[Paper Review] Lightcurve and spectral modelling of the Type IIb SN 2020acat. Evidence for a strong Ni bubble effect on the diffusion time

M. Ergon, Peter Lundqvist+11 more|arXiv (Cornell University)|Aug 14, 2023

Gamma-ray bursts and supernovaePhysics and Astronomy3 citations

TL;DR

This study uses the JEKYLL lightcurve and spectral synthesis code to model SN 2020acat, a Type IIb supernova, showing that strong expansion of nickel-rich clumps—termed "Ni bubbles"—is essential to fit both the diffusion and nebular phases. The expansion reduces effective opacity, shortening the diffusion timescale, and resolves a tension between early and late-phase observations that previous models failed to address, suggesting a systematic underestimation of ejecta masses in stripped-envelope SNe if this effect is ignored.

ABSTRACT

We use the light curve and spectral synthesis code JEKYLL to calculate a set of macroscopically mixed Type IIb supernova (SN) models, which are compared to both previously published and new late-phase observations of SN 2020acat. The models differ in the initial mass, the radial mixing and expansion of the radioactive material, and the properties of the hydrogen envelope. The best match to the photospheric and nebular spectra and lightcurves of SN 2020acat is found for a model with an initial mass of 17 solar masses, strong radial mixing and expansion of the radioactive material, and a 0.1 solar mass hydrogen envelope with a low hydrogen mass-fraction of 0.27. The most interesting result is that strong expansion of the clumps containing radioactive material seems to be required to fit the observations of SN 2020acat both in the diffusion phase and the nebular phase. These "Ni bubbles" are expected to expand due to heating from radioactive decays, but the degree of expansion is poorly constrained. Without strong expansion there is a tension between the diffusion phase and the subsequent evolution, and models that fit the nebular phase produce a diffusion peak that is too broad. The diffusion phase lightcurve is sensitive to the expansion of the "Ni bubbles", as the resulting Swiss-cheese-like geometry decreases the effective opacity and therefore the diffusion time. This effect has not been taken into account in previous lightcurve modelling of stripped-envelope SNe, which may lead to a systematic underestimate of their ejecta masses. It should be emphasized, though, that JEKYLL is limited to a geometry that is spherically symmetric on average, and large-scale asymmetries may also play a role. The relatively high initial mass found for the progenitor of SN 2020acat places it at the upper end of the mass distribution of Type IIb SN progenitors, and a single star origin can not be excluded.

Motivation & Objective

• To resolve the tension between the observed diffusion-phase lightcurve and nebular-phase evolution of SN 2020acat, which previous models fail to reconcile.
• To investigate the role of macroscopic mixing and expansion of radioactive material in shaping the lightcurve and spectral evolution of Type IIb supernovae.
• To assess whether the effective opacity and diffusion time are significantly altered by small-scale asymmetries due to expanding Ni-rich clumps.
• To constrain the progenitor mass and hydrogen envelope properties of SN 2020acat using self-consistent photospheric and nebular phase modeling.
• To evaluate the implications of neglecting Ni bubble expansion in standard lightcurve modeling, which may lead to systematic underestimation of ejecta masses in stripped-envelope SNe.

Proposed method

• The JEKYLL code is used to model the photospheric and nebular phase lightcurves and spectra of SN 2020acat, incorporating non-local thermodynamic equilibrium (NLTE) effects.
• A suite of macroscopically mixed Type IIb SN models is computed, varying initial mass, radial mixing and expansion of radioactive material, and hydrogen envelope mass and composition.
• The models include a Swiss-cheese-like geometry from expanding Ni-rich clumps, which reduces effective opacity and shortens the diffusion timescale.
• Model parameters are tuned to match both published and new late-phase observations of SN 2020acat, including photometry and spectroscopy in B, V, and K bands.
• The effect of Ni bubble expansion on the diffusion time is quantified by comparing models with and without strong expansion of radioactive clumps.
• The modeling accounts for macroscopic mixing and clump expansion, but assumes spherically symmetric average geometry, limiting the ability to assess large-scale asymmetries.

Experimental results

Research questions

• RQ1 Does the expansion of Ni-rich clumps ("Ni bubbles") significantly affect the diffusion timescale and lightcurve shape in Type IIb supernovae like SN 2020acat?
• RQ2 Why do models that fit the nebular phase fail to reproduce the observed diffusion peak width in SN 2020acat, and can this tension be resolved by including Ni bubble expansion?
• RQ3 To what extent does the effective opacity decrease due to the Swiss-cheese-like structure from expanding Ni clumps, and how does this impact lightcurve modeling?
• RQ4 What are the implications of neglecting Ni bubble expansion in standard lightcurve modeling for the inferred ejecta masses of stripped-envelope SNe?
• RQ5 Could large-scale asymmetries or other effects explain the observed discrepancies, or does small-scale clump expansion alone suffice to resolve the tension in SN 2020acat?

Key findings

• The best-fit model for SN 2020acat requires an initial progenitor mass of 17 M⊙, a 0.1 M⊙ hydrogen envelope with a low hydrogen mass fraction of 0.27, strong radial mixing, and significant expansion of the radioactive material.
• Strong expansion of Ni-rich clumps is essential to reconcile the observed diffusion peak width with the nebular phase evolution, as it reduces effective opacity and shortens the diffusion timescale.
• Without strong Ni bubble expansion, models produce a diffusion peak that is too broad, creating a tension between the early and late phases that cannot be resolved by other parameters alone.
• The effect of Ni bubble expansion on the diffusion time has not been accounted for in previous lightcurve modeling of stripped-envelope SNe, potentially leading to systematic underestimation of ejecta masses.
• The model shows a growing excess in the K band after ~100 days, reminiscent of dust formation seen in SN 2011dh, though not fully reproduced by the current modeling.
• The flat-topped He i 1.083 μm line profile observed after ~100 days in SN 2020acat is not reproduced by the model, suggesting extreme mixing or non-standard nuclear burning conditions may be at play.