A Type Ien supernova is a proposed type of supernova event. It is characterized by spectra dominated by elements such as ionized silicon, sulfur, and argon while lacking prominent features from elements such as carbon, oxygen, helium, and hydrogen. This classification was proposed after the discovery of SN 2021yfj in 2021, the first known of this type, whose emission lines were unique. This discovery also means that a placeholder intermediate type of supernova named SN Idn could exist with spectra dominated by either oxygen, neon, or magnesium.
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