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] λ5893 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] λ5893 line and remove its well-behaved time dependence to infer the initial mass of ⁵⁶Ni (M_Ni) produced in the explosion.We then examine ⁵⁶Ni yields for different SN Ia ejected masses (M_ej - 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),M_Ni is clustered near M_Ni ≈ 0.4 M_⊙ and shows a shallow increase as M_ej increases from ~1 to 1.4 M_⊙; at high stretch, M_ej clusters at the Chandrasekhar mass (1.4 M_⊙) while M_Ni spans a broad range from 0.6 to 1.2 M_⊙. This could constitute evidence for two distinct SN Ia explosion mechanisms.