SIMBAD references

Context. Radial transport of icy solid material from the cold outer disk to the warm inner disk is thought to be important for planet formation. However, the efficiency at which this happens is currently unconstrained. Efficient radial transport of icy dust grains could significantly alter the composition of the gas in the inner disk, enhancing the gas-phase abundances of the major ice constituents such as H₂O and CO₂. Aim. Our aim is to model the gaseous CO₂ abundance in the inner disk and use this to probe the efficiency of icy dust transport in a viscous disk. From the model predictions, infrared CO₂ spectra are simulated and features that could be tracers of icy CO₂, and thus dust, radial transport efficiency are investigated.
Methods. We have developed a 1D viscous disk model that includes gas accretion and gas diffusion as well as a description for grain growth and grain transport. Sublimation and freeze-out of CO₂ and H₂O has been included as well as a parametrisation of the CO₂ chemistry. The thermo-chemical code DALI was used to model the mid-infrared spectrum of CO₂, as can be observed with JWST-MIRI.
Results. CO₂ ice sublimating at the iceline increases the gaseous CO₂ abundance to levels equal to the CO₂ ice abundance of ∼10^–5, which is three orders of magnitude more than the gaseous CO₂ abundances of ∼10^–8 observed by Spitzer. Grain growth and radial drift increase the rate at which CO₂ is transported over the iceline and thus the gaseous CO₂ abundance, further exacerbating the problem. In the case without radial drift, a CO₂ destruction rate of at least 10^–11s^–1 or a destruction timescale of at most 1000yr is needed to reconcile model prediction with observations. This rate is at least two orders of magnitude higher than the fastest destruction rate included in chemical databases. A range of potential physical mechanisms to explain the low observed CO₂ abundances are discussed.
Conclusions. We conclude that transport processes in disks can have profound effects on the abundances of species in the inner disk such as CO₂. The discrepancy between our model and observations either suggests frequent shocks in the inner 10 AU that destroy CO₂, or that the abundant midplane CO₂ is hidden from our view by an optically thick column of low abundance CO₂ due to strong UV and/or X-rays in the surface layers. Modelling and observations of other molecules, such as CH₄ or NH₃, can give further handles on the rate of mass transport.