Continuum-driven mass loss from super-Eddington stars
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Line driven mass loss can account for the stellar winds of most hot, massive stars. However, some stars (notably those identified as LBV (Luminous Blue Variable) stars) have episodes of mass loss, several orders of magnitude higher than can be explained through line driving.
As a star approaches the Eddington limit, at which the radiative force excerted through electron scattering exceeds the gravitational force which holds the star together, the mass loss rate predicted for line driving diverges. At the same time the wind velocity approaches zero. This is contrary to observations of the Homonculous nebula, which infer wind speeds of the order of 500-800 km/s (Smith et al. 2003). In any case, a divergence of the mass loss rate is prohibited by the fact that the radiation field contains only a finite amount of energy, which places an upper limit on the mechanical luminocity of the star (the so-called photon-tiring effect).
An alternative to line driving is continuum driving. However, this only becomes effective, once the star has crossed the Eddington limit. The main problem with this concept lies in the fact that both radiative acceleration and gravity have a similar scaling with the radius. This means that their ration should be constant throughout the star, which would imply that a star that crosses the Eddington limit would become gravitationally unbound, in which case we would no longer observe a steady-state, surface wind.
A solution to this problem lies in the 'porosity' (See Radiation transport through porous media) of the gas. In other words, the gas would be clumped rather than have a constant density at given radius. This decreases the coupling between radiation and matter (Shaviv 1998, 2000), since photons could escape through the low density regions between the clumps. If the porosity is radius dependend as star could formally exceed the Eddington limit, while maintaining a quasi-stable wind at the surface, where the clumps become optically thin.
A star that exceeds the Eddington limit locally does not necessarily have a powerful wind. If the Eddington limit is exceeded in the stellar interior the material becomes convectively unstable (Joss et al. 1973). Efficient convective energy transport reduces the radiative luminocity which brings the Eddington parameter below unity. This suggests that a radiatively driven outflow has to be generated outside the region where convection can be efficient.
Novae
Another astronomical object that exceeds the Eddington Limit is the Nova Shaviv 2001. Novae are accreting white dwarf stars that undergo nuclear fusion in the outer layers of accreted material. This causes the outer layers to expand, while the luminosity increases rapidly. Since white dwarf mass is limited to the Chandrasekhar mass, it is easier to determine whether Novae exceed the Eddington Limit than in the case of a Luminous Blue variable.
