Distribution of Ni Yields of Type Ia Supernovae and Implications for Progenitors

The amount of Ni produced in type Ia supernova (SN Ia) explosion is probably the most important physical parameter underlying the observed correlation of SN Ia luminosities with their light curves. Based on an empirical relation between the Ni mass and the light curve parameter △m15, we obtained rough estimates of the Ni mass for a large sample of nearby SNe Ia with the aim of exploring the diversity in SN Ia. We found that the derived Ni masses for different SNe Ia could vary by a factor of ten (e.g., MNi = 0.1− 1.3M⊙), which cannot be explained in terms of the standard Chandrasekhar-mass model (with a Ni mass production of 0.4 – 0.8M⊙). Different explosion and/or progenitor models are clearly required for various SNe Ia, in particular, for those extremely nickel-poor and nickel-rich producers. The nickel-rich (with MNi > 0.8M⊙) SNe Ia are very luminous and may have massive progenitors exceeding the Chandrasekhar-mass limit since extra progenitor fuel is required to produce more Ni to power the light curve. This is also consistent with the finding that the intrinsically bright SNe Ia prefer to occur in stellar environments of young and massive stars. For example, 75% SNe Ia in spirals have ∆m15 < 1.2 while this ratio is only 18% in E/S0 galaxies. On the other hand, the nickel-poor SNe Ia (with MNi < 0.2M⊙) may invoke the sub-Chandrasekhar model, as most of them were found in early-type E/S0 galaxies dominated by the older and low-mass stellar populations. This indicates that SNe Ia in spiral and E/S0 galaxies have progenitors of different properties.