Explosions of pulsating red supergiants: a natural pathway for the diversity of Type II-P/L supernovae

TL;DR

This paper investigates how pulsations in the envelopes of red supergiants (RSGs) can shape the light curves of hydrogen-rich Type II supernovae. By self-consistently modeling a $15\,M_\odot$ RSG through its final evolution with MESA, then simulating envelope pulsations, dust effects, and SN explosions, the authors show that a $\kappa\gamma$-mechanism drives large-amplitude, long-period pulsations that restructure the envelope and alter SN shocks. Explosions at different pulsation phases yield a continuum of light-curve morphologies, from flat II-P-like plateaus to fast-declining II-L-like rises, with features traceable to density inversions tied to the ionization structure. The results suggest that SN diversity partly reflects pre-SN pulsational states, especially for more massive RSGs, and highlight the need to consider nonhydrostatic progenitors in SN modeling, while noting 1D limitations and the potential role of convection and binary evolution in shaping these outcomes.

Abstract

Red supergiants (RSGs), which are progenitors of hydrogen-rich Type II supernovae (SNe), have been known to pulsate from both observations and theory. The pulsations can be present at core collapse and affect the resulting SN. However, SN light curve models of such RSGs commonly use hydrostatic progenitor models and ignore pulsations. Here, we model the final stages of a 15 solar-mass RSG and self-consistently follow the hydrodynamical evolution. We find the growth of large amplitude radial pulsations in the envelope. After a transient phase where the envelope restructures, the pulsations settle to a steady and periodic oscillation with a period of 817 days. We show that they are driven by the $κγ$-mechanism, which is an interplay between changing opacities and the release of recombination energy of hydrogen and helium. This leads to complex and non-coherent expansion and contraction in different parts of the envelope, which greatly affect the SN progenitor properties, including its location in the Hertzsprung-Russell diagram. We simulate SN explosions of this model at different pulsations phases. Explosions in the compressed state result in a flat light curve (Type II-P). In contrast, the SN light curve in the expanded state declines rapidly, reminiscent of a Type II-L SN. For cases in between, we find light curves with various decline rates. Features in the SN light curves are directly connected to features in the density profiles. These are in turn linked to the envelope ionization structure, which is the driving mechanism of the pulsations. We predict that some of the observed diversity in Type II SN light curves can be explained by RSG pulsations. For more massive RSGs, we expect stronger pulsations that might even lead to dynamical mass ejections of the envelope and to an increased diversity in SN light curves.

Figures (18)

• Figure 1: Radial pulsation of the RSG. The radius evolution is shown in panel (a) with a zoom-in on one pulsation cycle in panel (b). The black dash-dotted line indicates the radius of the hydrostatic model. Panel (c) shows one pulsation cycle in the Hertzsprung-Russell diagram (HRD). The arrow indicates the direction of the loop in the HRD, and the markers are spaced equally in time every $1/20$ of the pulsation period. The hydrostatic model, the time-averaged effective temperature and luminosity, and their uncertainties are shown as well. For comparison, we show the typical uncertainty of the luminosity and effective temperature of Type II SN progenitors from smartt2015a as a gray marker. The colored markers labeled A--F in panels (b) and (c) indicate characteristic stages during the pulsation at which we calculate SN light curves. The pulsation phase $\phi$ is defined to be zero at maximum expansion.
• Figure 2: Restructuring of the envelope during the initial transient. Panel (a) shows the radius of the envelope (similar to Fig. \ref{['fig:pulsations_HRD']}a). The partial ionization zones and the mass coordinates of the recombination fronts of hydrogen and helium are shown in panel (b), as well as the total mass of the star. The recombination fronts are defined where the ionization fraction of the respective species reaches 0.5. The partial ionization zones are defined to be the layers where the ionization fraction of the respective species is between 10 and $90\,\mathrm{\%}$. Panel (c) shows the specific entropy in units of Avogadro's number $N_\mathrm{A}$ and the Boltzmann constant $k_\mathrm{B}$, as well as the density. Both quantities are reported at a mass coordinate of $5.5\,\mathrm{{\rm M}_{\odot}}$, which is close to the bottom of the convective envelope. Panel (d) shows the radiative cooling timescale in units of the dynamical timescale. The total energy of the convective envelope is shown in panel (e). The gray band indicates the catastrophic cooling event at $t \approx 32.5\,\mathrm{yr}$.
• Figure 3: Specific entropy $s$ in units of Avogadro's number $N_\mathrm{A}$ and the Boltzmann constant $k_\mathrm{B}$ at various mass coordinates inside the convective envelope. The gray band indicates the restructuring event at $t \approx 32.5\,\mathrm{yr}$.
• Figure 4: Recombination structure of the RSG envelope at points A--F. The six colored circles show the radius of the RSG at points A--F. The coloring inside the circles shows the specific released recombination energy $\epsilon_\mathrm{rec}$ as $\mathrm{logmod}(\epsilon_\mathrm{rec} / 10^3\,\mathrm{erg\,s^{-1}\,g^{-1}})$ on a linear radial scale, with $\mathrm{logmod}(x) = \mathrm{sign}(x)\log_{10}(|x|+1)$john1980a. Red corresponds to energy released by recombination, while blue corresponds to energy being removed via ionization. The square insets at each point show the recombination luminosity $L_{\mathrm{rec},i} = \int_{M_\mathrm{env}} \epsilon_{\mathrm{rec},i} \,\mathrm{d}m$ for the species $i = \mathrm{H^+,\ He^+}$ and He$^{2+}$ on a linear scale. Hatched regions indicate the partial ionization zone for hydrogen and helium, defined where the ionization fraction of the corresponding species is between $1\,\mathrm{\%}$ and $99\,\mathrm{\%}$. Light-gray shading at points A and B corresponds to neutral layers (ionization fractions less than $1\,\mathrm{\%}$). The central plot shows the radius evolution of one pulsation cycle and indicates the 6 highlighted points A--F. The temporal evolution of the recombination structure is available as an https://zenodo.org/records/16876815/preview/Fig4_recombination_evolution.mp4?include_deleted=0.
• Figure 5: Density profiles of the RSG envelope at points A--F during the pulsation cycle in terms of mass coordinate (panel a) and radius coordinate (panel b). The highlighted regions show hydrogen and helium partial ionization zones where the ionization fraction is between $10\,\mathrm{\%}$ and $90\,\mathrm{\%}$. The density profile of the hydrostatic model is shown for comparison. Arrows along the profiles indicate the radial velocity, both the direction (negative/inwards or positive/outwards) and the magnitude, and highlight layers that are expanding/contracting. The inset in panel (a) shows the density structure of the interior layers, up to where the core was cut out. The shading indicates the core region of the RSG. The inset in panel (b) shows the radius evolution of one pulsation cycle and indicates the points A--F. The temporal evolutions are available as an https://zenodo.org/records/16876815/preview/Fig5a_density_profile_evolution_mass.mp4?include_deleted=0 and an https://zenodo.org/records/16876815/preview/Fig5b_density_profile_evolution_radius.mp4?include_deleted=0.