Abstract
We suggest that clumpy-dense outflowing equatorial rings around evolved giant
stars, such as in supernova 1987A and the Necklace planetary nebula, are formed
by bipolar jets that compress gas toward the equatorial plane. The jets are
launched from an accretion disk around a stellar companion. Using the FLASH
hydrodynamics numerical code we perform 3D numerical simulations, and show that
bipolar jets expanding into a dense spherical shell can compress gas toward the
equatorial plane and lead to the formation of an expanding equatorial ring.
Rayleigh-Taylor instabilities in the interaction region break the ring to
clumps. Under the assumption that the same ring-formation mechanism operates in
massive stars and in planetary nebulae, we find this mechanism to be more
promising for ring formation than mass loss through the second Lagrangian
point. The jets account also for the presence of a bipolar nebula accompanying
many of the rings.
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