The Origin of inequality: isolated formation of a 30+10Msun binary black-hole merger

Abstract: The LIGO/Virgo collaboration has reported the detection of GW190412, a BH-BH merger with the most unequal masses to date: 24.4-34.7 Msun and 7.4-10.1 Msun (a mass ratio of q=0.21-0.41). Additionally, GW190412's effective spin was estimated to be Xeff=0.14-0.34, with the spin of the primary BH in the range a=0.17-0.59. Based on this and prior detections, about 10 percent of BH-BH mergers have q<0.4. Major BH-BH formation channels tend to produce BH-BH mergers with comparable masses (typically with 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 (masses and spins). 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<0.41 the rate density is: 6.8 Gpc^-3 yr^-1. As GW190412 shows some weak evidence for misaligned spins, we provide distribution of precession parameter in our models and conclude that if among the new LIGO/Virgo detections the evidence of system precession is strong and more than 10 percent of BH-BH mergers have large in-plane spin components (Xp>0.5) then common envelope isolated binary BH-BH formation channel can be excluded as their origin.