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Abstract

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
c_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 c_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 c_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.