2022ApJ...925...69B

The LIGO/Virgo gravitational-wave observatories have detected at least 50 double black hole (BH) coalescences. This sample is large enough to have allowed several recent studies to draw conclusions about the implied branching ratios between isolated binaries versus dense stellar clusters as the origin of double BHs. It has also led to the exciting suggestion that the population is highly likely to contain primordial BHs. Here we demonstrate that such conclusions cannot yet be robust because of the large current uncertainties in several key aspects of binary stellar evolution. These include the development and survival of a common envelope, the mass and angular-momentum loss during binary interactions, mixing in stellar interiors, pair-instability mass loss, and supernova outbursts. Using standard tools such as the rapid population synthesis codes StarTrack and COMPAS and the detailed stellar evolution code MESA, we examine as a case study the possible future evolution of Melnick 34, the most massive known binary star system (with initial component masses of 144 M _☉ and 131 M _☉). We show that, despite its fairly well-known orbital architecture, various assumptions regarding stellar and binary physics predict a wide variety of outcomes: from a close BH-BH binary (which would lead to a potentially detectable coalescence), through a wide BH-BH binary (which might be seen in microlensing observations), or a Thorne-Zytkow object, to a complete disruption of both objects by a pair-instability supernova. Thus, because the future of massive binaries is inherently uncertain, sound predictions about the properties of BH-BH systems formed in the isolated binary evolution scenario are highly challenging at this time. Consequently, it is premature to draw conclusions about the formation channel branching ratios that involve isolated binary evolution for the LIGO/Virgo BH-BH merger population.