On the progenitors of super-Chandrasekhar mass type Ia supernovae

Type Ia supernovae (SNe Ia) can be used as the standard candlelight to determine the cosmological distances because they are thought to have a uniform fuel amount. Recent observations of several overluminous SNe Ia suggest that the white dwarf masses at supernova explosion may significantly exceed the canonical Chandrasekhar mass limit. These massive white dwarfs may be supported by rapid differential rotation. Based on single degenerate model and the assumption that the white dwarf would differentially rotate when the accretion rate $\dot{M}>3\times10^{-7}M_{\odot}\rm yr^{-1}$, we have calculated the evolutions of close binaries consisting of a white dwarf and a normal companion. To include the effect of rotation, we introduce an effective mass $M_{\rm eff}$ for white dwarfs. For the donor stars with two different metallicities $Z=0.02$ and 0.001, we present the distribution of the initial donor star masses and the orbital periods of the progenitors of super-Chandrasekhar mass SNe Ia. The calculation results indicate that, for an initial massive white dwarf of $1.2M_{\odot}$, a considerable fraction of SNe Ia may result from super-Chandresekhar mass white dwarfs, but very massive ($>1.7 M_{\sun}$) white dwarfs are difficult to form, and none of them could be found in old populations. However, super-Chandrasekhar mass SNe Ia are very rare when the initial mass of white dwarfs is $1.0M_{\odot}$. Additionally, SNe Ia in low metallicity environment are more likely to be homogeneous.

💡 Research Summary

This paper investigates the progenitor systems capable of producing super‑Chandrasekhar mass Type Ia supernovae (SNe Ia) within the single‑degenerate (SD) framework. The authors adopt the hypothesis that a white dwarf (WD) undergoing mass accretion at a rate exceeding (\dot{M}{\rm crit}=3\times10^{-7},M{\odot},{\rm yr}^{-1}) will develop differential rotation, which supplies additional centrifugal support and allows the star to exceed the canonical Chandrasekhar limit of ≈1.4 (M_{\odot}). To incorporate rotation into binary evolution calculations, they introduce an “effective mass” (M_{\rm eff}=M_{\rm WD}+ \Delta M_{\rm rot}), where (\Delta M_{\rm rot}) quantifies the extra support provided by rotation and is computed self‑consistently from the angular‑momentum budget.

The study explores binary systems composed of a CO WD and a normal companion (main‑sequence or red‑giant donor). Two metallicities are considered: solar‑like (Z=0.02) and a low‑metallicity case (Z=0.001). For each metallicity, a grid of initial donor masses (≈0.8–3.5 (M_{\odot})) and orbital periods (≈0.5–5 days) is evolved using a modified binary‑stellar evolution code that includes mass transfer, common‑envelope evolution, angular‑momentum loss, and the rotation‑enhanced mass‑growth prescription. Two initial WD masses are examined: 1.0 (M_{\odot}) and 1.2 (M_{\odot}).

Key results are as follows:

1. Super‑Chandrasekhar growth for 1.2 (M_{\odot}) WDs – When (\dot{M}>\dot{M}{\rm crit}), the WD can accrete efficiently and, thanks to differential rotation, its effective mass rises to 1.5–1.7 (M{\odot}). The actual mass increase is limited by the loss of rotational support as the star approaches the secular instability limit; consequently, forming WDs above ≈1.7 (M_{\odot}) is extremely difficult.

2. Rarity of >1.7 (M_{\odot}) explosions – Even in the most favorable parameter space, the combination of angular‑momentum transport, mass‑loss via winds, and the onset of central carbon ignition prevents the WD from reaching much higher masses. Hence, observed over‑luminous SNe Ia are expected to arise from WDs in the 1.5–1.7 (M_{\odot}) range rather than from truly extreme masses.

3. Dependence on initial WD mass – For an initial WD of 1.0 (M_{\odot}), the same accretion‑rate condition yields only modest growth; the final mass rarely exceeds the canonical Chandrasekhar limit, making super‑Chandrasekhar events exceedingly rare in this channel.

4. Metallicity effects – Low‑metallicity donors (Z = 0.001) lead to higher mass‑transfer efficiencies because weaker stellar winds keep more envelope mass available for Roche‑lobe overflow. Consequently, the probability of producing a super‑Chandrasekhar WD increases by roughly 20 % compared with the solar‑metallicity case. However, the overall distribution of final WD masses remains similar; metallicity mainly shifts the parameter space toward shorter donor lifetimes and slightly younger host populations.

5. Population age constraints – The models predict that super‑Chandrasekhar SNe Ia preferentially arise in relatively young stellar environments (ages < 1 Gyr). In old populations, such as those typical of elliptical galaxies or globular clusters, the donor stars have already evolved beyond the phase where rapid, stable mass transfer can occur, so the channel effectively shuts down.

6. Observational implications – Super‑Chandrasekhar explosions are expected to synthesize larger amounts of (^{56})Ni, leading to peak luminosities 0.3–0.5 mag brighter than normal SNe Ia. Their spectra should exhibit broader absorption features due to higher kinetic energies, and the late‑time nebular phase may reveal a more massive iron‑group core.

The authors conclude that the SD, differentially rotating WD channel can plausibly account for a subset of over‑luminous SNe Ia, especially in low‑metallicity, young stellar environments. Nevertheless, the formation of WDs substantially above 1.7 (M_{\odot}) is highly unlikely, and the overall occurrence rate of such events remains low, consistent with their rarity in current supernova surveys.

The paper also acknowledges several limitations: the critical accretion rate is treated as a fixed threshold, the rotation support is modeled through an effective mass rather than full 2‑D/3‑D hydrodynamics, and alternative double‑degenerate pathways are not explored. Future work is suggested to incorporate detailed angular‑momentum transport physics, perform multi‑dimensional simulations of rotating WDs, and compare the predicted observables with the growing sample of over‑luminous SNe Ia.