CO₂-rich protoplanetary discs as a probe of dust radial drift and trapping

Abstract

Context . Mid-infrared spectra indicate considerable chemical diversity in the inner regions of protoplanetary discs, with some being H 2 O-dominated and others CO 2 -dominated. Sublimating ices from radially drifting dust grains are often invoked to explain some of this diversity, particularly with regards to H 2 O-rich discs. Aims . We model the contribution made by radially drifting dust grains to the chemical diversity of the inner regions of protoplanetary discs. These grains transport ices – including those of H 2 O and CO 2 – inwards to snow lines, thus redistributing the molecular content of the disc. As radial drift can be impeded by dust trapping in pressure maxima, we also explore the difference between smooth discs and those with dust traps due to gas gaps, quantifying the effects of gap location and formation time. Methods . We used a 1D protoplanetary disc evolution code to model the chemical evolution of the inner disc resulting from gas viscous evolution and dust radial drift. We post-processed these models to produce synthetic spectra, which we analyse with 0D LTE slab models to understand how this evolution may be expressed observationally. Results . Discs evolve through an initial H 2 O-rich phase as a result of sublimating ices, followed by a CO 2 -rich phase as H 2 O vapour is advected onto the star and CO 2 is advected into the inner disc from its snow line. The introduction of traps hastens the transition between the phases, temporarily raising the CO 2 /H 2 O ratio. However, whether or not this evolution can be traced in observations depends on the contribution of dust grains to the optical depth. If the dust grains become coupled to the gas after crossing the H 2 O snow line – for example if bare grains fragment more easily than icy grains – then the dust that delivers the H 2 O adds to the continuum optical depth and obscures the H 2 O, preventing any evolution in its visible column density. However, the CO 2 /H 2 O visible column density ratio is only weakly sensitive to assumptions about the dust continuum obscuration, making it a more suitable tracer of the impact of transport on chemistry than either individual column density. This can be investigated with spectra that show weak features that probe deep enough into the disc. The least effective gaps are those that open close to the star on timescales competitive with dust growth and drift as they block too much CO 2 ; gaps opened later or further out lead to higher CO 2 /H 2 O. This leads to a potential correlation between CO 2 /H 2 O and gap location that occurs on million-year timescales for fiducial parameters. Conclusions . Radial drift, especially when combined with dust trapping, produces CO 2 -rich discs on timescales longer than the viscous timescale at the H 2 O snow line (while creating H 2 O-rich discs at earlier times). Population analyses of the relationship between observed inner disc spectra and large-scale disc structure are needed to test the predicted role of traps.

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