Molecules with ALMA at Planet-forming Scales (MAPS). III. Characteristics of Radial Chemical Substructures

Abstract

The Molecules with ALMA at Planet-forming Scales (MAPS) Large Program provides a detailed, high-resolution (∼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.

No Significant Correlation between Line-emission and Continuum Substructures in the Molecules with ALMA at Planet-forming Scales Program

“…This power-law approximately matches the distribution of the radial locations of substructure shown in Fig. 17 of Law et al (2021). We have also varied the slope between 0.5 and 1.5, which did not affect our conclusions.…”

“…These rings and gaps are found at nearly all radii in the observed disks. Many of these line emission substructures coincide with the dust rings and gaps in the continuum emission, suggesting possible spatial associations between chemical lines and dust continuum (Law et al 2021). However, statistical tests are needed in order to robustly establish the physical nature of such associations.…”

“…We generally follow the approach of Law et al (2021) in quantifying the correlation between line emission and continuum substructures. We define an overlap as when the radial location of the line emission substructure, r 0,chem 1 , falls within the width of the annular continuum substructures…”

“…The MAPS program surveyed approximately 50 molecular lines from 20 different chemical species around five sources-IM Lup, GM Aur, AS 209, HD 163296, and MWC 480-where dust substructures have been detected in their continuum counterparts (Long et al 2018;Andrews et al 2018;Huang et al 2020). In total, more than 200 chemical substructures corresponding to certain molecular lines are identified at high spatial resolutions (7-30 au) (Law et al 2021). Almost all of these line emission substructures show signs of an axisymmetric structure (Law et al 2021).…”

“…In total, more than 200 chemical substructures corresponding to certain molecular lines are identified at high spatial resolutions (7-30 au) (Law et al 2021). Almost all of these line emission substructures show signs of an axisymmetric structure (Law et al 2021). In other words, rings and gaps are also the most abundant morphology in line emission substructures.…”