SN 2023ixf in Messier 101: Photo-ionization of Dense, Close-in Circumstellar Material in a Nearby Type II Supernova

Abstract

We present UV and/or optical observations and models of SN 2023ixf, a type II supernova (SN) located in Messier 101 at 6.9 Mpc. Early time (flash) spectroscopy of SN 2023ixf, obtained primarily at Lick Observatory, reveals emission lines of H i, He i/ii, C iv, and N iii/iv/v with a narrow core and broad, symmetric wings arising from the photoionization of dense, close-in circumstellar material (CSM) located around the progenitor star prior to shock breakout. These electron-scattering broadened line profiles persist for ∼8 days with respect to first light, at which time Doppler broadened the features from the fastest SN ejecta form, suggesting a reduction in CSM density at r ≳ 1015 cm. The early time light curve of SN 2023ixf shows peak absolute magnitudes (e.g., M u = −18.6 mag, M g = −18.4 mag) that are ≳2 mag brighter than typical type II SNe, this photometric boost also being consistent with the shock power supplied from CSM interaction. Comparison of SN 2023ixf to a grid of light-curve and multiepoch spectral models from the non-LTE radiative transfer code CMFGEN and the radiation-hydrodynamics code HERACLES suggests dense, solar-metallicity CSM confined to r = (0.5–1) × 1015 cm, and a progenitor mass-loss rate of M ̇ = 10 − 2 M ⊙ yr−1. For the assumed progenitor wind velocity of v w = 50 km s−1, this corresponds to enhanced mass loss (i.e., superwind phase) during the last ∼3–6 yr before explosion.

Early Spectroscopy and Dense Circumstellar Medium Interaction in SN 2023ixf

“…The disappearance of the narrow emission lines and evolution of Hα are better matched in the r1w6 model. This is consistent with the findings of Jacobson-Galan et al (2023), who find a best-fit model in their custom grid is the r1w6 model with a larger radius. Given the challenges of reproducing the observed spectra with published models, we conservatively conclude that the mass-loss rate for the progenitor of SN 2023ixf was between 10 −3 and 10 −2 M e yr −1 , based on the presence of persistent lines after the initial shock breakout spectrum.…”

“…Over the following day, these fade away as other higher-ionization features (such as He II (λ4685.5), N IV, C IV) strengthen. This is also discussed by Smith et al (2023), Jacobson-Galan et al (2023. This implies that we are witnessing a gradual or delayed ionization of the CSM, an effect never observed before.…”

“…Given the rapid evolution of this feature, this could be a function of the model sampling. We note that we do not see evidence of the rise of N V described by Jacobson-Galan et al (2023) in our spectra. Rather, we find the asymmetric blue wings of He II (λ4685.5) to be more consistent with N III (λλ4634.0,4640.6)/C III (λλ4647.5,4650.0).…”

“…All of the comparison SNe are Type IIP/L SNe with the exception of SN 2013cu, which is a Type IIb (Leonard et al 2000;Gal-Yam et al 2014;Terreran et al 2016Terreran et al , 2022Yaron et al 2017;Tartaglia et al 2021). Their light curves show similarly shallow linear declines in V band, with SN 2013cu being the steepest (Bianciardi et al 2023;Hiramatsu et al 2023;Jacobson-Galan et al 2023;Teja et al 2023;Yamanaka et al 2023).…”

High-resolution Spectroscopy of SN 2023ixf’s First Week: Engulfing the Asymmetric Circumstellar Material

“…We note that some lower-resolution spectra of SN 2023ixf do show broader wings of He II λ4686 at very early epochs comparable to our observations(Jacobson-Galan et al 2023). Because PEPSI spectra are cross-dispersed, the "continuum" level that is normalized in the pipeline reduction may include some of the broadest emission wings if it stretches across more than one order.…”