2020ApJ...898...99H

The gravitational wave source GW190412 is a binary black hole (BBH) merger with three unique properties: (i) its mass ratio is about 0.28, the lowest found so far, (ii) it has a relatively high positive effective spin parameter χ_eff=0.25, and (iii) it is observed to be precessing due to in-plane projected spin of the binary with an in-plane precession parameter χ_p=0.3. The two main formation channels of BBH formation fail to account for GW190412: field formation scenarios cannot explain the observed precession unless by invoking large natal kicks, and dynamical assembly in dense stellar systems is inefficient in producing such low-mass-ratio BBH mergers. Here, we investigate whether "double mergers" in wide hierarchical quadruple systems in the "3 + 1" configuration could explain the unique properties of GW190412. In this scenario, a compact object quadruple system experiences two mergers: first, two compact objects in the innermost orbit merge due to secular chaotic evolution. At a later time, the merged compact object coalesces with another compact object due to secular Lidov-Kozai oscillations. We find that our scenario is consistent with GW190412. In particular, we find a preferential projected spin around χ_p=0.2. However, the likelihood of a double merger is small and the formation efficiency of these systems is uncertain. If GW190412 originated from a double merger in a 3 + 1 quadruple, we find a strong constraint that the first merger likely occurred between roughly equal-mass BHs in the innermost orbit, since the recoil velocity from unequal-mass BHs would otherwise have disrupted the system.