Chemical transformation of CO in evolving protoplanetary discs across stellar masses: a route to C-rich inner regions
arXiv:2507.11631 · doi:10.1051/0004-6361/202555195
Abstract
Protoplanetary discs around Very Low Mass Stars (VLMS) show hydrocarbon-rich MIR spectra indicative of C/O>1 in their inner discs, in contrast to discs around higher-mass hosts which mostly show O-bearing species. One scenario proposed to elevate C/O in VLMS inner discs is the advection of O-depleted gas from the outer disc. However, if CO gas remains abundant, C/O can be at most ~1. We test if chemical transformation of CO into other species allows this transport scenario to produce C/O significantly above 1. We track the evolving inner disc C/H and O/H with a 1D disc evolution code. We model the transport of molecules in gas and ice and add conversions of species to represent key reaction pathways at the midplane. We explore the role of disc mass, size, ionization rate, and substructures. The inner disc C/O increases over time due to sequential delivery where O-rich species (e.g. H2O) give way to C-rich species (e.g. CH4). To reach C/O>1, separating C and O is key, hence the liberation of C from gaseous CO by He+ is critical. Ionization drives this chemistry and needs rates >~10^-17 s^-1 for VLMSs for sufficient chemical evolution within a disc lifetime. However, <~10^-17 s^-1 is needed to ensure that C/O stays <1 for the first few Myr in T Tauri discs. While C/O is usually higher for VLMS than T Tauri stars due to faster sequential delivery, C/O significantly above 1 is only produced by combining gas-phase CO destruction with gas advection and radial drift. Sufficient O depletion and hydrocarbon production around VLMSs can then be achieved but may imply higher ionization rates than T Tauris. Observations of older discs may distinguish whether higher ionization rates are indeed needed or faster physical evolution timescales alone are sufficient. CH3OH ice photodissociation at a dust trap between the CH3OH and CH4 snowlines may also liberate C as CH4 gas that can enrich the inner disc.
arXiv comment12 pages + 8 pages of appendices; 15 figures; submitted 17 April 2025 and accepted 14 July 2025 for publication in A&A
References in corpus (116)
- Observations of the Icy Universe
- The effects of snowlines on C/O in planetary atmospheres
- A simple model for the evolution of the dust population in protoplanetary disks
- The Spitzer ice legacy: Ice evolution from cores to protostars
- The Origin of T Tauri X-ray Emission: New Insights from the Chandra Orion Ultradeep Project
- Formation rates of complex organics in UV irradiated CH3OH-rich ices I: Experiments
- The XMM-Newton Extended Survey of the Taurus Molecular Cloud (XEST)
- The active lives of stars: a complete description of rotation and XUV evolution of F, G, K, and M dwarfs
- Chemical enrichment of giant planets and discs due to pebble drift
- EUV Flare Activity in Late-Type Stars
- Constraining the X-ray and Cosmic Ray Ionization Chemistry of the TW Hya Protoplanetary Disk: Evidence for a Sub-interstellar Cosmic Ray Rate
- Exclusion of Cosmic Rays in Protoplanetary Disks: Stellar and Magnetic Effects
- A Millimeter Continuum Size-Luminosity Relationship for Protoplanetary Disks
- The molecular composition of the planet-forming regions of protoplanetary disks across the luminosity regime
- Volatile carbon locking and release in protoplanetary disks. A study of TW Hya and HD 100546
- Lupus disks with faint CO isotopologues: low gas/dust or large carbon depletion?
- A Significantly Low CO Abundance Toward the TW Hya Protoplanetary Disk: A Path to Active Carbon Chemistry?
- The Different Evolution of Gas and Dust in Disks around Sun-like and Cool Stars
- Tracing the formation history of giant planets in protoplanetary disks with Carbon, Oxygen, Nitrogen and Sulphur
- Hydrocarbon emission rings in protoplanetary disks induced by dust evolution
- Cometary Chemistry and the Origin of Icy Solar System Bodies, the view after Rosetta
- Mass measurements in protoplanetary disks from hydrogen deuteride
- Setting the volatile composition of (exo)planet-building material. Does chemical evolution in disk midplanes matter?
- Magnetic flaring in the pre-main sequence Sun and implications for the early solar system
- Molecular abundances and C/O ratios in chemically evolving planet-forming disk midplanes
- Early Planet Formation in Embedded Disks (eDisk). I. Overview of the Program and First Results
- Signs of Early-Stage Disk Growth Revealed with ALMA
- Scaling Relations Associated with Millimeter Continuum Sizes in Protoplanetary Disks
- Protostars: Forges of cosmic rays?
- External photoevaporation of protoplanetary discs in sparse stellar groups: the impact of dust growth
- CO Depletion in Protoplanetary Disks: A Unified Picture Combining Physical Sequestration and Chemical Processing
- Transport of CO in Protoplanetary Disks: Consequences of Pebble Formation, Settling, and Radial Drift
- Rotationally-supported disks around Class I sources in Taurus: disk formation constraints
- CO destruction in protoplanetary disk midplanes: inside versus outside the CO snow surface
- Magnetically self-regulated formation of early protoplanetary discs
- Excess C/O and C/H in outer protoplanetary disk gas
- MINDS. The detection of CO with JWST-MIRI indicates abundant CO in a protoplanetary disk
- The Atomic and Molecular Content of Disks Around Very Low-mass Stars and Brown Dwarfs
- A stellar mass dependence of structured disks: a possible link with exoplanet demographics
- Hints for icy pebble migration feeding an oxygen-rich chemistry in the inner planet-forming region of disks
- A survey of C2H, HCN, and C18O in protoplanetary disks
- Bright C2H emission in protoplanetary disks in Lupus: high volatile C/O>1 ratios
- Exclusion of Cosmic Rays in Protoplanetary Disks. II. Chemical Gradients and Observational Signatures
- Formation of Organic Molecules and Water in Warm Disk Atmospheres
- Chemical Diversity in Protoplanetary Disks and Its Impact on the Formation History of Giant Planets
- Cosmic-ray acceleration in young protostars
- Methanol Along the Path from Envelope to Protoplanetary Disc
- The Gould's Belt Very Large Array Survey I: The Ophiuchus complex
- The HCN-Water Ratio in the Planet Formation Region of Disks
- A primordial origin for molecular oxygen in comets: A chemical kinetics study of the formation and survival of O ice from clouds to disks
- The Solar Nebula on Fire: A Solution to the Carbon Deficit in the Inner Solar System
- Rapid Evolution of Volatile CO from the Protostellar Disk Stage to the Protoplanetary Disk Stage
- Dust and gas mixtures with multiple grain species - a one-fluid approach
- Abundant hydrocarbons in the disk around a very-low-mass star
- Destruction of Refractory Carbon in Protoplanetary Disks
- Gas and multi-species dust dynamics in viscous protoplanetary discs: the importance of the dust back-reaction
- The star formation history of Upper Scorpius and Ophiuchus
- Size and structures of disks around very low mass stars in the Taurus star-forming region
- MINDS. Abundant water and varying C/O across the disk of Sz 98 as seen by JWST/MIRI
- The spatial distribution of carbon dust in the early solar nebula and the carbon content of planetesimals
- Stellar energetic particle ionization in protoplanetary disks around T Tauri stars
- The Gould Belt Very Large Array Survey IV: The Taurus-Auriga complex
- Close-in ice lines and the super-stellar C/O ratio in discs around very low-mass stars
- Probing Planet Forming Zones with Rare CO Isotopologues
- X-ray Super-Flares From Pre-Main Sequence Stars: Flare Energetics And Frequency
- An evolutionary study of volatile chemistry in protoplanetary disks
- A dusty origin for the correlation between protoplanetary disc accretion rates and dust masses
- Carbon depletion observed inside T Tauri inner rims: Formation of icy, kilometer size planetesimals by 1 Myr
- A novel way of measuring the gas disk mass of protoplanetary disks using N2H+ and C18O
- Gas mass tracers in protoplanetary disks: CO is still the best
- Molecules with ALMA at Planet-forming Scales (MAPS) XIII: HCO and disk ionization structure
- MINDS. Hydrocarbons detected by JWST/MIRI in the inner disk of Sz28 consistent with a high C/O gas-phase chemistry
- Radial and vertical dust transport inhibit refractory carbon depletion in protoplanetary disks
- Mass accretion to young stars triggered by flaring activity in circumstellar disks
- The ionising effect of low energy cosmic rays from a class II object on its protoplanetary disk
- Chemical Feedbacks of Pebble Growth: Impacts on CO depletion and C/O ratios
- Mind the gap: Distinguishing disc substructures and their impact on the inner disc composition
- Evidence for a Cosmic Ray Gradient in the IM Lup Protoplanetary Disk
- Destruction of refractory carbon grains drives the final stage of proto-planetary disk chemistry
- Tracing pebble drift and trapping using radial carbon depletion profiles in protoplanetary disks
- Desorption Kinetics and Binding Energies of Small Hydrocarbons
- Unlocking CO Depletion in Protoplanetary Disks II. Primordial C/H Predictions Inside the CO Snowline
- CO emission as an effective measure of gas masses of protoplanetary disks
- New Constraints on Protoplanetary Disk Gas Masses in Lupus
- Depletion of gaseous CO in protoplanetary disks by surface-energy-regulated ice formation
- The Mean Magnetic Field Strength of CI Tau
- MINDS: The very low-mass star and brown dwarf sample. Detections and trends in the inner disk gas
- The Galactic cosmic ray intensity at the evolving Earth and young exoplanets
- MINDS. A multi-instrument investigation into the molecule-rich JWST-MIRI spectrum of the DF Tau binary system
- A high-resolution survey of protoplanetary disks in Lupus and the nature of compact disks
- If you like C/O variations, you should have put a ring on it
- Measuring the atomic composition of planetary building blocks
- CO2-rich protoplanetary discs as a probe of dust radial drift & trapping
- Burned to ashes: How the thermal decomposition of refractory organics in the inner protoplanetary disc impacts the gas-phase C/O ratio
- MINDS. JWST-MIRI reveals a peculiar CO-rich chemistry in the drift-dominated disk CX Tau
- The Gould's Belt Very Large Array Survey V: The Perseus Region
- Testing the models of X-ray driven photoevaporation with accreting stars in the Orion Nebula Cluster
- An ALMA Survey of Chemistry in Disks around M4-M5 Stars
- Water Enrichment from Pebble Drift in Disks with Gap-forming Planets
- The impact of pre-main sequence stellar evolution on midplane snowline locations and C/O in planet forming discs
- MINDS: The very low-mass star and brown dwarf sample -- Hidden water in carbon-dominated protoplanetary disks
- Planet gap opening across stellar masses
- A new view of the corona of classical T Tauri stars: Effects of flaring activity in circumstellar disks
- A transition from HO to CH dominated spectra with decreasing stellar luminosity
- Changing disc compositions via internal photoevaporation I: Solar-mass stars
- The rapid formation of macromolecules in irradiated ice of protoplanetary disk dust traps
- Cosmic-ray ionization rate in protoplanetary disks with sheared magnetic fields
- Vertical CO surfaces as a probe for protoplanetary disk mass and carbon depletion
- Refractory carbon depletion by photolysis through dust collisions and vertical mixing
- Galactic cosmic ray propagation through M dwarf planetary systems
- Hidden under a warm blanket: If planets existed in protostellar disks, they would hardly produce observable substructures
- Dust evolution by chemisputtering during protostellar formation
- Changing disc compositions via internal photoevaporation II: M dwarf systems
- Tracking the Chemical Evolution of Hydrocarbons Through Carbon Grain Supply in Protoplanetary Disks
- C/O ratios in self-gravitating protoplanetary discs with dust evolution
- Dancing on the Grain: Variety of CO and its isotopologue fluxes as a result of surface chemistry and T Tauri disk properties