Mass Loss by Cool Carbon Stars

Extended model atmospheres, both static and expanding, are calculated for cool carbon stars. With = l. , Mbol = -6, C/H = 1.22 x l0- , and C/O s= 1.76, the structure of static models is such that carbon vapor is supersaturated in the outer layers of models with photo spheric temperature T* 3) usually considered to imply rapid condensation. For this temperature range, therefore, expanding atmospheres are constructed, the basic assumptions being (I) that the outflow is stationary; (2) that sudden formation of small grains occurs when S reaches a critical value, 5 and (3) that there is momentum-coupling between the grains and the gas. With these assumptions, grain formation occurs at the sonic point, and the resulting increase in the gradient of radiation pressure then drives the gas-grain mixture from the star. The mass-loss rates predicted with such models increase from l0- -1 at T* = 2500K to l0- -' at T* = 2000K. However, from the flux distributions of these models, it is concluded that the redistribution of photospheric radiation by the circumstellar grains is excessive. Several possibilities for diminishing this effect are discussed, the most spromising being that the assumption of small grains is wrong. Subject headings: radiative transfer - stars: carbon - stars: circumstellar shells - stars: mass loss