Abundance stratification in type Ia supernovae -- VII. The peculiar, C-rich iPTF16abc: highlighting diversity among luminous events

Observations of Type Ia supernovae (SNe\, Ia) reveal diversity, even within assumed subcategories. Here, the composition of the peculiar iPTF16abc (SN\, 2016bln) is derived by modeling a time series of optical spectra. iPTF16abc's early spectra combine traits of SNe 1999aa and 1991T known for weak \ 6355 and prominent \ features. However, it differs with weak early \ lines, and persistent \ lines post-peak. It also exhibits a weak \ H\&K feature aligning it with SN\, 1991T, an observation supported by their bolometric light curves. The early attenuation of \ results from abundance effect. The weakening of the \ 6355 line, stems from silicon depletion in the outer shells, a characteristic shared by both SNe 1999aa and 1991T, indicating a common explosion mechanism that terminates nuclear burning at around 12000 \, unseen in normal events. Beneath a thin layer of intermediate mass elements (IMEs) with a total mass of 0. 18, extends a \ rich shell totaling 0. 76 \ and generating a bolometric luminosity as high as L₏₄₀₊=1. 60 0. 1 10^43 ergs s^-1. Inner layers, typical of SNe\, Ia, hold neutron-rich elements, (\ and), totaling 0. 20 M. Stable iron, exceeding solar abundance, and carbon, coexist in the outermost layers, challenging existing explosion models. The presence of carbon down to v 9000\, , totalling 0. 01 \, unprecedented in this class, links iPTF16abc to SN\, 2003fg-like events. The retention of 91T-like traits in iPTF16abc underscores its importance in understanding the diversity of SNe\, Ia.