Ni-56 dredge-up in Supernova 1987A

We use early-time observations of He I 10830 Angstrom to measure the extent of upward mixing of radioactive material in SN 1987A. This work develops a nd extends the work of Graham, and places constraints on actual explosion m odels. The presence of the He I 10830-Angstrom (2s(3)S-2p(3)P) line at grea ter than or equal to 10 days post-explosion implies reionization by gamma-r ays from upwardly mixed radioactive material produced during the explosion. Using the unmixed explosion model 10H by Woosley as well as mixed versions of it, we estimated the gamma-ray energy deposition by applying a purely a bsorptive radiative transfer calculation. The deposition energy was used to find the ionization balance as a function of radius, and hence the 2s(3)S population density profile. This was then applied to a spectral synthesis m odel, and the synthetic spectra were compared with the observations. Neithe r model 10H nor the mixed version, 10HMM, succeeded in reproducing the obse rved He I 10830-Angstrom line. The discrepancy with the data found for 10HM M is particularly significant, as this model has successfully reproduced th e X-ray and gamma-ray observations and the UVOIR light curve. We find that a match to the He I line profile is achieved by reducing the extent of mixi ng in 10HMM. Our reduced-mixing models also reproduce the observed gamma-ra y line light curves and the iron-group velocities deduced from late-time in frared line profiles. We suggest that the He I line method provides a more sensitive measure of the extent of mixing in a type II supernova explosion.