SN 2021tsz: A luminous, short photospheric phase Type II supernova in a low-metallicity host

TL;DR

This study presents SN 2021tsz, a luminous, fast-declining Type II supernova in a low-metallicity dwarf host, analyzed through multi-band photometry, spectroscopy, and comprehensive host-galaxy diagnostics. Hydrodynamical modelling with SNEC shows the early luminosity is dominated by ejecta-CSM interaction, requiring a dense CSM shell of about $0.65 M_\\odot$ out to $R_{CSM} \\approx 3100 R_\\odot$, an explosion energy of $E_{exp} = 1.3\\times10^{51}$ erg, and a pre-SN structure with a $\\sim 4 M_\\odot$ hydrogen envelope on a $\\sim 9 M_\\odot$ progenitor. The host's metallicity ($Z \\approx 0.3 Z_\\odot$) and the inferred high mass loss support a binary progenitor scenario, offering a plausible path to the observed envelope stripping in a low-metallicity environment. The analysis highlights a diversity among short-photospheric-phase SNe II driven by varying degrees of CSM interaction and suggests SN 2021tsz as a transitional case bridging classic SNe II and IIb-like explosions, with implications for the role of binary evolution in massive-star deaths.

Abstract

We present the analysis of the luminous Type II Supernova (SN) 2021tsz, which exploded in a low-luminosity galaxy. It reached a peak magnitude of -18.88 $\pm$ 0.13 mag in the $r$ band and exhibited an initial rapid decline of 4.05 $\pm$ 0.14 mag (100 d)$^{-1}$ from peak luminosity till $\sim$30 d. The photospheric phase is short, with the SN displaying bluer colours and a weak H$α$ absorption component--features consistent with other luminous, short-photospheric phase Type II SNe. A distinct transition from the photospheric to the radioactive tail phase in the $V$ band--as is common in hydrogen-rich Type II SNe--is not visible in SN 2021tsz, although a modest $\sim$1 mag drop is apparent in the redder filters. Hydrodynamic modelling suggests the luminosity is powered by ejecta-circumstellar material (CSM) interaction during the early phases (<30 days). Interaction with 0.6 M$_\odot$ of dense CSM extending to 3100 R$_\odot$ reproduces the observed luminosity, with an explosion energy of 1.3$\times$10$^{51}$ erg. The modelling indicates a pre-SN mass of 9 M$_\odot$, which includes a hydrogen envelope of 4 M$_\odot$, and a radius of $\sim$1000 R$_\odot$. Spectral energy distribution analysis and strong-line diagnostics reveal that the host galaxy of SN 2021tsz is a low-metallicity, dwarf galaxy. The low-metallicity environment and the derived high mass loss from the hydrodynamical modelling strongly support a binary progenitor system for SN 2021tsz.

Figures (12)

• Figure 1: Top panel: Spectrum of the host galaxy SDSS J233758.39-002629.6 of SN 2021tsz, overlaid with the best-fit stellar continuum model obtained using FIREFLY. Prominent emission lines, including H$\alpha$, H$\beta$, [Oiii] $\lambda\lambda$4959,5007, and [Nii] $\lambda$6583, are marked. Bottom panel: Host galaxy spectrum after subtraction of the stellar continuum model, highlighting the nebular emission lines used for redshift, star formation rate (SFR), and metallicity diagnostics.
• Figure 2: $BgVri$, ZTF $gri$, and ATLAS $c$, $o$-band light curves of SN 2021tsz offset by values as shown in the legend. ZTF $gri$ magnitudes are in the SDSS photometric system. The absolute magnitudes are corrected for distance and reddening as listed in Table \ref{['Tab5:SN2021tsz']}. Parametrised fit to the $r$ and $i$ band light curves are also shown Olivares2010. The vertical gray lines denote the epochs at which a spectrum of SN 2021tsz was obtained.
• Figure 3: $B-V$ colour curve of SN 2021tsz compared to those of the comparison sample SNe. The colour of SN 2021tsz and the comparison sample are extinction-corrected. Prior to estimating the colour, LOESS (locally estimated scatterplot smoothing) regression was applied to smooth out the $B$ and $V$ band light curves of all SNe. In the background, the unreddened sub-sample of 19 SNe II from dejaeger2018a is shown in blue, while the rest of the sample SNe with extinction correction applied are shown in pink.
• Figure 4: Comparison of absolute $r$ band light curve of SN 2021tsz with those of the comparison sample. The magnitudes are corrected for distance and reddening as listed in Table \ref{['comp_sample']}.
• Figure 5: Spectral evolution of SN 2021tsz from 8.0 to 61.0 days after explosion, with prominent lines marked. Right panel: Evolution of the H$\alpha$ profile in velocity space (centred at the rest wavelength of H$\alpha$) for spectra with SNR $>$ 5. The broad component is shown after subtraction of the narrow Gaussian (FWHM $\sim$10 Å) with its best-fit profile overplotted with black dashed lines. The corresponding narrow component, attributed to the host galaxy, is indicated by gray dashed lines.