NICKEL BUBBLE INSTABILITY AND MIXING IN SN-1987A

At least three stages where dynamical instabilities can cause macrosco pic mixing have been identified in the hydrodynamics of SN 1987A. With a specific aim to investigate the mixing of Ni-56, we consider the la test stage which occurs in nickel bubbles inflated by heating due to t he Ni-56 --> Co-56 --> Fe-56 radioactive decays. One-dimensional hydro dynamics simulations show that a snowplowed shell around each inflatin g blob of Ni-56 is Rayleigh-Taylor unstable and, after t congruent-to 3-5 days, should be broken and mixed into the expanding nickel in the form of O-C-He-H clumps. To evaluate the additional acceleration of ni ckel resulting from its percolation through the ruptured shell, we ado pt a simple model based on the thin-shell snowplow approximation and a relaxation-type equation for the mass transfer by mixing. Our calcula tions indicate that, despite the beneficial role of mixing, the increa se in the nickel radial velocity at the stage of nickel bubble instabi lity is relatively modest and, when combined with the published result s for the preceding stage of instabilities in SN 1987A, not sufficient to explain the observed Ni/Co/Fe velocities of congruent-to 3000 km s -1. The observed velocities could however be explained if the precedin g stage ended (at t congruent-to 10(4) s) by ejection of a few nickel clumps with masses M(n) congruent-to 0.1M(Ni) congruent-to 0.007 M. an d radial velocities congruent-to 1800 km s-1. The values of the Ni/Co/ Fe volume filling factor, f(n) = 0.3-0.9, that we calculate with our m odel agree very well with those inferred by Li, McCray, & Sunyaev (199 3) from the emission-line data.