IK Pegasi and the Double Merger Path to Type Ia Supernovae

TL;DR

The paper identifies IK Peg-type systems—MS stars enriched in s-process elements orbiting unusually massive white dwarfs—as empirical tracers of inner-binary mergers in hierarchical triples, a pathway that can lead to Type Ia supernovae via a double-merger. By defining robust abundance-based criteria and applying them to Gaia MS+WD samples, the authors argue that a few dozen such systems likely exist in the current data, representing observable intermediate stages toward SNe Ia. They outline plausible formation histories, including dynamical evolution and selective chemical enrichment of the tertiary, and discuss validation strategies in open clusters and targeted rate estimates. The work positions chemical enrichment as a scalable fossil record to reconstruct complex evolutionary channels and connect Gaia populations to the SN Ia progenitor problem, with near-term opportunities through Gaia DR4 and dedicated spectroscopic campaigns.

Abstract

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.

Figures (8)

• Figure 1: IK Peg A element abundance relative to iron, from Smalley_1996, and from Stateva_2012. Open triangles mark upper limits.
• Figure 2: The relationship between WD mass, companion metallicity, and Ba abundance. Gaia MS+WD systems with measured Ba abundances are plotted as diamonds Rekhi_2024 and circles Rekhi_2026. Known Ba dwarfs (pre-red giant branch; pre-RGB) hosting WD companions from Escorza_2023 are shown as triangles, while star symbols denote Ba giants. Filled symbols indicate Ba-enriched systems, defined as $\text{[Ba/Fe]} > +0.25$ for the Rekhi_2024Rekhi_2026 samples, or classified as Ba dwarfs/strong Ba giants in the Escorza_2023 sample. The red dashed line delineates the tentative empirical limit beyond which barium enrichment is inconsistent with a single-progenitor scenario. The green dotted line marks the most conservative limit, based on single-progenitor evolution models (see Figure \ref{['fig:BaFeVsMwd']}).
• Figure 3: Maximal [Ba/Fe] abundance attained during the AGB phase for metallicities $\text{[Fe/H]} \approx -0.7$Karakas_2018, $-0.3$, $0$, and $+0.3$Karakas_2016, plotted against the final WD mass of a single progenitor using the Cunningham_2024 IFMR. For comparison, the positions of IK Peg (star) and the IK Peg--type candidate Gaia DR3 5550946678313327744 (diamond; Rekhi_2026) are indicated. Both systems have near-Solar metallicity.
• Figure 4: Comparison of initial and final masses adapted from Ironi_2025. Arrows and gray dots mark the cluster turn-off mass and the median progenitor mass (assuming a Kroupa_2001 IMF), respectively. Open circles indicate systems consistent with the Cunningham_2024 IFMR (solid line; $1\sigma$ uncertainty in gray). The pentagon marks the estimated position of the Blue Lurker WOCS 14020 Leiner_2025.
• Figure 5: Comparison of the [Ba/Fe] abundances for the Rekhi_2024 sample as reported in GALAH DR3 versus those derived using pysme (following the methodology of Rekhi_2026). The red dashed line indicates the 1:1 relation for reference.