If you like C/O variations, you should have put a ring on it

Abstract

Context. The C/O ratio as traced with C2H emission in protoplanetary disks is fundamental for constraining the formation mechanisms of exoplanets and for our understanding of volatile depletion in disks, but current C2H observations show an apparent bimodal distribution that is not well understood, indicating that the C/O distribution is not described by a simple radial dependence. Aims. The transport of icy pebbles has been suggested to alter the local elemental abundances in protoplanetary disks through settling, drift, and trapping in pressure bumps, resulting in a depletion of volatiles in the surface layer and an increase in the elemental C/O. Methods. We combine all disks with spatially resolved ALMA C2H observations with high-resolution continuum images and constraints on the CO snow line to determine if the C2H emission is indeed related to the location of the icy pebbles. Results. We report a possible correlation between the presence of a significant CO-ice dust reservoir and high C2H emission, which is only found in disks with dust rings outside the CO snow line. In contrast, compact dust disks (without pressure bumps) and warm transition disks (with their dust ring inside the CO snow line) are not detected in C2H, suggesting that such disks may have never contained a significant CO ice reservoir. Conclusions. This correlation provides evidence for the regulation of the C/O profile by the complex interplay of CO snow line and pressure bump locations in the disk. These results demonstrate the importance of including dust transport in chemical disk models for a proper interpretation of exoplanet atmospheric compositions and a better understanding of volatile depletion in disks, in particular the use of CO isotopologs to determine gas surface densities.

High-resolution atmospheric retrievals of WASP-121b transmission spectroscopy with ESPRESSO: Consistent relative abundance constraints across multiple epochs and instruments

“…Developing robust techniques to constrain relative elemental abundances from high-resolution ground-based observations (Gandhi et al 2019;Line et al 2021;Pelletier et al 2021;Gibson et al 2022), such as carbon-to-oxygen (C/O) or refractory-to-volatile ratios, allows us to infer their formation and evolution mechanisms (Öberg et al 2011;Madhusudhan et al 2014;Mordasini et al 2016;Lothringer et al 2021), which has been a long-standing goal of exoplanet science. However, simply inferring planet formation processes from relative atmospheric abundances is a challenging task, due to the numerous complex processes which govern a planet's formation history (Van der Marel et al 2021;Mollière et al 2022). Although space-based observatories such as the James Webb Space Telescope (JWST) and ARIEL are expected to constrain these quantities to better than 0.2 dex (Greene et al 2016), they will be limited resources, and thus it is necessary to couple these results with high-resolution efforts, to gain a fuller understanding of these planetary systems.…”

Global Modeling of Nebulae with Particle Growth, Drift, and Evaporation Fronts. III. Redistribution of Refractories and Volatiles

“…There have also been a number of model studies aimed more specifically at the compositional variations one might expect, some including particle drift and diffusion, and mainly emphasizing the volatiles of the outer solar system (Öberg et al 2011;Ali-Dib et al 2014;Piso et al 2015Piso et al , 2016Thiabaud et al 2015;Öberg & Bergin 2016;Booth et al 2017;Cridland et al 2017;Bosman et al 2018;Krijt et al 2018Krijt et al , 2020. These results resonate with a growing number of observational studies using the Atacama Large Millimeter/submillimeter Array (ALMA) and other facilities (Qi et al 2013a(Qi et al , 2013bCleeves 2016;Zhang et al 2020aZhang et al , 2020bvan der Marel et al 2021;and others). However, the particle growth physics, radiative transfer, and global mass redistribution in most of these latter models has been significantly simplified to focus on the chemistry.…”

Gemini-LIGHTS: Herbig Ae/Be and Massive T Tauri Protoplanetary Disks Imaged with Gemini Planet Imager

“…Second, the temperature of the higher-mass disks may prevent them from forming rings and gaps. Disks around higher-mass stars may be hotter, which can prevent the most volatile ice species, like CO, from freezing out onto dust grains, as found recently in a comparison of carbon depletion in T Tauri versus Herbig AeBe disks (van der Marel et al 2021;Sturm et al 2022). If the dust rings are created by the accumulation of millimeter-sized grains at the edges of gaps, which may in turn be carved by planets, reduced amounts of CO or CO 2 ice in the warmer disks could inhibit planetesimal formation or grain growth.…”