EVIDENCE FOR NI-56 ]CO-56-]FE-56 DECAY IN TYPE-IA SUPERNOVAE

In the prevailing picture of Type Ia supernovae (SN Ia), their explosive burning produces Ni-56, and the radioactive decay chain Ni-56 --> Co-56 --> Fe-56 powers the subsequent emission. We test a central feature of this theory by measuring the relative strengths of a [Co III] emission feature near 5900 angstrom and a [Fe III] emission feature near 4700 angstrom. We measure 38 spectra from 13 SN Ia ranging from 48 to 310 days after maximum light. When we compare the observations with a simple multilevel calculation, we find that the observed Fe/Co flux ratio evolves as expected when the Fe-56/Co-56 abundance ratio follows from Ni-56 --> Co-56 --> Fe-56 decay. From this agreement, we conclude that the cobalt and iron atoms we observe through SN la emission lines arc produced by the radioactive decay of Ni-56, just as predicted by a wide range of models for SN Ia explosions.