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

Abstract: 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. Significant eccentricities in the LIGO band (higher than 0.01) are only obtained with zero supernova (SNe) kicks and are directly linked to the role of secular evolution. A similar outlier effect is seen in the $\chi_{\mathrm{eff}}$ distribution, with negative values obtained only with zero SNe kicks. When kicks are taken into account, there are no systems that evolve into a quadruple consisting of four compact objects. For our fiducial model, we estimate the merger rates (in units of $\Gpcyr$) in 2+2 quadruples (3+1 quadruples) to be 10.8 $\pm$ 0.9 (2.9 $\pm$ 0.5), 5.7 $\pm$ 0.6 (1.4 $\pm$ 0.4) and 0.6 $\pm$ 0.2 (0.7 $\pm$ 0.3) for BH-BH, BH-NS and NS-NS mergers respectively. The BH-BH merger rates represent a significant fraction of the current LIGO rates, whereas the other merger rates fall short of LIGO estimates.