Author: Amit Kashi,

Department of Physics, Ariel University, Ariel 4070000, Israel

SN 2023ixf is a very recent core-collapse supernova (CCSN) in M101 that showed signatures of interaction with circumstellar material (CSM) surrounding the progenitor star, which faded over the first 7 days. There was no indication of a pre-explosion outburst that might have ejected the CSM. The spectra included narrow lines broadened to become intermediate-width lines before disappearing from the spectrum within a few days. This was considered by other researchers to indicate a limited extent to the dense CSM of about 30 AU.

As there was no previous outburst, previous papers suggested a scenario that involved several years of high mass loss rate from the star prior to the CCSN. The Letter by Soker (2023) gives an alternative scenario. The scenario starts with the effervescent zone model that was suggested by Soker in earlier works (Soker 2008, 2021) for asymptotic giant branch (AGB) stars (e.g., the carbon AGB star IRC+10216 and the pulsating AGB Mira A), and red supergiant (RSG) stars (e.g., the enormous star VY CMa). The model proposes that the effervescent zone is composed of bound dense clumps that are lifted by stellar pulsation and envelope convection to distances that can reach tens of AU, and then fall back. The dense clumps provide most of the compact CSM mass and exist alongside the regular wind that escapes from the star. Soker uses observations of SN 2023ixf and quantities derived in previous works such as the CSM expansion velocity to scale the effervescent model parameters. The Letter estimates that for a compact CSM within 30 AU that contains 0.01 solar masses, the density of each clump is ~3000 times the density of the regular wind at the same radius and that the total volume filling factor of the clumps is ~6%. The Letter suggests that the clumps might cover only a small fraction of the CCSN photosphere in the first days after the explosion, accounting for the absence of strong narrow absorption lines. The model is thus able to explain the compact dense CSM around the progenitor of the CCSN SN 2023ixf. The long-lived effervescent zone is compatible with the absence of evidence for outbursts in the years prior to the explosion of SN 2023ixf. The lack of narrow blueshifted absorption lines is also explained by the effervescent zone.

The clumpiness of the CSM may be related to the mass loss and evolution of the progenitor star before the supernova explosion that involved a stage of strong pulsations. Massive stars can experience episodic mass loss through stellar winds or other processes, leading to the formation of clumps in the CSM. Clumping also exists as a non-eruptive (quiescence) wind of a massive star. The source for clumping in stellar wind has a few possible explanations, like the line-deshadowing instability (e.g., Owocki & Sundqvist 2018) and generation in subsurface convection zone (e.g., Driessen et al. 2022). Including clumping in wind modeling led to a reduction in the estimates of the mass loss rates of massive stars by a factor of a few (e.g., Bouret et al 2003, Fullerton et al. 2006, Sundqvist et al. 2019; Gr{\"a}fener 2021), so its effect on the evolution of these
stars is certainly important.

There are indications that the CSM in SN 2023ixf is aspherical. In many cases such deviation from spherical symmetry is an indication for the presence of a binary companion. Clumps have been shown to be formed in numerical simulations of stellar winds (Akashi et al 2012; Kashi 2017). The clumps also react differently to the stellar radiation, and favor accretion in binary systems because clumpy winds cannot be decelerated as effectively as smooth winds by the radiation and wind ram pressure of the accretor. Clumps have a strong effect in colliding wind binaries with a significant wind momentum ratio. Clumps and filaments are formed due to the instabilities and the secondary gravity then pulls the clumps that might get accreted in a sub-Bondi-Hoyle-Lyttleton accretion (Kashi et al. 2022). The clumpy effervescent zone can considerably enhance the mass transfer in a binary system by overfilling the Roche lobe and causing wind Roche lobe overflow. Hydrodynamical simulations also showed that accretion from a clumpy massive-star wind can take place in supergiant X-ray binaries (El Mellah et al. 2018).

As in other cases, let alone for such a close SN, follow up observations will eventually determine which of the suggested models, if any, is correct. Even if in the present case the model will turn out to be invalid for SN 2023ixf it might apply for future SNe observed by large surveys. If the effervescent model holds, my expectation is that SN 2023ixf has a close companion that might expose itself in the coming years.




References:

Akashi, M.-S., Kashi, A., & Soker, N. 2013, New Astronomy, 18, 23: Accretion of dense clumps in the periastron passage of Eta Carinae

Bouret, J.-C., Lanz, T., Hillier, D.-J., et al. 2003, ApJ, 595, 1182: Quantitative Spectroscopy of O Stars at Low Metallicity: O Dwarfs in NGC 346

Driessen, F.~A., Sundqvist, J.~O., & Dagore, A. 2022, Astronomy & Astrophysics, 663, A40: Theoretical wind clumping predictions from 2D LDI models of O-star winds at different metallicities

El Mellah, I., Sundqvist, J.~O., & Keppens, R. 2018, MNRAS, 475, 3240: Accretion from a clumpy massive-star wind in supergiant X-ray binaries

Fullerton, A.~W., Massa, D.~L., & Prinja, R.~K. 2006, ApJ, 637, 1025: The Discordance of Mass-Loss Estimates for Galactic O-Type Stars

Gr{\"a}fener, G. 2021, Astronomy & Astrophysics, 647, A13: Physics and evolution of the most massive stars in 30 Doradus. Mass loss, envelope inflation, and a variable upper stellar mass limit

Kashi, A. 2017, MNRAS, 464, 775: Accretion at the periastron passage of Eta Carinae

Kashi, A., Michaelis, A., & Kaminetsky, Y. 2022, MNRAS, 516, 3193: Accretion in massive colliding-wind binaries and the effect of the wind momentum ratio

Owocki, S.-P., & Sundqvist, J.-O. 2018, MNRAS, 475, 814: Characterizing the turbulent porosity of stellar wind structure generated by the line-deshadowing instability

Soker, N. 2021, ApJ, 906, 1: A Pre-explosion Extended Effervescent Zone around Core-collapse Supernova Progenitors

Soker, N. 2008, New Astronomy, 13, 491: A phenomenological model for the extended zone above AGB stars

Sundqvist, J.~O., Bj{\"o}rklund, R., Puls, J., & Najarro, F. 2019, Astronomy & Astrophysics, 632, A126: New predictions for radiation-driven, steady-state mass-loss and wind-momentum from hot, massive stars. I. Method and first results