Journal ArticleOnce a Triple, Not Always a Triple: The Evolution of Hierarchical Triples That Yield Merged Inner BinariesShariat, Cheyanne (ORCID:0000000312479349); Naoz, Smadar (ORCID:0000000298029279); El-Badry, Kareem (ORCID:0000000268711752); Rodriguez, Antonio C (ORCID:0000000341899668); Hansen, Bradley_M S (ORCID:0000000178403502); Angelo, Isabel (ORCID:0000000297512664); Stephan, Alexander P (ORCID:0000000182200548)<title>Abstract</title> <p>More than half of all main-sequence (MS) stars have one or more companions, and many of those with initial masses <8<italic>M</italic>_⊙are born in hierarchical triples. These systems feature two stars in a close orbit (the inner binary) while a tertiary star orbits them on a wider orbit (the outer binary). In hierarchical triples, three-body dynamics combined with stellar evolution drives interactions and, in many cases, merges the inner binary entirely to create a renovated “post-merger binary” (PMB). By leveraging dynamical simulations and tracking binary interactions, we explore the outcomes of merged triples and investigate whether PMBs preserve signatures of their three-body history. Our findings indicate that in 26%–54% of wide double white dwarf (DWD) binaries (<italic>s</italic>≳ 100 au), the more massive white dwarf (WD) is a merger product, implying that these DWD binaries were previously triples. Overall, we estimate that 44% ± 14% of observed wide DWDs originated in triple star systems and thereby have rich dynamical histories. We also examine MS+MS and MS+red giant mergers manifesting as blue straggler stars (BSSs). These PMBs have orbital configurations and ages similar to most observed BSS binaries. While the triple+merger formation channel can explain the observed chemical abundances, moderate eccentricities, and companion masses in BSS binaries, it likely only accounts for ∼20%–25% of BSSs. Meanwhile, we predict that the majority of observed single BSSs formed as collisions in triples and harbor long-period (>10 yr) companions. Furthermore, both BSS binaries and DWDs exhibit signatures of WD birth kicks.</p>The Astrophysical Journal2024-12-2410647811The Astrophysical Journal9781470004-637Xhttps://doi.org/10.3847/1538-4357/ad944a2206428National Science Foundation