AGB–WD Merger Remnants

• The paper introduces AGB–WD merger remnants as binary products where a white dwarf merges with an AGB star’s degenerate core, influencing Type Ia supernova progenitor scenarios.
• It details how core mass, envelope retention, and off-center burning episodes govern the remnant’s evolution, with models indicating core masses from 0.5–1.0 M⊙ and variable envelope masses.
• Observational diagnostics, such as C/O-rich and H/He-deficient nebulae along with HR diagram placements similar to AGB stars, enable systematic identification in surveys.

An Asymptotic Giant Branch–White Dwarf (AGB–WD) merger remnant is the product of a common-envelope evolution or direct coalescence in a binary system where a white dwarf (commonly carbon-oxygen, CO, or oxygen-neon, ONe) merges with the degenerate core of an AGB star. These systems constitute a key channel in stellar population evolution, Type Ia supernova progenitor scenarios, and the formation of several classes of luminous transient objects. The evolutionary tracks, surface properties, nucleosynthetic signatures, and final outcomes of AGB–WD merger remnants are governed by the interplay of degenerate core mass, envelope retention, off-center burning episodes, and the timescales of post-merger structural adjustment. Their physical states and observable signatures have been extensively characterized using one-dimensional stellar evolution models, radiative transfer calculations, and photoionization codes (Wu et al., 14 Dec 2025, Brooks et al., 2017, Soker, 2022, Yao et al., 2023, Canals et al., 2018).

1. Initial Structures and Formation Channels

The canonical AGB–WD merger event occurs through either a common-envelope evolution (CEE) phase or direct merger during late binary evolution. Key outcomes depend on the nature (CO or ONe) and mass of both the WD and the AGB core, as well as the fraction of the AGB envelope retained post merger. Population synthesis studies indicate two dominant channels: CO WD + CO core (“channel 4”) and ONe WD + CO core (“channel 6”), representing ≈65% and ≈35% of the post-CEE WD-forming mergers, respectively. Pure ONe–rich products are rare, <1% (Canals et al., 2018).

The remnant comprises a hot, highly degenerate core, typically in the range $M_\mathrm{core} \sim 0.5$–$1.0\,M_\odot$ for modeled systems (Wu et al., 14 Dec 2025), or $M_\mathrm{rem} \gtrsim 1.35\,M_\odot$ in Chandrasekhar-mass–selected samples (Canals et al., 2018). The envelope mass can range from negligible (fully stripped) to nearly the full initial AGB envelope; typical remnants in the CD (core-degenerate) Type Ia progenitor channel retain only $M_\mathrm{env} \sim 0.03\,M_\odot$ (Soker, 2022), while more generic mergers may preserve up to several $M_\odot$, depending on envelope ejection efficiency (Wu et al., 14 Dec 2025).

Merger remnants are initially assembled by “grafting” a hot, double-WD–type CO core onto the H-rich AGB envelope, preserving the composition jump at the H/He and He/CO boundaries (Wu et al., 14 Dec 2025). This produces an inner core with an isothermal region and off-center temperature maximum—the site of subsequent off-center ignition.

2. Post-Merger Evolution: Core Burning and Structural Adjustment

The evolutionary trajectory depends primarily on core mass and envelope retention. Immediately post merger, the CO core contracts on a Kelvin–Helmholtz ($\tau_{\rm KH}$) timescale:

$$\tau_{\rm KH} \approx \frac{G M_{\rm core}^2}{R_{\rm core} L}$$

During this phase, central and off-center temperatures rise until the carbon-fusion rate overtakes neutrino cooling at a mass coordinate $M_r \approx 0.8$–$0.99\,M_\odot$, typically at $T \sim (5$–$8) \times 10^8\,\mathrm{K}$ (Wu et al., 14 Dec 2025). This triggers off-center carbon ignition with a conductive carbon flame propagating inward. In high-mass remnants ($M_{\mathrm{core}} \gtrsim 1.05\,M_\odot$), the flame efficiently reaches the center, converting the core to ONe. For lower core mass, the flame stalls prior to center (“quenched”), and the remnant experiences quasi-steady shell burning instead. In the presence of a significant He envelope, as in He WD + (CO/ONe) WD mergers, a He-burning shell supports a red-giant–like configuration with $L_{\rm surf} \sim 3 \times 10^4\,L_\odot$, $T_{\rm eff} \sim 1.3 \times 10^4$ K, and radius $R \sim 30–60\,R_\odot$ (Brooks et al., 2017).

Late stages can involve ignition of further burning shells (off-center neon in massive ONe remnants), envelope inflation, and successive dredge-up or mixing events: for example, deep convection at $M_\mathrm{core} \sim 1.12$–$1.20\,M_\odot$ dredges up $\sim0.4\,M_\odot$ of He-burning ashes (mainly $^{12}$C and $^{24}$Mg), altering the envelope composition (Brooks et al., 2017).

3. HR Diagram Positions and Surface Properties

Throughout the phases of off-center ignition and shell burning, AGB–WD merger remnants occupy loci in the Hertzsprung–Russell diagram nearly coincident with those of normal AGB or super-AGB stars given the same total mass. Characteristic properties include $L \sim 10^3$–$10^5\,L_\odot$, $T_{\rm eff} \sim 3,500$–$5,000$ K (for envelope-rich models), and radii up to several hundred $R_\odot$ during envelope “puff-up” episodes (Wu et al., 14 Dec 2025).

Surface abundance signatures vary with initial envelope retention and degree of mass loss. Models with significant envelope stripping show lower $^{12}\mathrm{C}/^{14}\mathrm{N}$ due to deeper dredge-up, but subsequent wind-driven mass loss over $10^4$–$10^5$ yr leads to convergence with the envelope chemistry of normal AGBs. Consequently, the long-lived, giant-like AGB–WD merger remnants are generally indistinguishable photometrically or asteroseismically from single-star AGB stars; possible (but not yet definitive) discriminants are isotopic ratios ($^{18}\mathrm{O}/^{16}\mathrm{O}$) or transient surface anomalies (Wu et al., 14 Dec 2025).

For short-lived, highly inflated remnants, “born again” tracks are seen following He-shell flashes in scenarios with a post-merger H/He envelope ($M_\mathrm{env} \sim 0.03\,M_\odot$). Such episodes yield dramatic re-inflation ($R \sim 50$–$200\,R_\odot$, $L \sim 10^3$–$10^5\,L_\odot$) over $10^1$–$10^3$ yr (Soker, 2022).

4. Final Outcomes: Collapse, Thermonuclear Explosions, and Remnant Populations

The endpoint is determined by core mass growth and off-center burning progression:

• For $M_\mathrm{core} \gtrsim 1.05\,M_\odot$, the carbon flame reaches the center, producing an ONe core which can ignite off-center neon burning. This is expected to yield core-collapse or electron-capture supernovae on $10^1$–$10^2$ yr timescales after flame arrival (Wu et al., 14 Dec 2025).
• If $M_\mathrm{core} < 1.03\,M_\odot$, the remnant cools to form a hybrid CO/ONe white dwarf unless further shell burning increases $M_\mathrm{core}$ prior to envelope loss.
• Helium WD + massive WD mergers can ultimately collapse to a neutron star as the core approaches $M_\mathrm{Ch}$, releasing $E \sim 10^{50}$ erg; the shock traverses the extended, low-mass envelope, powering a bright, short-lived optical/UV transient ($M_V \sim -17$, $L_\mathrm{peak} \sim 10^{43}$ erg s${}^{-1}$, photometric width $\sim7$ days) (Brooks et al., 2017).

Alternatively, if mass and rotational support conspire, remnants can delay explosion for up to $\sim10^6$ yr, encountering circumstellar material (CSM) formed during the initial envelope ejection (Canals et al., 2018). In the core-degenerate Ia progenitor context, the fraction of events with pre-explosion helium flashes or “born-again” signatures is $\lesssim$few × $10^{-4}$ of all SNe Ia (Soker, 2022).

5. Observational Diagnostics and Nebular Signatures

Direct detection of AGB–WD merger remnants is hampered by their photometric and spectroscopic similarity to normal post-AGB stars or super-AGB stars during most of their evolution (Yao et al., 2023, Wu et al., 14 Dec 2025). The most robust discriminant is the presence of unusually C/O-rich, H/He-deficient circumstellar nebulae resulting from remnants of double WD or H/He–stripped AGB–WD mergers (Yao et al., 2023).

Key diagnostics include:

• Near-absence of hydrogen lines (suppressed by $>10^3$ vs. normal planetary nebulae);
• Strong mid-IR and optical lines of carbon ([C IV] 4658, 7727 Å), oxygen ([O III] 5007 Å; [O IV] 25.89 μm; [O V] 32.60 μm), and neon ([Ne VI] 7.64 μm), with C/O–rich mass fractions (C $20$–$30$%, O $70$–$80$%);
• Dust continuum with $D/G\sim0.1$, peaking at 10–30 μm and dominating the WISE W3/W4 bands;
• Expansion velocities $v_\mathrm{out}\sim$ 20–60 km s$^{-1}$ for the nebular shell.

Surveys targeting [O III] 5007 Å with 1 hr exposures on 4 m-class telescopes can identify shells in M31 with $F_{5007}\sim3\times10^{-16}$ erg s$^{-1}$ cm$^{-2}$ (Yao et al., 2023). JWST MIRI spectroscopy allows mid-IR line detection down to $2\times10^{-18}$ erg s$^{-1}$ cm$^{-2}$ in M87.

In rare cases where an explosion occurs during or immediately after these phases (e.g., “born-again” or SN Ia with collision of ejecta and slow CSM), early-time bumps and peculiar H/He pollution in light curves/spectra are predicted (Soker, 2022).

6. Associated Transients and Survey Implications

AGB–WD merger remnants are now linked to classes of transient events:

• Fast, luminous transients (peak $M_V\sim-17$, optical durations of 1 week, velocities $v_\mathrm{ph}\sim4,000-6,000$ km s$^{-1}$), powered by NS formation in a low-mass, extended envelope (Brooks et al., 2017);
• Hydrogen- or helium-polluted peculiar SNe Ia, when a super-Chandrasekhar core explodes within the inflated born-again envelope (Soker, 2022);
• “Born-again” phenomena observed as early flux excesses or spectroscopically as weak H$\alpha$ or He I lines superposed on SN ejecta.

Expected occurrence rates suggest a prevalence of $\sim10$–$100$ merger remnants detectable in a single massive galaxy at any time, but born-again/early-bump transients are expected at $\sim1$ event per year globally for SNe Ia (Yao et al., 2023, Soker, 2022).

7. Astrophysical Context and Alternative Interpretations

AGB–WD merger remnants provide alternative explanations for classes of objects and phenomena:

• Their HR diagram tracks and surface abundances overlap those of super-AGB stars and explain luminous red giants such as HV 2112, which have been considered Thorne–Żytkow object (TŻO) candidates (Wu et al., 14 Dec 2025).
• The inferred rates and progenitor mass ranges suggest likely overlap with events previously interpreted as channel products of other single- or binary-star evolution modes (Canals et al., 2018).
• The presence of C/O-rich, H/He-deficient circumstellar shells serves as a critical test for distinguishing between merger remnants and other late-stage stellar populations (Yao et al., 2023).

In summary, AGB–WD merger remnants undergo complex post-merger burning and structural evolution, exhibit transient and persistent signatures that mimic or contribute to the diversity of luminous red stars and fast transients, and are now accessible to systematic identification via tailored narrow-band, spectroscopic, and mid-IR survey strategies (Wu et al., 14 Dec 2025, Brooks et al., 2017, Soker, 2022, Yao et al., 2023, Canals et al., 2018).