2021ApJ...907L..19V

Stellar triples with massive stellar components are common and can lead to sequential binary black hole mergers. Here we outline the evolution toward these sequential mergers and explore these events in the context of gravitational-wave astronomy and the pair-instability mass gap. We find that binary black hole mergers in the pair-instability mass gap can be of triple origin and therefore are not exclusively formed in dense dynamical environments. We discuss the sequential merger scenario in the context of the most massive gravitational-wave sources detected to date: GW170729 and GW190521. We propose that the progenitor of GW170729 is a low-metallicity field triple. We support the premise that GW190521 could not have been formed in the field. We conclude that triple stellar evolution is fundamental to the understanding of gravitational-wave sources and likely other energetic transients as well.