56Ni dredge-up in Supernova 1987A

Abstract

We use early-time observations of He I 10830 A to measure the extent of upward mixing of radioactive material in SN 1987A. This work develops and extends the work of Graham, and places constraints on actual explosion models. The presence of the He I 10830-A (2s 3 S2p 3 P) line at $10 days post-explosion implies reionization by g-rays 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 g-ray energy deposition by applying a purely absorptive radiative transfer calculation. The deposition energy was used to nd the ionization balance as a function of radius, and hence the 2s 3 S population density prole. This was then applied to a spectral synthesis model, and the synthetic spectra were compared with the observations. Neither model 10H nor the mixed version, 10HMM, succeeded in reproducing the observed He I 10830-A line. The discrepancy with the data found for 10HMM is particularly signicant, as this model has successfully reproduced the X-ray and g-ray observations and the UVOIR light curve. We nd that a match to the He I line prole is achieved by reducing the extent of mixing in 10HMM. Our reducedmixing models also reproduce the observed g-ray line light curves and the iron-group velocities deduced from late-time infrared line proles. We suggest that the He I line method provides a more sensitive measure of the extent of mixing in a type II supernova explosion.