AstroNote 2024-132

SN 2024igg was discovered by ZTF (ZTF24aamlalc) on 2024-05-07.30 UTC and was subsequently classified as SN II by Meissner et al. (AstroNote 2024-124) based on a spectrum obtained with the Nordic Optical Telescope. Since the transient attained a luminosity of -19 in ZTF g, r and ATLAS o bands, quite luminous for a typical SN II, we obtained a followup spectrum with the 2m Liverpool Telescope (LT; Steele et al. 2004) on 2024-05-15.94 UTC. 

The spectrum reveals a prominent feature at ~6450A, initially identified as H-alpha that led to a SN II classification. We identify this feature instead as C II 6580A, consistent with the presence of other C II lines at 5890A and 7234A. The spectrum also shows weak Si II 6355A absorption, a signature of 03fg-like SNe Ia. Based on the overall spectrum and peak absolute magnitude of < -19, we reclassify SN 2024igg as a Ia-SC or 03fg-like SN Ia. The strong C II lines in both the LT and NOT spectra show evidence of significant unburnt carbon, suggesting these events explode within dense carbon-rich envelopes. The interaction of SN ejecta with the envelope could contribute additional luminosity, thereby making these events overluminous. The spectra of SN 2024igg show similarity wiith those of SN 2020esm (Dimitriadis et al. 2022), another Ia-SC that was initially classified as SN II.