Vertical evolution of exocometary gas: I. How vertical diffusion shortens the CO lifetime

Abstract

Bright debris discs can contain large amounts of CO gas. This gas was thought to be a protoplanetary remnant until it was recently shown that it could be released in collisions of volatile-rich solids. As CO is released, interstellar UV radiation photodissociates CO producing CI, which can shield CO allowing a large CO mass to accumulate. However, this picture was challenged because CI is inefficient at shielding if CO and CI are vertically mixed. Here, we study for the first time the vertical evolution of gas to determine how vertical mixing affects the efficiency of shielding by CI. We present a 1D model that accounts for gas release, photodissociation, ionisation, viscous evolution, and vertical mixing due to turbulent diffusion. We find that if the gas surface density is high and the vertical diffusion weak (${\alpha }_{\mathrm{v}}/\alpha <[H/r{]}^2$) CO photodissociates high above the midplane, forming an optically thick CI layer that shields the CO underneath. Conversely, if diffusion is strong (${\alpha }_{\mathrm{v}}/\alpha >[H/r{]}^2$) CI and CO become well mixed, shortening the CO lifetime. Moreover, diffusion could also limit the amount of dust settling. High-resolution ALMA observations could resolve the vertical distribution of CO and CI, and thus constrain vertical mixing and the efficiency of CI shielding. We also find that the CO and CI scale heights may not be good probes of the mean molecular weight, and thus composition, of the gas. Finally, we show that if mixing is strong the CO lifetime might not be long enough for CO to spread interior to the planetesimal belt where gas is produced.

Accepted for publication in MNRAS (18 pages, 14 figures)

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• ALMA Observations of the HD~110058 debris disk
• Deprojecting and constraining the vertical thickness of exoKuiper belts
• Primordial or Secondary? Testing models of debris disk gas with ALMA
• The ALMA survey to Resolve exoKuiper belt Substructures (ARKS) I: Motivation, sample, data reduction, and results overview
• The ALMA survey to Resolve exoKuiper belt Substructures (ARKS) III: The vertical structure of debris disks
• The ALMA survey to Resolve exoKuiper belt Substructures (ARKS) IV: CO gas imaging and overview
• The ALMA survey to Resolve exoKuiper belt Substructures (ARKS) VIII: A dust arc and non-Keplerian gas kinematics in HD 121617
• The ALMA survey to Resolve exoKuiper belt Substructures (ARKS). X. Interpreting the peculiar dust rings around HD 131835
• Discovery of carbon monoxide emission from five debris disks around young A-type stars
• High CO/H2 ratios supports an exocometary origin for a CO-rich debris disk
• HD 44892: The youngest (or oldest?) gas-harbouring debris disc around an intermediate mass star
• The ALMA survey to Resolve exoKuiper belt Substructures (ARKS) XI: Gas-dust interactions and radial offsets between micron and millimetre-sized grains