[Paper Review] The Progenitor Star of SN 2023ixf: A Massive Red Supergiant with Enhanced, Episodic Pre-Supernova Mass Loss

Yu-Jing Qin, Keming Zhang+11 more|arXiv (Cornell University)|Sep 18, 2023

Gamma-ray bursts and supernovae4 citations

TL;DR

This study identifies the progenitor of SN 2023ixf as a massive red supergiant (RSG) with enhanced, episodic pre-supernova mass loss, using high-precision Keck/NIRC2 and Hubble Space Telescope (HST) imaging. The progenitor exhibits strong infrared excess and periodic variability, best explained by a dusty RSG with a pulsation-driven dust cycle, resulting in a high mass-loss rate of $3.58 \times 10^{-4}\,\mathrm{M}_{\odot}\,\text{yr}^{-1}$ and a dense, confined circumstellar medium ejected during the final pulsation phase.

ABSTRACT

We identify the progenitor star of SN 2023ixf in the nearby galaxy Messier 101 using Keck/NIRC2 adaptive optics imaging and pre-explosion HST/ACS images. The supernova position, localized with diffraction-spike pattern and high precision relative astrometry, unambiguously coincides with a single progenitor candidate of m_F814W=24.96(-0.04)(+0.05). Forced photometry further recovers 2-sigma detections in the F673N and F675W bands and imposes robust flux limits on the bluer bands. Given the reported infrared excess and semi-regular variability of the progenitor, we fit a time-dependent spectral energy distribution (SED) model of a dusty red supergiant (RSG) to a combined dataset of HST photometry, as well as ground-based near-infrared and Spitzer/IRAC [3.6], [4.5] photometry from the literature. The progenitor closely resembles a RSG of T_eff=3343+/-27 K and logL=5.10+/-0.02, with a 0.11+/-0.01 dex (25.2+/-1.7 per cent) variation over the mean luminosity at a period of P=1128.3+/-6.5 days, heavily obscured by a dust envelope with an optical depth of tau=2.83+/-0.03 at 1 micron (or A_V=10.28+/-0.11 mag). Such observed signatures match a post-main sequence star of 18.1(-1.2)(+0.7) Msun, close to the most massive SN II progenitor, with a pulsation-enhanced mass-loss rate of M_dot=(3.58+/-0.15) x 10^(-4) Msun/yr. The dense and confined circumstellar material is likely ejected during the last episode of radial pulsation before the explosion. Notably, we find strong evidence for periodic variation of tau (or both T_eff and tau) along with luminosity, a necessary assumption to reproduce the wavelength dependence of the variability, which implies dust sublimation and condensation during radial pulsations. Given the observed SED, partial dust obscuration remains a possible scenario, but any unobstructed binary companion over 7.1 Msun can be ruled out.

Motivation & Objective

• To identify and characterize the progenitor star of SN 2023ixf in the nearby spiral galaxy M101 using pre-explosion HST and Keck/NIRC2 imaging.
• To resolve the 'RSG problem' by investigating why massive RSGs with $\log(L/L_\odot) > 5.1$ are rarely observed as SN II progenitors.
• To determine the physical properties of the progenitor, including luminosity, effective temperature, mass, and dust obscuration, using multi-wavelength photometry.
• To test whether periodic variability in the progenitor's luminosity and dust optical depth can be explained by pulsation-driven dust sublimation and condensation cycles.
• To rule out the presence of a massive, unobscured binary companion as an alternative explanation for the observed infrared excess and variability.

Proposed method

• Used Keck/NIRC2 adaptive optics imaging with diffraction-spike pattern and high-precision relative astrometry to localize the SN position to $19.5\,\text{mas}$ uncertainty.
• Performed forced photometry on pre-explosion HST/ACS images to recover $2\sigma$ detections in F673N, F675W, and F814W bands.
• Constructed a time-dependent spectral energy distribution (SED) model of a dusty red supergiant using combined HST, ground-based near-infrared, and Spitzer/IRAC photometry.
• Fitted a pulsation-modulated SED model with variable dust optical depth ($\tau$) and effective temperature ($T_{\text{eff}}$) to reproduce the wavelength-dependent variability.
• Incorporated a time-varying dust envelope model where $\tau$ varies due to periodic dust sublimation and condensation driven by radial pulsations.
• Used a MARCS stellar atmosphere model with dust opacity to derive physical parameters, including mass-loss rate $\dot{M}$, luminosity $L$, and $A_V$.

Experimental results

Research questions

• RQ1 What is the nature of the progenitor star of SN 2023ixf, and does it match the expected properties of a massive red supergiant?
• RQ2 What causes the observed infrared excess and semi-regular variability in the progenitor, and can it be explained by pulsation-driven dust cycles?
• RQ3 What is the mass-loss rate and circumstellar medium density of the progenitor, and how does it compare to typical RSGs?
• RQ4 Can the observed wavelength-dependent variability in the SED be explained by time-varying dust optical depth rather than stellar variability alone?
• RQ5 Is a massive, unobscured binary companion a viable alternative explanation for the observed emission and extinction?

Key findings

• The progenitor of SN 2023ixf is a massive red supergiant with $\log(L/L_\odot) = 5.10 \pm 0.02$ and $T_{\text{eff}} = 3343 \pm 27\,\text{K}$, consistent with a $18.1_{-1.2}^{+0.7}\,\mathrm{M}_{\odot}$ post-main sequence star.
• The progenitor is heavily obscured by a dust envelope with $\tau = 2.83 \pm 0.03$ at $1\,\mu\text{m}$ (or $A_V = 10.28 \pm 0.11$ mag), indicating a high pre-SN mass-loss rate of $(3.58 \pm 0.15) \times 10^{-4}\,\mathrm{M}_{\odot}\,\text{yr}^{-1}$.
• The circumstellar medium has a density of $(4.55 \pm 0.20) \times 10^{-15}\,\text{g}\,\text{cm}^{-3}$, consistent with ejection during the final pulsation phase.
• Strong evidence supports synchronized variation in dust optical depth ($\tau$) and luminosity, suggesting periodic dust sublimation and condensation driven by radial pulsations.
• The observed amplitude-wavelength relationship of variability is best reproduced by a model with time-varying $\tau$, not just stellar variability, indicating dynamic dust processes.
• Any unobscured binary companion with mass $> 7.1\,\mathrm{M}_{\odot}$ can be ruled out based on the photometric data, supporting a single massive RSG progenitor.