Helium star evolutionary channel to super-Chandrasekhar mass type Ia supernovae
Recent discovery of several overluminous type Ia supernovae (SNe Ia) indicates that the explosive masses of white dwarfs may significantly exceed the canonical Chandrasekhar mass limit. Rapid differential rotation may support these massive white dwarfs. Based on the single-degenerate scenario, and assuming that the white dwarfs would differentially rotate when the accretion rate $\dot{M}>3\times 10^{-7}M_{\odot}\rm yr^{-1}$, employing Eggleton’s stellar evolution code we have performed the numerical calculations for $\sim$ 1000 binary systems consisting of a He star and a CO white dwarf (WD). We present the initial parameters in the orbital period - helium star mass plane (for WD masses of $1.0 M_{\odot}$ and $1.2 M_{\odot}$, respectively), which lead to super-Chandrasekhar mass SNe Ia. Our results indicate that, for an initial massive WD of $1.2 M_{\odot}$, a large number of SNe Ia may result from super-Chandrasekhar mass WDs, and the highest mass of the WD at the moment of SNe Ia explosion is 1.81 $M_\odot$, but very massive ($>1.85M_{\odot}$) WDs cannot be formed. However, when the initial mass of WDs is $1.0 M_{\odot}$, the explosive masses of SNe Ia are nearly uniform, which is consistent with the rareness of super-Chandrasekhar mass SNe Ia in observations.
💡 Research Summary
The paper addresses the puzzling existence of several overluminous Type Ia supernovae (SNe Ia) whose inferred explosion masses exceed the canonical Chandrasekhar limit of ~1.44 M☉. The authors adopt the single‑degenerate (SD) scenario, wherein a carbon‑oxygen white dwarf (CO WD) accretes material from a non‑degenerate companion. They propose that if the mass‑transfer rate (Ṁ) exceeds 3 × 10⁻⁷ M☉ yr⁻¹, the accreting WD will rotate differentially. Differential rotation supplies additional centrifugal support, allowing the WD to remain stable well beyond the non‑rotating Chandrasekhar mass.
To test this hypothesis, they employ Eggleton’s one‑dimensional stellar evolution code to simulate roughly one thousand binary configurations consisting of a helium star (He star) donor and a CO WD accretor. Two initial WD masses are examined: 1.0 M☉ and 1.2 M☉. For each WD mass, a grid of initial orbital periods and He‑star masses is explored, covering the parameter space that could lead to successful mass growth under the assumed rotation condition.
During the evolution, the He‑star expands and fills its Roche lobe, initiating Roche‑lobe overflow (RLOF). The transferred helium is burned into carbon and oxygen on the WD surface, while the high accretion rate maintains differential rotation. The simulations track the WD’s mass, angular momentum distribution, and the evolution of the donor star until either the WD reaches a critical mass for thermonuclear runaway or the mass‑transfer rate drops below the threshold, at which point differential rotation would cease.
The results reveal a clear dependence on the initial WD mass. For an initial WD of 1.2 M☉, many binary systems evolve to produce a WD mass at explosion as high as 1.81 M☉. This upper bound is set by the point at which the mass‑transfer rate inevitably declines, causing the differential rotation to weaken and the star to become unable to support further mass growth. Consequently, WDs more massive than ~1.85 M☉ are not produced in any of the simulated channels. This range of final masses aligns well with the inferred masses of observed overluminous SNe Ia such as SN 2003fg and SN 2006gz.
In contrast, when the initial WD mass is 1.0 M☉, the final masses at explosion cluster tightly around the canonical Chandrasekhar value (≈1.4–1.5 M☉). The differential rotation does not enable a substantial mass increase because the accretion history and angular momentum budget are insufficient to sustain the higher centrifugal support needed for much larger masses. This outcome naturally explains why super‑Chandrasekhar SNe Ia are rare in the observed sample.
The study also maps the viable region in the initial orbital‑period versus He‑star‑mass plane for each WD mass. Systems with relatively short initial periods and He‑star masses around 1.0–1.2 M☉ provide the most efficient mass‑transfer rates, ensuring that Ṁ stays above the critical threshold for a sufficient duration. Wider orbits and lower‑mass donors tend to produce lower accretion rates, leading to early cessation of differential rotation and preventing the WD from reaching super‑Chandrasekhar masses.
While the results are compelling, the authors acknowledge several limitations. The use of a 1‑D stellar evolution code necessitates simplified treatments of rotation, angular momentum transport, and magnetic fields. Real WDs are likely to experience complex internal shear, possible magnetic torques, and multi‑dimensional instabilities that could alter the longevity of differential rotation. Moreover, the study focuses exclusively on He‑star donors; alternative channels involving hydrogen‑rich companions or double‑degenerate mergers are not addressed. Metallicity effects on mass‑transfer stability and wind mass loss are also omitted. Future work involving three‑dimensional hydrodynamic simulations, detailed magnetohydrodynamic modeling, and observational searches for pre‑explosion He‑star companions would be essential to validate and refine the proposed channel.
In summary, the paper demonstrates that a helium‑star donor binary can, under the assumption of sustained differential rotation at high accretion rates, grow a CO white dwarf to masses up to ~1.81 M☉, providing a plausible evolutionary pathway for the observed overluminous, super‑Chandrasekhar SNe Ia. The rarity of such events is explained by the stringent initial conditions required, especially the need for an already massive WD (≈1.2 M☉) and a donor that can maintain a high mass‑transfer rate for an extended period. This work thus strengthens the case for rotation‑supported super‑Chandrasekhar explosions within the single‑degenerate framework, while highlighting the need for more sophisticated modeling to fully capture the physics of rapidly rotating white dwarfs.