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

TL;DR

SN 2021ukt is a rare transitional supernova that evolves from an early IIn-like, CSM-interaction-dominated spectrum to a later Ib-like, ejecta-dominated appearance, with a short ~25-day optical plateau. The authors combine multi-epoch spectroscopy and photometry with MESA+STELLA light-curve modeling and nebular diagnostics to constrain the progenitor: a low-to-intermediate mass star with a H-rich envelope partially stripped before explosion. Nebular [Ca II]/[O I] flux ratios imply $M_{ ext{ZAMS}}$ around $12\,M_{}$, while light-curve modeling favors a higher mass near $25\,M_{}$ and a substantial H-envelope of $M_{ ext{H,env}}\approx2.1\,M_{}$, though the tail requires low $M_{ ext{Ni}}$ or high ejecta velocities for efficient trapping. The persistent Hα emission signals ongoing interaction with extended, H-rich CSM, suggesting a mass-loss history that places SN 2021ukt in a continuum between standard SN II and stripped-envelope SNe, with implications for progenitor evolution and binary mass transfer scenarios.

Abstract

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 Hα 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] 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 (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.

Figures (9)

• Figure 1: Spectroscopic evolution of SN 2021ukt. The values on the right of the spectra correspond to the phases between the estimated time of explosion (MJD 59424 $\pm$ 2.5) and the spectroscopic observation date. A SNIFS instrumental artifact in the range $\sim 5000$--5200 Å was manually discarded. Telluric lines in the spectra are labeled with solid green vertical lines. Left: The early spectra obtained between days +3 and +11 exhibit narrow, SN IIn-like Balmer lines. The H$\;$ emission persists as the ejecta cool and the photosphere recedes. Middle: Between days +8 and +51, the spectra manifest broad P Cygni profiles. H$\alpha$ emission is superimposed on broad He$\;$$\lambda$6678. Right: After $\sim$ day 61, the spectra are dominated by O$\;$, Ca$\;$, and H$\alpha$ emission features, which may be attributed to the ejecta interacting with a radially extended CSM profile. SN 2021ukt appears to be fully nebular by $\sim 6$ months, possibly as early as +78 days.
• Figure 2: Spectra of SN 2021ukt (green) at different epochs compared with the spectra of SN Ib 2009jf (blue), SN IIb 2013df (dark blue), and SN IIn 2010jl (purple) at similar phases. Vertical dashed lines mark rest-frame wavelengths of various features. The comparison with SN IIb 2013df is especially striking in the bottom panel, suggesting that SN 2021ukt was an SN IIb at this stage; however, the better-developed He$\;$ lines in SN 2021ukt, and the bump at the rest wavelength of H$\alpha$ instead possibly being the emission component of the He$\;$$\lambda$6678 line, argue that it was actually an SN Ib as concluded by yesmin2025spectral. The spectra of SNe 2009jf, 2013df, and SN 2010jl were obtained from the Berkeley SuperNova DataBase (SNDB; 2012MNRAS.425.1789S; 2016MNRAS.461.3057S). The phases of the spectra of SNe 2021ukt, 2009jf, and 2013df correspond to the days between first light and the observation date, while the phases of SN 2010jl correspond to the days between first observation and the observation date. For the purpose of presentation, all spectra were binned to 10 Å, and shifted and scaled arbitrarily.
• Figure 3: Velocity profiles of the Balmer lines of SN 2021ukt at days +3, +5, and +180 as presented in the upper, middle, and lower panels, respectively. The FWHM in the first two epochs are measured by fitting a Lorentzian function centered on H$\alpha$$\lambda$6563 and H$\beta$$\lambda$4861, while the H$\alpha$ line profile in the nebular spectrum is fitted with a Gaussian function. The H$\alpha$ feature broadens from a FWHM $\approx 900$ km $\text{s}^{-1}$ at day +3 to a FWHM $\approx 2200$ km $\text{s}^{-1}$ at day +180.
• Figure 4: Nebular spectrum of SN 2021ukt (LRIS, +180 days) compared with the nebular spectra of SNe IIb 1993J and 2008ax, SN Ib 1990U, SN Ib/Ic 1990aj, and SN Ib--IIn 2014C obtained at similar phases. For the purpose of presentation, the underlying continuum of each spectrum was fitted by a low-order polynomial and divided. All presented spectra were arbitrarily shifted. The phases correspond to the time between first light and the observation date, except for SNe 1990aj and 1990U, which are given relative to the time of the first detection. All spectra presented in this figure were sourced from the Berkeley SuperNova DataBase (SNDB; 2012MNRAS.425.1789S; 2016MNRAS.461.3057S) and 2019MNRAS.482.1545S.
• Figure 5: Velocity profiles of emission lines [Ca$\;$] $\lambda \lambda$7291, 7324 and [O$\;$] $\lambda \lambda$6300, 6364 at day +180. We first estimate the underlying continuum of the lines of interest by linearly interpolating the flux spectrum from adjacent wavelength region and subtracting it from the observed flux. We then integrate the flux below the best fit to the line profile after adopting a multicomponent Gaussian function to fit the emission features and account for noise. The velocities are estimated by with the rest-frame wavelength of the the midpoint of the doublets.