4. Chemical Structure
The chemistry in the outer layers can show quite unusual pathways as compared to standard astro-chemical models for interstellar clouds, because the the dust settling can lead to quite dense yet almost dust-free local conditions, where one of the most important reactions: dust + H + H → dust + H2 becomes very slow because of the lacking dust surface.
The model predicts the snowline (transition between gaseous and frozen water) to be situated at about 0.5 AU, but viscous heating in the midplane (ignored in this model) might shift it further outward. Anyway, at higher altitudes, the snowline bends and becomes almost a horizontal line, because the water ice is UV photo-desorbed in the directly irradiated layers.
Return to:
1. Spectral Energy Distribution
2. Disc Shape and Dust Settling
3. Gas and Dust Temperatures
Go on with:
5. Predicted Continuum Observations
6. Predicted Line Observations