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 ${}^{56}$Ni to ${}^{56}$Co at early times, and the decay of ${}^{56}$Co to ${}^{56}$Fe from ~60 days after explosion. We examine the evolution of the [Co III] 5892 A 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 ${}^{56}$Co as a function of time. This result indicates both efficient local energy deposition from positrons produced in ${}^{56}$Co decay, and long-term stability of the ionization state of the nebula. We compile 77 nebular spectra of 25 SN Ia from the literature and present 17 new nebular spectra of 7 SNe Ia, including SN2014J. From these we measure the flux in the [Co III] 5892 A line and remove its well-behaved time dependence to infer the initial mass of ${}^{56}$Ni ($M_{Ni}$) produced in the explosion. We then examine ${}^{56}$Ni yields for different SN Ia ejected masses ($M_{ej}$ - calculated using the relation between light curve width and ejected mass) and find the ${}^{56}$Ni 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$M_{\odot }$ and shows a shallow increase as $M_{ej}$ increases from ~1-1.4$M_{\odot }$; at high stretch, $M_{ej}$ clusters at the Chandrasekhar mass (1.4$M_{\odot }$) while $M_{Ni}$ spans a broad range from 0.6-1.2$M_{\odot }$. This could constitute evidence for two distinct SN Ia explosion mechanisms.