A series of optical and one near-infrared nebular spectra covering the first year of the Type Ia supernova SN 2011fe are presented and modelled .
The density profile that proved best for the early optical/ultraviolet spectra , “ \rho -11fe ” , was extended to lower velocities to include the regions that emit at nebular epochs .
Model \rho -11fe is intermediate between the fast deflagration model W7 and a low-energy delayed-detonation .
Good fits to the nebular spectra are obtained if the innermost ejecta are dominated by neutron-rich , stable Fe-group species , which contribute to cooling but not to heating .
The correct thermal balance can thus be reached for the strongest [ Fe ii ] and [ Fe iii ] lines to be reproduced with the observed ratio .
The ^ { 56 } Ni mass thus obtained is 0.47 \pm 0.05 \textrm { M } _ { \odot } .
The bulk of ^ { 56 } Ni has an outermost velocity of \sim 8500 km s ^ { -1 } .
The mass of stable iron is 0.23 \pm 0.03 \textrm { M } _ { \odot } .
Stable Ni has low abundance , \sim 10 ^ { -2 } \textrm { M } _ { \odot } .
This is sufficient to reproduce an observed emission line near 7400 Ã .
A sub-Chandrasekhar explosion model with mass 1.02 \textrm { M } _ { \odot } and no central stable Fe does not reproduce the observed line ratios .
A mock model where neutron-rich Fe-group species are located above ^ { 56 } Ni following recent suggestions is also shown to yield spectra that are less compatible with the observations .
The densities and abundances in the inner layers obtained from the nebular analysis , combined with those of the outer layers previously obtained , are used to compute a synthetic bolometric light curve , which compares favourably with the light curve of SN 2011fe .