Carbon-Ignited Violent Merger

• Carbon-Ignited Violent Merger is defined as a channel where dynamic mass transfer in double CO white dwarf systems ignites carbon in shock-heated hotspots, initiating supersonic detonations.
• Numerical simulations reveal that key parameters such as the mass ratio, compactness, and initial spin states critically determine the onset of runaway burning and detonation conditions.
• Hydrodynamical models predict observable signatures including early optical/UV flashes, characteristic nucleosynthetic yields, and hypervelocity remnants, linking them to peculiar Type Ia SNe.

A carbon-ignited violent merger (C-ignited VM) is a specific channel for thermonuclear explosions in compact binaries, in which two carbon–oxygen (CO) white dwarfs (WDs) inspiral under gravitational wave emission until the less massive component is dynamically disrupted and accreted onto its companion. Under conditions of sufficiently high mass ratio and total mass, the accretion proceeds so violently that carbon ignition is triggered dynamically in a local, shock-heated hotspot on the primary's surface. This mechanism leads to a supersonic detonation that propagates through the CO material, synthesizing iron-group and intermediate-mass elements, powering a Type Ia supernova (SN Ia), and leaving behind, in some cases, a hypervelocity surviving remnant. The C-ignited VM scenario has become pivotal to understanding peculiar SNe Ia (notably 03fg/02es-like events) and the fastest known compact objects in the Galaxy.

1. Physical and Dynamical Prerequisites for Carbon-Ignited Violent Mergers

C-ignited VMs require compact double-degenerate systems where both components are CO WDs. The system's key control parameters are the individual WD masses ($M_1$, $M_2$), the mass ratio $q \equiv M_2/M_1$, and the binary separation at the onset of mass transfer.

Mass transfer initiates when the less massive WD fills its Roche lobe. The rate rapidly exceeds the Eddington limit, entering a super-Eddington regime where the accretor can process only a small fraction ($$\dot{M}_{\rm Edd}\sim 10^{-5} M_\odot\,{\rm yr}^{-1}$$) and most mass is ejected in a high-velocity wind. As the mass-transfer rate $\dot{M}_{\rm D}$ increases beyond $\sim 0.1{-}1\,M_\odot\,{\rm yr}^{-1}$, orbital angular momentum is efficiently removed, driving a runaway merger on the system's dynamical timescale ($\sim 100$ s). The tidal debris from the secondary strikes the surface of the primary, compressing a surface layer to $\rho \sim 10^6{-}10^7\,\mathrm{g\,cm^{-3}}$ and $T \sim 2\times10^9$ K.

A critical mass ratio threshold for violent merger is required: for non-synchronized (irrotational) binaries, $q_{\rm cr} \sim 0.8$ at $M_1 = 0.9 M_\odot$, while synchronous rotation pushes $q_{\rm cr}$ higher, approximately

$$q_{\rm cr} \approx 0.82 \left( \frac{M_1}{M_\odot} \right)^{-0.91}$$

for $0.75 < M_1/M_\odot < 1.1$ (Sato et al., 2016). The fully synchronous case yields $q_{\rm cr}$ up to $\sim 0.9$ for typical primaries.

Initial spin states and precise binary parameters are critical; tidally locked systems, initial separations, and inspiral durations modulate the violence of interaction and thus the detonation threshold (Sato et al., 2016, Zhu, 2014).

2. Microphysics of Ignition and Detonation

Carbon ignition is dictated by the balance between nuclear burning and local hydration: $\tau_{\rm CC} = \frac{C_P T}{\epsilon_{\rm CC}}$ must be shorter than the local dynamical time,

$\tau_{\rm dyn} = (24 \pi G \rho)^{-1/2}$

for detonation to proceed. Empirically, $\rho \gtrsim 2\times10^6\,\mathrm{g\,cm^{-3}}$ and $T \gtrsim 2.5\times10^9$ K over $\gtrsim 1$ dynamical time are necessary (Sato et al., 2016, Kromer et al., 2013, Fenn et al., 2016, Tanikawa et al., 2015).

The subsequent supersonic detonation wraps around the primary, propagating through CO fuel at speeds $\sim 10^9\,\mathrm{cm\,s^{-1}}$. Critical kernel sizes for sustained detonation are order $0.1{-}1$ km, well-resolved in state-of-the-art grid-based simulations (Fenn et al., 2016). Hydrodynamic instabilities, such as Kelvin–Helmholtz vortices, further amplify local heating and magnetic fields, facilitating burning even in sub-Chandrasekhar systems (Zhu et al., 2015, Zhu, 2014).

For sufficiently massive primaries ($M_1 \gtrsim 1.0-1.2\,M_\odot$), compressional heating, nuclear self-heating, and angular momentum transport raise the primary's core conditions to $T \gtrsim 1.4 \times 10^9$ K and $\rho \gtrsim 6 \times 10^7\,\mathrm{g\,cm^{-3}}$, triggering a centrally concentrated detonation and yielding SN Ia explosions at the bright end of the luminosity function (Fenn et al., 2016).

3. Hydrodynamical Simulation Frameworks and Key Signatures

Simulations utilize smoothed particle hydrodynamics (SPH) for the inspiral and early merger, grid-based Eulerian hydrodynamics for detonation and ejecta propagation, and codes such as Arepo for MHD and nuclear burning. Magnetic field amplification during merger (from initial $\sim 10^3$ G to $\gtrsim 10^{10}$ G within $\sim 100$ s through rapidly growing Kelvin–Helmholtz instabilities) enforces efficient angular momentum transport, converting rotational energy to core heating and catalyzing ignition within $\sim 10^4-10^8$ s (Zhu et al., 2015).

Hydrodynamical models robustly predict:

• Unbound ejecta masses $M_{\rm ej} \sim 1.5-1.7\,M_\odot$
• $^{56}$Ni synthesis ranging from $0.18$ to $0.86\,M_\odot$ depending on primary mass (Kromer et al., 2013, Fenn et al., 2016)
• Kinetic energies $E_{\rm kin} \sim 1.1-1.7 \times 10^{51}$ erg
• Strong asymmetries in the density and composition profiles of the ejecta, with pronounced toroidal voids (from disrupted secondary) and strong high-velocity features (Tanikawa et al., 2015, Pakmor et al., 13 Oct 2025).

A fraction of the donor WD may survive as a low-mass ($0.10-0.16\,M_\odot$) hypervelocity WD remnant, ejected at $v\sim 2800$ km/s, furnishing a natural origin for the fastest D6-type stars observed (Pakmor et al., 13 Oct 2025, Bhat et al., 14 Oct 2025). Simultaneously, $10^{-3}-10^{-2} M_\odot$ of C/O/He-rich material is ejected as pre-explosion circumstellar material (CSM) at $v \gtrsim 1000$ km/s, forming a CSM shell at radii $\sim 10^{12-15.5}$ cm (Inoue et al., 10 Dec 2025, Pakmor et al., 13 Oct 2025).

4. Circumstellar Material and Early-Time Observational Features

The super-Eddington phase yields a steep CSM density profile,

$$\rho_{\rm CSM}(r) = D \left( \frac{r}{10^{14}\,\mathrm{cm}} \right)^{-3.5},\qquad D \sim 10^{-14} \text{--} 10^{-13}\,\mathrm{g\,cm^{-3}}$$

with total CSM mass $M_{\rm CSM} \sim (1-3) \times 10^{-2}\,M_\odot$ (Inoue et al., 10 Dec 2025).

Hydrodynamic models coupling SN ejecta to this CSM predict a prompt optical/UV/X-ray flash within $1-4$ days of explosion, peaking at $L_{\rm peak}\sim10^{42.5}-10^{43.5}$ erg s$^{-1}$ and $g$-band $-15$ to $-16$ mag, with characteristic durations of $1-4$ days. The color rapidly evolves from $g-r \sim +0.4$ to $g-r \sim -0.4$, then reddens as $^{56}$Ni decay takes over. X-ray emission at $E\sim 10$ keV is expected, peaking at $t\sim 1$–$3$ days with $L_X\sim 2\times10^{42}$ erg s$^{-1}$ keV$^{-1}$, detectable up to 100 Mpc (Inoue et al., 10 Dec 2025).

The distribution and mass of the CSM naturally explain the early excesses observed in 03fg/02es-like SNe Ia and persistent C/O emission lines attributed to CSM/ejecta interaction (Inoue et al., 10 Dec 2025). C-ignited VMs do not require the presence of thick He envelopes on the primary, ensuring clear diagnostic spectral features distinguishing them from He-ignited double-detonation scenarios (Inoue et al., 10 Dec 2025, Tanikawa et al., 2015).

5. Nucleosynthetic Yields, Remnants, and Population Implications

Nuclear burning in C-ignited VM explosions synthesizes a broad mix of Fe-group, intermediate-mass elements (IMEs), and leaves unburned central O when the secondary is only partially disrupted. $^{56}$Ni yields span $0.18-0.86\,M_\odot$, with IMEs like Si, S, and Ca totalling $\sim0.1-0.4\,M_\odot$ and significant unburned O ($\sim0.2-0.5\,M_\odot$) for subluminous events (Kromer et al., 2013, Fenn et al., 2016).

Remnants include:

• A hypervelocity, low-mass ($0.10-0.16\,M_\odot$) CO WD, surface-polluted with $^{56}$Ni and He, explaining the properties of D6-1/D6-3 (Pakmor et al., 13 Oct 2025, Bhat et al., 14 Oct 2025).
• In some parameter regimes, especially with more massive primaries, the violent merger leaves no bound remnant, consistent with the absence of surviving WDs in normal SNe Ia remnants (Pakmor et al., 13 Oct 2025).

These events produce pronounced asphericity in the ejecta, observable as spectral line polarization ($\sim 0.5$–$1$\%) and nebular phase line profiles. Pre-explosion CSM may explain the variability in Na I D absorption features and the diversity of early-time SNe Ia light curves (Tanikawa et al., 2015, Inoue et al., 10 Dec 2025).

C-ignited VMs account for the rates of peculiar SNe Ia ($\sim1-3\%$), aligning with the observed abundance of peculiar 03fg/02es-like SNe. More typical SNe Ia likely arise from He-shell (quadruple) detonation channels in lower-mass or less-violent mergers (Pakmor et al., 13 Oct 2025, Bhat et al., 14 Oct 2025).

6. Astrophysical Context, Limitations, and Open Problems

The C-ignited VM scenario provides a physically robust channel for sub-Chandrasekhar SNe Ia and hypervelocity compact remnants, but remains sensitive to:

• Progenitor initial spins: tidal locking delays or prevents detonation (Sato et al., 2016, Zhu, 2014).
• Numerical resolution: higher resolution reveals lower $q_{\rm cr}$ and better captures hotspot physics (Sato et al., 2016, Tanikawa et al., 2015).
• Nuclear networks: simplified $\alpha$-chains suffice for ignition; detailed yields require post-processed, large isotope networks (Kromer et al., 2013).
• CSM properties: early spectral features and UV/optical flashes depend on the steepness, extent, and composition of CSM (Inoue et al., 10 Dec 2025).
• Viewing angle and asymmetry: polarization and spectral signatures vary with geometry (Tanikawa et al., 2015).

Direct comparisons to observations reveal both matches (e.g. SN 2010lp, iPTF14atg, D6-1/D6-3) and tensions (envelope sizes, polarization), highlighting the need for multi-dimensional radiative transfer, higher-fidelity nuclear burning, and multi-physics integration extending from inspiral to late light curve phases (Kromer et al., 2013, Kromer et al., 2016, Inoue et al., 10 Dec 2025).

Ongoing and future studies coupling 3D hydrodynamics, MHD, nuclear reaction networks, and radiative transfer are essential to fully map the diversity and statistical yields of the C-ignited violent merger channel in the context of cosmic SNe Ia demographics and compact object surveys.