With recent advances in gravitational-wave astronomy , the direct detection of gravitational waves from the merger of two stellar-mass compact objects has become a realistic prospect .
Evolutionary scenarios towards mergers of various double compact objects generally invoke so-called common-envelope evolution which is poorly understood , leading to large uncertainties in the predicted merger rates .
Here we explore , as an alternative , the scenario of massive overcontact binary ( MOB ) evolution , which involves two very massive stars in a very tight binary which remain fully mixed due to their tidally induced high spin .
While many of these systems merge early-on , we find large numbers of MOBs which swap mass several times but survive as a close binary until the stars collapse .
The simplicity of the MOB scenario allows us to use the efficient , public stellar-evolution code MESA for exploring it systematically by means of detailed numerical calculations .
We find that , at low metallicity , MOBs produce double-black-hole ( BH+BH ) systems that will merge within a Hubble time with mass-ratios close to one , in two mass ranges , \sim 25 \dots 60 \mathrm { M } _ { \odot } and \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ > $ } } } 130 % \mathrm { M } _ { \odot } , with pair-instability supernovae ( PISNe ) being produced at intermediate masses .
Our models are also able to reproduce counterparts of various stages in the MOB scenario in the local Universe , providing direct support for the scenario .
We map the initial binary parameter space that produces BH+BH mergers , determine the expected chirp mass distribution , merger times , the expected Kerr parameters and predict event rates .
We typically find that for Z \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ < $ } } } Z _ % { \odot } / 10 , there is one BH+BH merger event for \sim 1 , 000 core-collapse supernovae .
The advanced LIGO ( aLIGO ) detection rate is more uncertain and depends on the metallicity evolution .
Deriving upper and lower limits from a local and a global approximation for the metallicity distribution of massive stars , we estimate aLIGO detection rates ( at the aLIGO design limit ) of \sim 19 - 550 yr ^ { -1 } for BH-BH mergers below the PISN gap and of \sim 2.1 - 370 yr ^ { -1 } above the PISN gap .
Even with conservative assumptions , we find that aLIGO should soon detect BH+BH mergers from the MOB scenario and that these could be the dominant source for aLIGO detections .