Making GW190412: isolated formation of a 30+10 Msun binary black-hole merger

The LIGO/Virgo collaboration has reported the detection of GW190412, a black hole-black hole (BH-BH) merger with the most unequal masses to date: m_1=24.4--34.7 Msun and m_2=7.4--10.1 Msun, corresponding to a mass ratio of q=0.21--0.41 (90 per cent probability range). Additionally, GW190412's effective spin was estimated to be X_eff=0.14--0.34, with the spin of the primary BH in the range a_spin=0.17--0.59. Based on this and prior detections, >10 per cent of BH-BH mergers have q 0.5). Here we test whether the classical isolated binary evolution channel can produce mergers resembling GW190412. We show that our standard binary evolution scenario, with the typical assumptions on input physics we have used in the past, produces such mergers. We provide an explicit example of an unequal mass BH-BH merger, which forms at low metallicity (Z=0.002) from two massive stars (77 and 35 Msun) and results in a BH-BH merger with m_1=27.0 Msun, m_2=9.9 Msun (q=0.37), with primary black hole spin a_spin=0.19 and an effective spin parameter X_eff=0.335. For this particular model of the input physics the overall BH-BH merger rate density in the local Universe (z=0) is: 73.5 Gpc^-3 yr^-1, while for systems with q<0.21, 0.28, 0.41, and 0.59 the rate density is: 0.01, 0.12, 6.8, and 22.2 Gpc^-3 yr^-1, respectively. The results from our standard model are consistent with the masses and spins of the black holes in GW190412, as well as with the LIGO/Virgo estimate of the fraction of unequal-mass BH-BH mergers.