explained.
• The J=1–0 and J=2–1 lines of 13CO and C18O show consistent wing emission, indicating that the lower-frequency lines do not probe deeper into the disk midplane than the higher-frequency lines.