Evolution Models of CO WD -- AGB Star Merger Remnants

TL;DR

The paper tackles the uncertain outcomes of common envelope evolution by modeling post-merger remnants formed from CO WD merging with an AGB core. It employs 1D merger-remnant models, built with a double-WD-merger-like core plus AGB envelope across a grid of $M_{ m core}=0.5$–$1.0\,M_0odot$ and four envelope-loss scenarios, using MESA to follow their evolution with an approx$^{21}$ network and OPAL opacity. A key result is that off-center carbon burning ignites soon after merger, with the carbon flame typically reaching the center in $10^{4}$–$10^{5}$ years for many models; in the most massive remnants, off-center neon burning is triggered and a core-collapse SN is expected, while less massive remnants continue H/He shell burning and evolve along AGB-like tracks. The surface abundances, particularly the $^{12}{ m C}/^{14}{ m N}$ ratio, depend on envelope mass loss and wind treatment, with potential observational overlap between merger remnants and normal AGB/SAGB stars, suggesting that some giant-like stars (e.g., HV 2112) could be AGB-WD merger remnants. Overall, the work highlights the viability of the merger channel in producing CCSN progenitors and SAGB-like objects and provides observable predictions for future tests.

Abstract

Common envelope evolution is a critical but still poorly understood phase in binary evolution. It plays a key role in forming close binaries such as hot subdwarfs, double white dwarfs, X-ray binaries, and double neutron stars. However, its outcomes remain highly uncertain. Depending on the efficiency of envelope ejection, a system may either survive as a close binary or undergo a complete merger. In this work, we investigate the post merger evolution of systems where a CO WD mergers with the core of an AGB star. A grid of merger remnant models with various core and envelope masses is constructed. At the onset of evolution, the CO core contracts and undergoes off-center carbon ignition, producing an inwardly propagating carbon flame. For remnants with relatively low mass of CO core, the flame phase is followed by core contraction and subsequent H-shell burning. For more massive CO cores, the carbon flame reaches the center and is soon followed by off-center neon burning, which is expected to eventually lead to core-collapse supernovae. The merger remnants occupy nearly the same region on HR diagram as ordinary AGB or super-AGB stars, exhibiting similar surface properties. Although their surface abundance may differ slightly from those of normal AGB stars depending on the initial core and envelope masses, these differences are strongly reduced once mass-loss is taken into account. We suggest that some giant-like stars, including candidates for Thorne-Zytkow objects (e.g., HV 2112), might alternatively be explained as AGB-WD merger remnants.

Figures (8)

• Figure 1: Structure profiles for constructing the merger remnant of an $8.0{M}_{\odot}$ AGB star and a $1.0{M}_{\odot}$ CO WD. Panel (a): elemental abundance distribution and entropy profile of an $8.0{M}_{\odot}$ AGB star when CO core mass reaches $1.0{M}_{\odot}$. Panel (b): elemental abundance distribution and entropy profile of a double $1.0{M}_{\odot}$ CO WD merger remnant. Panel (c): elemental abundance distribution and entropy profile of the merger remnant formed from the combination of an $8.0{M}_{\odot}$ AGB and a $1.0{M}_{\odot}$ CO WD.
• Figure 2: Structure profiles of the $8.0{M}_{\odot}$ AGB star + $1.0{M}_{\odot}$ CO WD merger remnant at different evolutionary stages. Panel (a1) and (a2): temperature, density, nuclear energy generation rate, and neutrino cooling rate (a1), and elemental abundance distribution (a2) at the onset of off-center carbon ignition; Panel (b1) and (b2): same as above, when the inwardly propagating carbon flame reaches the center; Panel (c1) and (c2): same as above, at the onset of off-center neon ignition.
• Figure 3: Evolution of the $8.0{M}_{\odot}$ AGB star + $1.0{M}_{\odot}$ CO WD merger remnant. Panel (a): HR diagram, where blue, golden, sea-green, cyan and red filled cycles mark different evolutionary stages. Magenta arrows indicate the evolutionary directions, with the corresponding timescale labeled. Gray diagonal lines denote constant stellar radii. Panel (b): time evolution of luminosity (red), radius (sea-green), central temperature (blue), and the mass coordinate of the maximum temperature (black). Cyan vertical dotted line marks the end of rapid envelope expansion phase, corresponding to the cyan filled cycle in panel (a).
• Figure 4: Similar to Fig. 2, but for the $3.0{M}_{\odot}$ AGB star + $0.5{M}_{\odot}$ CO WD merger remnant. Panel (a1) and (a2): H/He shell burning phase prior to off-center carbon ignition; Panel (b1) and (b2): at the onset of off-center carbon ignition; Panel (c1) and (c2): carbon shell burning phase.
• Figure 5: Evolutionary tracks on the HR diagram for the $24$ merger remnant models. Panel (a)-(f) represent models with different AGB core masses (CO WD masses). In each panel, lines with different colours correspond to models with different envelope masses (while the core mass remains constant). Black dashed, dotted and dash-dotted lines in each panel represent the evolutionary tracks of $5.0{M}_{\odot}$, $7.0{M}_{\odot}$ and $9.0{M}_{\odot}$ stars, respectively.