R Coronae Borealis Variables - Physics

Physics

Two main models for carbon dust formation near the R Coronae Borealis stars have been proposed, one model that presumes the dust forms at a distance of 20 star radii from the center of the star, and one model that presumes that the dust forms in the photosphere of the star. The rationale for the 20 radii formation is that the carbon condensation temperature is 1,500 K, while the photospheric dust model was formulated by the 20 radii models failure to explain the fast decline of the RCBs' light curves just before reaching minimum. The 20 radii model requires a large and thereby long-time buildup of the obstructing dust cloud, making the fast light decline hard to comprehend.

The alternate theory of photospheric buildup of carbon dust in a 4,500-6,500 K temperature environment could be explained by condensations in the low pressure parts of shock fronts – being detected in the atmosphere of RY Sagittarii – a condensation that causes local runaway cooling, allowing carbon dust to form.

The formation of the stars themselves is also unclear. Standard stellar evolution models do not produce large luminous stars with essentially zero hydrogen. The two main theories to explain these stars are both somewhat exotic, perhaps befitting such rare stars. In one, a merger occurs between two White Dwarf stars, one a Helium White Dwarf and the other a Carbon-Oxygen White Dwarf. White Dwarfs are naturally lacking in hydrogen and the resultant star would also lack that element. The second model postulates a massive convective event at the onset of burning of an outer Helium shell, causing the little remaining atmospheric Hydrogen to be turned over into the interior of the star. It is possible that the diversity of R CrB stars is caused by a diversity of formation mechanisms, relating them to Extreme Helium stars and hydrogen-deficient Carbon stars.