Efficiency of radial transport of ices in protoplanetary disks probed with infrared observations: the case of CO2
Abstract
The efficiency of radial transport of icy solid material from outer disk to the inner disk is currently unconstrained. Efficient radial transport of icy dust grains could significantly alter the composition of the gas in the inner disk. 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. Features in the simulated CO2 spectra are investigated for their dust flux tracing potential. We have developed a 1D viscous disk model that includes gas and grain motions as well as dust growth, sublimation and freeze-out and 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. 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 further increase the gaseous CO2 abundance. A CO2 destruction rate of at least 10-11 s-1 is needed to reconcile model prediction with observations. This rate is at least two orders of magnitude higher than the fastest known chemical destruction rate. A range of potential physical mechanisms to explain the low observed CO2 abundances are discussed. Transport processes in disks can have profound effects on the abundances of species in the inner disk. 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 in to the disk surface XDR/PDR. Other molecules, such as CH4 or NH3, can give further handles on the rate of mass transport.
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