[Paper Review] Normal type Ia supernovae from disruptions of hybrid He-CO white-dwarfs by CO white-dwarfs

Hagai B. Perets, Yossef Zenati+2 more|arXiv (Cornell University)|Oct 16, 2019

Gamma-ray bursts and supernovae62 references20 citations

TL;DR

This paper proposes that mergers between carbon-oxygen (CO) white dwarfs and hybrid helium-carbon-oxygen (He-CO) white dwarfs can produce normal type Ia supernovae. The He-rich layer detonates first, compressing the CO core to densities exceeding 10⁷ g cm⁻³, triggering a second detonation that produces ⁵⁶Ni yields consistent with observed peak luminosities of M_B ≈ −18.4 to −19.2, explaining the diversity and rates of normal SNe Ia.

ABSTRACT

Type Ia supernovae (SNe) are thought to originate from the thermonuclear explosions of carbon-oxygen (CO) white dwarfs (WDs). The proposed progenitors of standard type Ia SNe have been studied for decades and can be, generally, divided into explosions of CO WDs accreting material from stellar non-degenerate companions (single-degenerate; SD models), and those arising from the explosive interaction of two CO WDs (double-degenerate; DD models). However, current models for the progenitors of such SNe fail to reproduce the diverse properties of the observed explosions, nor do they explain the inferred rates and the characteristics of the observed populations of type Ia SNe and their expected progenitors. Here we show that the little-studied mergers of CO-WDs with hybrid Helium-CO (He-CO) WDs can provide for a significant fraction of the normal type Ia SNe. Here we use detailed thermonuclear-hydrodynamical and radiative-transfer models to show that a wide range of mergers of CO WDs with hybrid He-CO WDs can give rise to normal type Ia SNe. We find that such He-enriched mergers give rise to explosions for which the synthetic light-curves and spectra resemble those of observed type Ia SNe, and in particular, they can produce a wide range of peak-luminosities, MB(MR)~ 18.4 to 19.2 (~ 18.5 to 19:45), consistent with those observed for normal type Ia SNe. Moreover, our population synthesis models show that, together with the contribution from mergers of massive double CO-WDs (producing the more luminous SNe), they can potentially reproduce the full range of type Ia SNe, their rate and delay-time distribution.

Motivation & Objective

• To address the long-standing challenge of explaining the origin of normal type Ia supernovae, which remain poorly reproduced by standard single-degenerate and double-degenerate progenitor models.
• To investigate whether hybrid He-CO white dwarfs—previously overlooked in SN Ia progenitor studies—can serve as viable progenitors for normal SNe Ia.
• To determine if mergers involving these hybrid WDs can reproduce the observed peak luminosities, light curves, and delay-time distribution of normal type Ia SNe.
• To assess the role of shock compression from He-layer detonation in enabling detonation of low-mass CO cores (down to 0.7 M⊙) that would otherwise fail to ignite.

Proposed method

• Used 2D axisymmetric hydrodynamical simulations to model the merger of CO white dwarfs with hybrid He-CO white dwarfs, including detailed nuclear burning and energy deposition.
• Employed radiative-transfer calculations to synthesize light curves and spectra from the simulated explosion dynamics and composition.
• Tracked the evolution of density, temperature, and composition, focusing on shock compression of the CO core following He-layer detonation.
• Applied a shock compression model based on the adiabatic index (γ = 4/3 for degenerate matter) to estimate density amplification across the shock front.
• Used population synthesis models with Kroupa initial mass function, uniform mass ratio, log-uniform orbital separation, and thermal eccentricity distribution to estimate merger rates and delay-time distributions.
• Compared synthetic light curves and spectra with observed normal type Ia SNe to validate the model's consistency with observations.

Experimental results

Research questions

• RQ1 Can mergers between CO white dwarfs and hybrid He-CO white dwarfs produce normal type Ia supernovae with observed peak luminosities?
• RQ2 What is the role of shock compression from a He-layer detonation in enabling detonation of low-mass CO cores (≤0.7 M⊙)?
• RQ3 Do such mergers reproduce the observed delay-time distribution and total rate of normal type Ia supernovae?
• RQ4 Can the synthetic light curves and spectra from these mergers match the diversity and characteristics of observed normal SNe Ia?

Key findings

• Mergers of CO white dwarfs with hybrid He-CO white dwarfs produce synthetic light curves and spectra that closely match observed normal type Ia supernovae, with peak absolute magnitudes in the range M_B ≈ −18.4 to −19.2.
• The He-rich layer detonates first, generating a shock that compresses the CO core to densities exceeding 10⁷ g cm⁻³, enabling subsequent detonation and efficient ⁵⁶Ni production.
• Even CO white dwarfs as low as 0.7 M⊙ can be sufficiently compressed to achieve high-density conditions necessary for explosive burning, challenging previous assumptions about minimum mass thresholds.
• The model successfully reproduces a wide range of peak luminosities, including both normal and slightly more luminous SNe, suggesting a unified origin for the normal SN Ia population.
• Population synthesis models indicate that such mergers, combined with massive CO-CO mergers, can reproduce the observed total rate and delay-time distribution of normal type Ia supernovae.
• The 2D simulations suggest a lower limit of ~0.7 M⊙ for CO WDs to produce ⁵⁶Ni-rich explosions via this mechanism, though 3D effects may raise this threshold slightly.