2024MNRAS.529.3838A

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.