Abundance stratification in type Ia supernovae -- VI: the peculiar slow decliner SN\,1999aa

The abundance distribution in the ejecta of the peculiar slowly declining Type Ia supernova (SN,Ia) SN,1999aa is obtained by modelling a time series of optical spectra. Similar to SN,1991T, SN,1999aa was characterised by early-time spectra dominated by \FeIII\ features and a weak \SiII,6355,Å line, but it exhibited a high-velocity \CaII,H&K line and morphed into a spectroscopically normal SN,Ia earlier. Three explosion models are investigated, yielding comparable fits. The innermost layers are dominated by $\sim 0.3$,\Msun\ of neutron-rich stable Fe-group elements, mostly stable iron. Above that central region lies a \Nifs-dominated shell, extending to $v\approx 11,000$ – $12,000$,\kms, with mass $\sim 0.65$,\Msun. These inner layers are therefore similar to those of normal SNe,Ia. However, the outer layers exhibit composition peculiarities similar to those of SN,1991T: the intermediate-mass elements shell is very thin, containing only $\sim 0.2$,\Msun, and is sharply separated from an outer oxygen-dominated shell, which includes $\sim 0.22$,\Msun. These results imply that burning suddenly stopped in SN,1999aa. This is a feature SN,1999aa shares with SN,1991T, and explain the peculiarities of both SNe, which are quite similar in nature apart from the different luminosities. The spectroscopic path from normal to SN,1991T-like SNe,Ia cannot be explained solely by a temperature sequence. It also involves composition layering differences, suggesting variations in the progenitor density structure or in the explosion parameters.