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 modelling a time series of optical spectra. iPTF16abc's early spectra combine traits of SNe 1999aa and 1991T known for weak Si II λ 6355 and prominent Fe III features. However, it differs with weak early Fe III lines, and persistent C II lines post-peak. It also exhibits a weak Ca II H&K feature aligning it with SN 1991T, an observation supported by their bolometric light curves. The early attenuation of Fe III results from abundance effect. The weakening of the Si II λ 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 12 000 km s^-1 unseen in normal events. Beneath a thin layer of intermediate mass elements (IMEs) with a total mass of 0.18 M_⊙, extends a ⁵⁶Ni rich shell totaling 0.76 M_⊙ and generating a bolometric luminosity as high as L_peak = 1.60 ± 0.1 × 10⁴³ ergs s^-1. Inner layers, typical of SNe Ia, hold neutron-rich elements, (⁵⁴Fe and ⁵⁸Ni), 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 km s^-1, totalling ~0.01 M_⊙ 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.