Ascertain the theoretical outcomes of stellar mergers and the properties of merger remnants

Ascertain the physical outcomes of stellar mergers in young multiple-star systems, including the resulting remnant’s composition, structure, rotation, and subsequent evolution, and develop models that reliably predict these outcomes across varying progenitor masses and merger histories.

Background

The simulation reveals that mergers are common, especially in massive systems, and can contribute substantially to stellar mass growth. However, accurately modelling post-merger remnants is difficult due to the sporadic and repeated nature of events and the diversity of progenitor masses.

The authors connect merger outcomes to broader astrophysical implications (e.g., magnetic massive stars, mass function shaping), explicitly noting that these theoretical outcomes remain insufficiently understood, underscoring the need for improved modelling and interpretation.

References

However, the theoretical outcome of these mergers is not yet fully understood, and modelling their remnants in our simulation is challenging due to their sporadic nature, repeated occurrences, and wide range of progenitor masses.

— Formation of massive multiple-star systems: early migration and mergers  (2601.06251 - Chon et al., 9 Jan 2026) in Section 3.4 (Mergers)

This channel, however, remains physically uncertain.

First, when two non-degenerate stars merge during CE evolution, the final product is uncertain because the stellar-merger process is not fully understood.

— A Census of Stellar-mass Black Holes in the Milky Way with POPKIN. I. Isolated Black Holes  (2609.29211 - He et al., 24 Sep 2026) in Section 8.4, “Uncertainties in CE Mergers”

We note, however, that slow envelope mixing after the merger can not fundamentally be ruled out, which could raise the N/C-surface value.

— Chemical fingerprints of binary mass transfer in massive stars  (2608.11940 - Jin et al., 12 Aug 2026) in Mergers, paragraph beginning “Stellar mergers involve complex physics”

A detailed calculation of the global evolution of a double-degenerate WD merger remnant, including the hydrostatic giant phase, will be an important subject for future work.

— The Ultrafast Line-Driven Wind from the Double-Degenerate Merger Remnant WD J005311  (2608.19037 - Kato et al., 19 Aug 2026) in Section 6, “Conclusion”

Which WD mergers end in such a collapse -- rather than in a thermonuclear explosion or a stable massive WD -- is not reliably known; it has been suggested that super-Chandrasekhar Carbon-Oxygen (CO)+CO mergers with sufficiently unequal masses (for which carbon ignites off-center and burns the remnant to Oxygen-Neon (ONe) rather than exploding it), as well as mergers involving ONe WDs, follow this path (e.g. \citealt{Yoon07,Shen12,Schwab16,Kremer26}, and references therein).

— 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 5.2, “A double WD merger with a delayed collapse”

The main uncertainty is quantitative: existing calculations do not robustly demonstrate the ejection of the required $\sim0.3$--$3\,M_\odot$ at tens to hundreds of km\,s${-1}$.

— 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 6, Discussion

Because detailed abundance calculations were not performed in that work, it remains unclear whether carbon and oxygen would be significantly enriched in the hot subdwarf formed from such a merger.

— On the origin of carbon- and oxygen-rich hot subdwarfs: an sdB merger scenario  (2609.37320 - Tang et al., 29 Sep 2026) in Section 1, Introduction