Nickel Bubble Instability and Mixing in SN 1987A

At least three stages where dynamical instabilities can cause macroscopic mixing have been identified in the hydrodynamics of SN 1987A. With a specific aim to investigate the mixing of ^56^Ni, we consider the latest stage which occurs in nickel bubbles inflated by heating due to the ^56^Ni-^56^Co-^56^Fe radioactive decays. One-dimensional hydrodynamics simulations show that a snowplowed shell around each inflating blob of ^56^Ni is Rayleigh-Taylor unstable and, after t~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 nickel resulting from its percolation through the ruptured shell, we adopt a simple model based on the thin-shell snowplow approximation and a relaxation-type equation for the mass transfer by mixing. Our calculations indicate that, despite the beneficial role of mixing, the increase in the nickel radial velocity at the stage of nickel bubble instability is relatively modest and, when combined with the published results for the preceding stage of instabilities in SN 1987A, not sufficient to explain the observed Ni/Co/Fe velocities of ~3000 km s^-1^. The observed velocities could however be explained if the preceding stage ended (at t ~ 10^4^ s) by ejection of a few nickel clumps with masses M_n_ ~ 0.1M_Ni_~ 0.007 M_sun_ and radial velocities ~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 model agree very well with those inferred by Li, McCray, & Sunyaev (1993) from the emission-line data.