Halley's comet - Its origin and composition

The results are presented of an improved set of calculations for the evolution of chemically inhomogeneous solar models with cometlike cores in which the motions of the convective elements and their contribution to the turbulent stress is computed directly. Overshooting convective jets are found to create a turbulent stress whose size, on average, is about 20 percent of the total pressure in the inner 20 percent of the stellar mass. This slows down the rate of H burning in the low-Z envelope surrounding the core during the first four billion years of evolution and leads to a final solar model having a lower mean temperature and hence lower neutrino luminosity. Successful models rule out the existence of a presolar nucleus which accreted from solid grains rich in CH4 and NH3 ices, and favor a grain composition in which C is present predominantly as CO, and N as N2. Three possible core compositions are examined in order to predict the chemical composition of the comet Halley.