We present spectroscopic and photometric observations of supernova (SN) 2021ukt, a peculiar short-plateau object that was originally identified as a Type IIn SN and later underwent an unprecedented transition to a Type Ib (possibly Type IIb) SN. The early-time light curves of SN 2021ukt exhibit a ~25 day plateau. Such a short phase of hydrogen recombination suggests a rather thin H-rich outer envelope of the progenitor star. The relatively narrow Balmer emission lines in spectra of SN 2021ukt during the first week indicate the interaction between the expanding ejecta and the immediate circumstellar material (CSM). This H{\alpha} line is observed throughout its helium-rich ejecta-dominated phase and nebular phase, suggesting persistent interaction with a radially extended CSM profile. We explore the synthetic light-curve model among grids of parameters generated by MESA+STELLA. We also compare the spectrophotometric evolution of SN 2021ukt with several well-sampled supernovae 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] {\lambda}{\lambda} 7291, 7324 and [O I] {\lambda}{\lambda} 6300, 6364 during its nebular phase. Our analysis suggests that the progenitor star of SN 2021ukt has a zero-age main-sequence (ZAMS) mass of about 12 solar masses, a mass of radioactive nickel-56 synthesized in the SN ejecta of about 0.04 solar masses, and a mass of the H-rich envelope of about 0.5 solar masses. This study adds to the growing sample of transitional supernovae, reinforcing evidence for a continuum of underrepresented progenitors whose evolutionary pathways lie between those of standard SN models.
@article{arxiv.2511.22923,
title = {SN 2021ukt: A Transitional Supernova with a Short Plateau and Persistent Interaction},
author = {Neil R. Pichay and Sergiy S. Vasylyev and Audrey M. Liddle and Alexei V. Filippenko and WeiKang Zheng and Thomas G. Brink and Yi Yang and Matthew Graham and Daniel Stern and Daichi Hiramatsu and Claudia P. Gutiérrez and K. Azalee Bostroem and Estefania Padilla Gonzalez and D. Andrew Howell and Curtis McCully and Megan Newsome and Craig Pellegrino and Giacomo Terreran and Ivan Altunin and Raphael Baer-Way and Vidhi Chandler and Asia A. deGraw and Connor F. Jennings and Michael B. May},
journal= {arXiv preprint arXiv:2511.22923},
year = {2025}
}