How important is secular evolution for black hole and neutron star mergers in 2+2 and 3+1 quadruple-star systems?

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Mergers of black holes (BHs) and neutron stars (NSs) result in the emission of gravitational waves that can be detected by LIGO.

• In this paper, we look at 2+2 and 3+1 quadruple-star systems, which are common among massive stars, the progenitors of BHs and NSs.
• We carry out a detailed population synthesis of quadruple systems using the MSE code, which seamlessly takes into consideration stellar evolution, binary and tertiary interactions, $N$-body dynamics, and secular evolution.
• We find that, although secular evolution plays a role in compact object (BH and NS) mergers, (70--85) \% (depending on the model assumptions) of the mergers are solely due to common envelope (CE) evolution.