Super-Chandrasekhar-Mass Light Curve Models for the Highly Luminous Type Ia Supernova 2009dc

Several highly luminous Type Ia supernovae (SNe Ia) have been discovered. Their high luminosities are difficult to explain with the thermonuclear explosions of the Chandrasekhar-mass white dwarfs (WDs). In the present study, we estimate the progenitor mass of SN 2009dc, one of the extremely luminous SNe Ia, using the hydrodynamical models as follows. Explosion models of super-Chandrasekhar-mass (super-Ch-mass) WDs are constructed, and multi-color light curves (LCs) are calculated. The comparison between our calculations and the observations of SN 2009dc suggests that the exploding WD has a super-Ch mass of 2.2-2.4 solar masses, producing 1.2-1.4 solar masses of Ni-56, if the extinction by its host galaxy is negligible. If the extinction is significant, the exploding WD is as massive as \sim2.8 solar masses, and \sim1.8 solar masses of Ni-56 is necessary to account for the observations. Whether the host-galaxy extinction is significant or not, the progenitor WD must have a thick carbon-oxygen layer in the outermost zone (20-30% of the WD mass), which explains the observed low expansion velocity of the ejecta and the presence of carbon. Our estimate on the mass of the progenitor WD, especially for the extinction-corrected case, is challenging to the current scenarios of SNe Ia. Implications on the progenitor scenarios are also discussed.

💡 Research Summary

The paper addresses the challenge of explaining the exceptionally high luminosity of the Type Ia supernova SN 2009dc, which cannot be reproduced by the standard Chandrasekhar‑mass (≈1.4 M☉) white‑dwarf (WD) explosion scenario. The authors construct a series of hydrodynamic explosion models for super‑Chandrasekhar‑mass (super‑Ch) WDs with total masses ranging from 2.0 to 3.0 M☉. Each model is parameterised by the mass of the carbon‑oxygen (C‑O) envelope, the amount of radioactive ^56Ni synthesised, and the explosion energy. Radiative‑transfer calculations are then performed to generate multi‑band light curves (UBVRIJHK) that can be directly compared with the observed photometry and spectra of SN 2009dc.

Two distinct extinction assumptions for the host galaxy are examined. If host‑galaxy extinction is negligible, the best‑fit models require a WD mass of 2.2–2.4 M☉ and a ^56Ni mass of 1.2–1.4 M☉. In this case a thick C‑O layer comprising about 20–30 % of the total WD mass is essential; it slows the outer ejecta, reproducing the observed low expansion velocities (~8000 km s⁻¹) and the persistent C II absorption features. If significant host extinction is assumed, the corrected peak luminosity demands a more massive progenitor (≈2.8 M☉) and a larger ^56Ni yield (~1.8 M☉). Even in this scenario a substantial C‑O envelope is required to keep the kinetic energy per unit mass low enough to match the observed velocity evolution.

The analysis demonstrates that the combination of a very massive WD, a large amount of ^56Ni, and a thick outer C‑O mantle is the only configuration that simultaneously reproduces the peak brightness, the slow decline, the low line velocities, and the carbon signatures of SN 2009dc. The authors discuss possible formation channels for such a progenitor. Rapid rotation can raise the effective Chandrasekhar limit, allowing a WD to accrete beyond 1.4 M☉ while remaining in hydrostatic equilibrium. Alternatively, the merger of two WDs (the double‑degenerate scenario) can produce a super‑Ch mass object, especially if the merger remnant retains a significant C‑O envelope and undergoes delayed detonation after angular momentum redistribution. Both pathways, however, face theoretical difficulties in naturally creating the required thick C‑O outer layer and in preventing premature carbon burning that would reduce the ^56Ni yield.

The paper concludes that SN 2009dc provides strong evidence for a subclass of Type Ia supernovae originating from super‑Chandrasekhar‑mass white dwarfs. Whether the progenitor was a rapidly rotating single WD or the product of a double‑degenerate merger, the inferred masses (2.2–2.8 M☉) and ^56Ni yields (1.2–1.8 M☉) exceed the limits of conventional single‑degenerate models. This challenges current theoretical frameworks for SNe Ia, suggesting that additional physical processes—such as rotational support, merger dynamics, or asymmetric explosions—must be incorporated to fully explain the most luminous events.