Evidence for Ni-56 yields Co-56 yields Fe-56 decay in type IA supernovae

In the prevailing picture of Type Ia supernovae (SN Ia), their explosive burning produces ^56^Ni, and the radioactive decay chain ^56^Ni - ^56^Co - ^56^Fe 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 A and a [Fe III] emission feature near 4700 A. 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 ^56^Fe/^56^Co abundance ratio follows from ^56^Ni - ^56^Co - ^56^Fe decay. From this agreement, we conclude that the cobalt and iron atoms we observe through SN Ia emission lines are produced by the radioactive decay of ^56^Ni, just as predicted by a wide range of models for SN Ia explosions.