[Paper Review] Molecules with ALMA at Planet-forming Scales (MAPS) III: Characteristics of Radial Chemical Substructures

Charles J. Law, Ryan A. Loomis+33 more|arXiv (Cornell University)|Sep 13, 2021

Astrophysics and Star Formation Studies192 references112 citations

TL;DR

This study presents a systematic analysis of radial chemical substructures in 18 molecular lines across five protoplanetary disks using high-resolution ALMA data from the MAPS Large Program. It identifies over 200 substructures with diverse morphologies (rings, gaps, plateaus) across all observed radii, revealing that chemical and dust substructures are spatially coincident within 150 au but that outer and inner disk substructures often lack direct dust associations, indicating additional drivers like snowlines and radiation fields.

ABSTRACT

The Molecules with ALMA at Planet-forming Scales (MAPS) Large Program provides a detailed, high resolution (${\sim}$10-20 au) view of molecular line emission in five protoplanetary disks at spatial scales relevant for planet formation. Here, we present a systematic analysis of chemical substructures in 18 molecular lines toward the MAPS sources: IM Lup, GM Aur, AS 209, HD 163296, and MWC 480. We identify more than 200 chemical substructures, which are found at nearly all radii where line emission is detected. A wide diversity of radial morphologies - including rings, gaps, and plateaus - is observed both within each disk and across the MAPS sample. This diversity in line emission profiles is also present in the innermost 50 au. Overall, this suggests that planets form in varied chemical environments both across disks and at different radii within the same disk. Interior to 150 au, the majority of chemical substructures across the MAPS disks are spatially coincident with substructures in the millimeter continuum, indicative of physical and chemical links between the disk midplane and warm, elevated molecular emission layers. Some chemical substructures in the inner disk and most chemical substructures exterior to 150 au cannot be directly linked to dust substructure, however, which indicates that there are also other causes of chemical substructures, such as snowlines, gradients in UV photon fluxes, ionization, and radially-varying elemental ratios. This implies that chemical substructures could be developed into powerful probes of different disk characteristics, in addition to influencing the environments within which planets assemble. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.

Motivation & Objective

• To characterize the radial distribution and morphological diversity of molecular chemical substructures in protoplanetary disks at planet-forming scales.
• To investigate the spatial relationship between chemical substructures and millimeter continuum dust substructures across multiple disks.
• To determine the physical mechanisms driving chemical substructures beyond dust morphology, including snowlines, radiation fields, and elemental gradients.
• To assess the role of chemical substructures as probes of disk conditions and planet formation environments.

Proposed method

• High angular resolution (10–20 au) ALMA observations of 18 molecular lines toward five protoplanetary disks: IM Lup, GM Aur, AS 209, HD 163296, and MWC 480.
• Systematic visual and profile-based identification of radial chemical substructures (rings, gaps, plateaus, shoulders, asymmetries) in molecular line emission profiles.
• Comparison of chemical substructure locations with known millimeter continuum dust substructures from prior DSHARP and MAPS observations.
• Use of radial profile fitting and visual inspection to determine substructure radii, widths, and depths, with uncertainty estimates based on fitting procedures.
• Classification of substructures into categories (e.g., plateau, shoulder, radial asymmetry) based on radial profile morphology.
• Correlation analysis between chemical substructures and dust substructures to assess spatial coincidence and potential physical links.

Experimental results

Research questions

• RQ1 What types of radial chemical substructures (e.g., rings, gaps, plateaus) are present in the inner and outer regions of protoplanetary disks at 10–20 au resolution?
• RQ2 To what extent are chemical substructures spatially coincident with dust substructures in the millimeter continuum within 150 au?
• RQ3 What physical mechanisms—beyond dust morphology—can explain chemical substructures in regions where dust and chemistry do not align?
• RQ4 How do chemical substructures in the inner disk (<50 au) compare to those in the outer disk (>150 au) in terms of morphology and origin?
• RQ5 Can chemical substructures serve as independent probes of disk conditions such as snowlines, UV flux gradients, and elemental ratios?

Key findings

• More than 200 radial chemical substructures were identified across the five MAPS disks, spanning nearly all radii where molecular line emission is detected.
• A wide diversity of radial morphologies—including rings, gaps, plateaus, shoulders, and radial asymmetries—was observed both within individual disks and across the sample.
• Within 150 au, the majority of chemical substructures are spatially coincident with millimeter continuum dust substructures, indicating a strong physical and chemical link between the disk midplane and warm molecular layers.
• Chemical substructures in the inner disk (<50 au) and most substructures beyond 150 au show no direct correspondence with dust substructures, suggesting alternative drivers such as snowlines, UV radiation gradients, ionization, and radial variations in elemental abundances.
• Tentative substructures were identified in 14 lines across the sample, including plateaus, shoulders, and asymmetric features, with radial locations estimated visually and reported in mas and au.
• The radial locations of chemical substructures show complex spatial relationships with NIR rings and continuum disk edges, particularly in IM Lup, AS 209, and HD 163296, with some substructures extending beyond the outermost NIR rings.