IK Pegasi and the Double Merger Path to Type Ia Supernovae

Barium abundance
Maximal barium abundance attained during the AGB phase for different metallicities (Karakas et al. 2016, 2018), plotted against the final WD mass of a single progenitor using the Cunningham et al. 2024 IFMR. For comparison, the positions of IK Peg (star) and the IK Peg–type candidate Gaia DR3 5550946678313327744 (diamond; Rekhi et al. 2026) are indicated. Both systems have near-Solar metallicity. (Hallakoun et al. 2026).

Recent Gaia astrometry has revealed thousands of main-sequence + white-dwarf binaries (MS+WD) at separations of ~0.1-10 au, including a subset hosting unusually massive (>~0.8 Msun) WDs. We argue that s-process enrichment in the non-degenerate companion provides a powerful diagnostic for identifying WDs that formed via mergers in hierarchical triple systems. For a massive WD, standard single-star evolution requires a massive (>~4 Msun) progenitor, yet such progenitors produce negligible s-process yields. We define IK Peg-type systems as those exhibiting this mass-yield tension: barium-enhanced companions orbiting WDs too massive to have descended from efficient s-process producers. The well-known system IK Peg exemplifies this class. Applying this framework to published spectroscopic data reveals several additional candidates, and we estimate that a few dozen such systems should exist in the current Gaia sample. If these systems trace inner-binary mergers in primordial triples, they represent observable intermediate stages towards eventual Type Ia supernovae via the double-merger pathway, as predicted by recent population-synthesis models.

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