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In the movies, the top row shows gas-phase species, bottom row shows ice-on-small grains (excluding ice species on pebbles). The plots showing time evolution at points 1-3 (see top-left panel of movies for where these points are located) show the sum of gas + ice-on-small grains for all 5 molecules.

Chemistry only (assuming a constant CR ionization rate of 1e-17/s):

These results are very similar to Bosman et al. (2018); CO is destroyed and the carbon is put in to either CO2, CH4, or CH3OH. In the coldest regions (Point 3 for example), the formation of CH3OH is most efficient, while in warmer regions (e.g., Point 1) CO2 and CH4 are preferentially formed.

Chemistry + turbulent diffusion (assuming \alpha=1e-4):

With vapor and ice moving around, some of the gradients that developed in the “chemistry only” case are smeared out. This leads to an enhancement of CH3OH in Point 2 (which is being mixed up from the midplane) and to more CO2 in Point 3 (which is being mixed down from the warm-ish layer in which it forms).

Turbulent diffusion + (stationary) pebble formation, no chemistry:

This case  is similar to case M0b in Krijt et al. (2018); pebbles form throughout the disk (from the inside out), are not (yet) allowed to move radially, and act as a sink for ice species. At Point 1 for example, CO2, H2O, and CH3OH are locked up in pebbles locally while the CO abundance stays constant because CO is primarily present as gas. At Point 2, CO is also present as vapor, but it is being mixed down and locked up in pebbles, resulting in the (eventual) depletion of CO.

Turbulent diffusion + (stationary) pebble formation + chemistry:

Here we have added chemistry to the previous simulation with pebble formation. At Points 2 and 3, the combined effects of chemistry and pebble formation result in more CO being removed then in any of the previous models.

Turbulent diffusion + pebble formation + radial drift, bu no chemistry:

Here we allow (for the first time) pebbles to drift inward radially, while ignoring chemistry (similar to case M1 in Krijt et al., 2018). Compared to the model with stationary pebbles (and no chemistry) the main difference is an increase in CO at Point 1, caused by the evaporation of CO-ice-rich pebbles that have drifted through the midplane CO snowline. The enhancement peaks around 1 Myr (when the pebble flux is high) and is subsequently smeared out.

Turbulent diffusion + pebble formation + radial drift + chemistry:

Here we have added chemistry to the model with drifting pebbles. Focussing on Point 1, it looks like the enhancement in CO is essentially gone after 3 Myr. Two effects contribute to this: (i) CO is chemically destroyed at Point 1 itself, resulting in the formation of CO2 and CH4, and (ii) CO-ice is destroyed chemically in the region outside the snowline, leading to drifting pebbles that are relatively CO-ice-poor, lowering the influx of CO (but not necessarily C!) to the inner disk.

Also including everything, but CR ionization rate = 1e-16/s:

Increasing the CR ionization rate (x10) makes the timescales for chemistry shorter. The faster chemistry results in the CO abundance at Point 1 to be heavily depleted after 3 Myr, even though the pebble behavior (formation efficiency, drift rate, etc.) is identical compared to the previous calculation.