Close binaries in hierarchical stellar systems

Many (but not all) close binaries are inner pairs in triple and higherorder hierarchical systems. The statistical relation between close binaries and hierarchical systems is well established. Although close binaries can form by a combination of Kozai-Lidov cycles with tides, the study of Moe & Kratter (2018) indicated that only a fraction of them could be produced by this mechanism and that many close binaries form even before stars contract onto the main sequence. The predicted statistical signatures of tidal formation, such as accumulation of periods just below the tidal cutoff, are not observed. Instead, the main channel of close-binary formation is likely accretion-driven migration. The relation between close binaries and triple stars is not casual, it derives from the common factor which is the accretion strength. High accretion creates massive stars that have a large fraction of close binaries and, at the same time, a large fraction of hierarchical systems.