Helium star donor channel for the progenitors of type Ia supernovae

Type Ia supernovae (SNe Ia) play an important role in astrophysics, especially in the study of cosmic evolution. There are several progenitor models for SNe Ia proposed in the past years. In this paper, we have carried out a detailed study of the He star donor channel, in which a carbon-oxygen white dwarf (CO WD) accretes material from a He main sequence star or a He subgiant to increase its mass to the Chandrasekhar mass. Employing Eggleton’s stellar evolution code with an optically thick wind assumption, and adopting the prescription of Kato & Hachisu (2004) for the mass accumulation efficiency of the He-shell flashes onto the WDs, we performed binary evolution calculations for about 2600 close WD binary systems. According to these calculations, we mapped out the initial parameters for SNe Ia in the orbital period–secondary mass ($\log P^{\rm i}-M^{\rm i}_2$) plane for various WD masses from this channel. The study shows that the He star donor channel is noteworthy for producing SNe Ia (i.e. $\sim 1.2\times10^{-3} {\rm yr}^{-1}$ in the Galaxy), and that the progenitors from this channel may appear as supersoft X-ray sources. Importantly, this channel can explain SNe Ia with short delay times ($\la 10^{8}$ yr), which is consistent with recent observational implications of young populations of SN Ia progenitors.

💡 Research Summary

Type Ia supernovae (SNe Ia) are essential tools for cosmology, yet the nature of their progenitor systems remains unsettled. In this paper the authors investigate the helium‑star donor channel, in which a carbon‑oxygen white dwarf (CO WD) grows in mass by accreting helium‑rich material from a helium main‑sequence star or a helium subgiant until it reaches the Chandrasekhar limit and explodes as a SN Ia.

The study employs Eggleton’s one‑dimensional stellar evolution code, incorporating the optically thick wind model to regulate mass transfer when the accretion rate exceeds a critical value. For the treatment of helium‑shell flashes on the WD surface, the authors adopt the mass‑accumulation efficiency prescription of Kato & Hachisu (2004), which provides a realistic dependence of the accumulation efficiency (η_He) on both the mass‑transfer rate and the WD mass. When the transfer rate lies within a narrow “stable burning” window (≈10⁻⁷–10⁻⁵ M☉ yr⁻¹), η_He approaches unity, allowing most of the transferred helium to be retained. Outside this window, either helium flashes or wind losses reduce the net growth of the WD.

A comprehensive grid of binary evolution calculations is performed, covering four initial WD masses (0.9, 1.0, 1.1, 1.2 M☉), secondary masses from 0.6 to 2.5 M☉, and orbital periods ranging from roughly 0.2 to 5 days (log P from –0.7 to +0.7). In total about 2 600 close WD+He‑star systems are simulated. For each model the authors track the mass‑transfer history, the occurrence of helium flashes, wind losses, and the eventual approach to the Chandrasekhar mass. The successful SN Ia progenitors are then plotted in the initial orbital period–secondary mass (log P⁰–M₂⁰) plane, revealing well‑defined regions where the channel works. For example, with an initial WD mass of 1.0 M☉, successful systems typically have secondary masses of 0.9–1.8 M☉ and initial periods of 0.6–1.6 days, where the transfer rate stays within the stable burning regime for ≈10⁶–10⁷ yr, allowing the WD to gain ≈0.3–0.4 M☉.

To estimate the Galactic SN Ia rate contributed by this channel, the authors combine their binary‑population results with a Milky Way star‑formation rate of ≈3 M☉ yr⁻¹ and a standard initial binary‑parameter distribution (mass ratios, separations, and orbital eccentricities). The resulting birthrate is ≈1.2 × 10⁻³ yr⁻¹, corresponding to roughly 30–40 % of the total observed SN Ia rate (≈3 × 10⁻³ yr⁻¹). This demonstrates that the helium‑star donor channel is not a marginal pathway but a substantial contributor to the SN Ia population.

An important observational implication is that the progenitor systems should appear as supersoft X‑ray sources (SSSs) during the mass‑accretion phase. The models predict X‑ray luminosities of 10³⁶–10³⁸ erg s⁻¹ and effective temperatures around 5 × 10⁵ K, consistent with known SSSs such as CAL 83 or RX J0513.9‑6951. Detecting and monitoring such sources, especially in star‑forming regions, would provide a direct test of the channel.

Because the time from binary formation to SN Ia explosion is short—typically less than 10⁸ yr—the channel naturally explains the “prompt” component of SN Ia populations inferred from observations of young stellar environments and from delay‑time distribution studies. This aligns with recent evidence that a significant fraction of SNe Ia arise from relatively young progenitors, a feature that is difficult to reproduce with double‑degenerate or classic single‑degenerate (hydrogen‑donor) models alone.

The paper also discusses the sensitivity of the results to key assumptions. The efficiency of the optically thick wind, the exact boundaries of the stable helium‑burning regime, and the initial WD mass distribution all influence the size of the successful region in parameter space. In particular, systems with initial WD masses below 1.0 M☉ have a dramatically reduced success rate, emphasizing the importance of the initial WD mass in determining the channel’s viability.

In summary, the authors provide a thorough theoretical framework for the helium‑star donor channel, map its viable initial conditions, quantify its contribution to the Galactic SN Ia rate, and highlight observable signatures (SSSs and short delay times). Their work strengthens the case that helium‑rich donors are a major pathway to Type Ia supernovae, especially for the prompt component, and sets clear directions for future observational tests and refinements of the wind and flash physics.