Dust and molecules in the shells of carbon stars - the inverse greenhouse effect

The expanding, cooling atmosphere of the carbon star IRC + 10216 provides the first evidence for the inverse greenhouse effect: calculations using the Planck mean absorption coefficient show that silicon carbide grains close to the star would be cooler, and graphite grains would be hotter, than if they were blackbodies, thus inhibiting graphite grain formation and enhancing silicon carbide grain formation. This invalidates the conventional use of the supersaturation ratios to determine when the solids condense. As a result SiC is expected to condense close to the star, but graphite to condense much further out. The abundance of 18 molecules observed in the extended envelope suggest that, at about T = 1250 K, C has not condensed but SiC has. This is based on a molecular freeze-out model, in which reactions suddenly cease as the expanding gas cools. It fits in with recent infrared models which suggest that graphite is not present within |5R*| of the stellar surface.