摘要：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 CO2abundance in the inner disk and use this to probe the efficiency of icy dust transport in a viscous disk. From the model predictions, infrared CO2spectra 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 CO2and H2O has been included as well as a parametrisation of the CO2chemistry. The thermo-chemical code DALI was used to model the mid-infrared spectrum of CO2, as can be observed with JWST-MIRI.Results.CO2ice sublimating at the iceline increases the gaseous CO2abundance to levels equal to the CO2ice abundance of ~10−5, which is three orders of magnitude more than the gaseous CO2abundances of ~10−8observed bySpitzer. Grain growth and radial drift increase the rate at which CO2is transported over the iceline and thus the gaseous CO2abundance, further exacerbating the problem. In the case without radial drift, a CO2destruction rate of at least 10−11s−1or a destruction timescale of at most 1000 yr 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 CO2abundances 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 CO2is hidden from our view by an optically thick column of low abundance CO2due to strong UV and/or X-rays in the surface layers. Modelling and observations of other molecules, such as CH4or NH3, can give further handles on the rate of mass transport.
