Delayed compact-object–core merger after common-envelope ejection

Determine whether a failed common-envelope ejection can produce a delayed compact-object–core merger with a post-common-envelope orbital-decay time of approximately 10^2–10^4 years and a long-lived, steady, low-power engine consistent with FRB 121102.

Background

The common-envelope scenario requires the envelope to be incompletely ejected, followed by orbital decay of the compact companion through residual envelope material and eventual merger with the stellar core. The delay would allow the slowly expanding envelope to form the confining shell before the engine turns on.

The paper states that the post-common-envelope inspiral time has not been reliably calculated and that the delayed, low-power merger variant lacks detailed models of its mass ejection, engine power, and longevity. It therefore identifies both the probability of realizing the delayed-merger channel and the ability of the resulting engine to remain steady for decades as explicit open questions.

References

However, the delay between core collapse and the compact-object merger depends on the formation and migration of the fragments and thus remains uncertain. It is also unclear whether this channel is preferentially associated with a particular core-collapse SN subtype.

— WFST Follow-up of S251112cm: Searching for an Optical Counterpart to a Subsolar-mass Compact-binary Merger Candidate  (2609.18523 - Liu et al., 16 Sep 2026) in Section 6, subsection “Superkilonova-like Counterparts”

The main open questions are whether the delayed-merger channel (as opposed to prompt merger during the CE, or successful ejection with no merger) is realized with a reasonable probability, and whether the engine can remain as steady as required over decades.

— FRB 121102: No supernova-like ejecta or magnetar power, hinting at a binary WD merger  (2608.26567 - Waxman et al., 27 Aug 2026) in Section 5.3, “Common envelope ejection with a delayed compact-object--core merger”