Supernovae from Red Supergiants with Extensive Mass Loss

We calculate multicolor light curves (LCs) of supernovae (SNe) from red supergiants (RSGs) exploded within dense circumstellar medium (CSM). Multicolor LCs are calculated by using a multi-group radiation hydrodynamics code STELLA. If CSM is dense enough, the shock breakout signal is delayed and smeared by CSM and kinetic energy of SN ejecta is efficiently converted to thermal energy which is eventually released as radiation. We find that explosions of RSGs are affected by CSM in early epochs when mass-loss rate just before the explosions is higher than 10^{-4} Msun/yr. Their characteristic features are that the LC has a luminous round peak followed by a flat LC, that multicolor LCs are simultaneously bright in ultraviolet and optical at the peak, and that photospheric velocity is very low at these epochs. We calculate LCs for various CSM conditions and explosion properties, i.e., mass-loss rates, radii of CSM, density slopes of CSM, explosion energies of SN ejecta, and SN progenitors inside, to see their influence on LCs. We compare our model LCs to those of ultraviolet-bright Type IIP SN 2009kf and show that the mass-loss rate of the progenitor of SN 2009kf just before the explosion is likely to be higher than 10^{-4} Msun/yr. Combined with the fact that SN 2009kf is likely to be an energetic explosion and has large 56Ni production, which implies that the progenitor of SN 2009kf is a massive RSG, our results indicate that there could be some mechanism to induce extensive mass loss in massive RSGs just before their explosions.

💡 Research Summary

The paper investigates how a dense circumstellar medium (CSM) formed by intense mass loss from red supergiant (RSG) progenitors influences the early light curves (LCs) of core‑collapse supernovae (SNe). Using the multi‑group radiation hydrodynamics code STELLA, the authors perform one‑dimensional simulations of RSG explosions embedded in CSM with a range of physical parameters: mass‑loss rates (Ṁ = 10⁻⁵–10⁻³ M☉ yr⁻¹), CSM outer radii (R_CSM ≈ 10¹⁴–10¹⁶ cm), density slopes (ρ ∝ r⁻ˢ with s = 1.5–2.5), explosion energies (E_exp = 1–5 × 10⁵¹ erg), and synthesized ⁵⁶Ni masses (0.05–0.15 M☉). The key finding is that when the pre‑explosion mass‑loss rate exceeds roughly 10⁻⁴ M☉ yr⁻¹, the CSM becomes sufficiently massive (≳0.1 M☉) to trap the shock breakout. The shock is delayed and smeared as it propagates through the CSM, converting a large fraction (30–50 %) of kinetic energy into thermal radiation. This process produces a luminous, rounded peak that is simultaneously bright in the ultraviolet (UV) and optical bands and lasts for several days, unlike the sharp, brief shock‑breakout flash expected for a bare RSG.

During this CSM‑powered phase the photospheric velocity drops to 3–4 × 10³ km s⁻¹, markedly lower than the ≈6 × 10³ km s⁻¹ typical of ordinary Type IIP SNe. The shape and duration of the peak are sensitive to the CSM radius: larger radii (R_CSM ≈ 2–3 × 10¹⁵ cm) yield a flatter, longer‑lasting plateau, while compact CSM (≤10¹⁴ cm) produces a sharper, shorter peak. The density slope also matters: a wind‑like profile (s ≈ 2) maximizes energy conversion, whereas shallower (s = 1.5) or steeper (s = 2.5) profiles modify the peak brightness and timescale in predictable ways.

Increasing the explosion energy raises the overall luminosity of the peak by ~0.3–0.5 mag and enhances the late‑time tail if more ⁵⁶Ni is synthesized. The progenitor mass influences the CSM mass for a given Ṁ, so more massive RSGs (≈20–25 M☉) naturally produce brighter, more extended CSM‑powered peaks.

The authors compare their model grid to the UV‑bright Type IIP SN 2009kf, which exhibited a simultaneous UV–optical peak and unusually low early photospheric velocities. The best‑matching model has Ṁ ≈ 2 × 10⁻⁴ M☉ yr⁻¹, R_CSM ≈ 2 × 10¹⁵ cm, s ≈ 2, E_exp ≈ 3 × 10⁵¹ erg, and ⁵⁶Ni ≈ 0.1 M☉. This suggests that SN 2009kf’s progenitor was a massive RSG that underwent a brief, intense mass‑loss episode shortly before core collapse.

The study’s implications are twofold. First, it provides a physical explanation for a subclass of Type IIP SNe that are UV‑bright and have low early velocities, linking them to pre‑explosion mass‑loss episodes. Second, it hints at an as‑yet‑poorly understood mechanism capable of driving extreme mass loss in massive RSGs—potentially pulsational instabilities, wave‑driven outflows, or binary interactions.

The authors acknowledge limitations of their 1‑D approach, noting that real CSM may be asymmetric (e.g., disks or clumps) and that multi‑dimensional radiation hydrodynamics could alter shock propagation and radiative transfer. Nonetheless, the work establishes a clear observational signature—early UV/optical luminosity combined with low photospheric velocity—that can be used by upcoming transient surveys (e.g., ULTRASAT, LSST) to identify SNe exploding in dense CSM and to probe the final stages of massive RSG evolution.