Measuring nickel masses in Type Ia supernovae using cobalt emission in nebular phase spectra

The light curves of Type Ia supernovae (SNe Ia) are powered by the radioactive decay of 56Ni to 56Co at early times, and the decay of 56Co to 56Fe from ?60 d after explosion. We examine the evolution of the [Co?iii] ?5893 emission complex during the nebular phase for SNe Ia with multiple nebular spectra and show that the line flux follows the square of the mass of 56Co as a function of time. This result indicates both efficient local energy deposition from positrons produced in 56Co decay and long-term stability of the ionization state of the nebula. We compile SN Ia nebular spectra from the literature and present 21 new late-phase spectra of 7 SNe Ia, including SN 2014J. From these we measure the flux in the [Co?iii] ?5893 line and remove its well-behaved time dependence to infer the initial mass of 56Ni (MNi) produced in the explosion. We then examine 56Ni yields for different SN Ia ejected masses (Mej – calculated using the relation between light-curve width and ejected mass) and find that the 56Ni masses of SNe Ia fall into two regimes: for narrow light curves (low stretch s ? 0.7–0.9), MNi is clustered near MNi ? 0.4?M? and shows a shallow increase as Mej increases from ?1 to 1.4?M?; at high stretch, Mej clusters at the Chandrasekhar mass (1.4?M?) while MNi spans a broad range from 0.6 to 1.2?M?. This could constitute evidence for two distinct SN Ia explosion mechanisms.