SN 2021ukt: A Transitional Supernova with a Short Plateau and Persistent Interaction

Abstract We present spectroscopic and photometric observations of SN 2021ukt, a peculiar short-plateau object that was originally identified as a Type IIn supernovae (SN) and later underwent an unprecedented transition to a Type Ib (possibly Type IIb) SN. The early time light curves of SN 2021ukt exhibit an ∼25 day plateau. Such a short phase of hydrogen recombination suggests a rather thin H-rich outer envelope of the progenitor star. The narrow Balmer emission lines in the spectra of SN 2021ukt during the first week indicate the interaction between the expanding ejecta and the immediate circumstellar material (CSM). This H α line is observed throughout its helium-rich ejecta-dominated phase and nebular phase, suggesting persistent interaction with a radially extended CSM profile. We explore synthetic light curves among grids of parameters generated by MESA + STELLA . We also compare the spectrophotometric evolution of SN 2021ukt with several SNe that exhibit a short plateau and persistent ejecta–CSM interaction. An estimate of the progenitor mass of SN 2021ukt is made based on the flux ratio between [Ca ii ] λλ 7291, 7324 and [O i ] λλ 6300, 6364 during its nebular phase. Our analysis suggests that the progenitor star of SN 2021ukt has a zero-age main-sequence mass of M ZAMS ≈ 12 M ⊙ , a mass of radioactive 56 Ni synthesized in the SN ejecta of M Ni ≈ 0.05 M ⊙ , and a mass of the H-rich envelope of M H,env ≲ 0.5 M ⊙ . This study adds to the growing sample of transitional SNe, reinforcing evidence for a continuum of underrepresented progenitors whose evolutionary pathways lie between those of standard SN models.