Solar-type binaries with short orbital periods ( P _ { close } \equiv 1 - 10 days ; a \lesssim 0.1 AU ) can not form directly via fragmentation of molecular clouds or protostellar disks , yet their component masses are highly correlated , suggesting interaction during the pre-main-sequence ( pre-MS ) phase .
Moreover , the close binary fraction of pre-MS stars is consistent with that of their MS counterparts in the field ( F _ { close } = 2.1 % ) .
Thus we can infer that some migration mechanism operates during the early pre-MS phase ( \tau \lesssim 5 Myr ) that reshapes the primordial separation distribution .
We test the feasibility of this hypothesis by carrying out a population synthesis calculation which accounts for two formation channels : Kozai-Lidov ( KL ) oscillations and dynamical instability in triple systems .
Our models incorporate ( 1 ) more realistic initial conditions compared to previous studies , ( 2 ) octupole-level effects in the secular evolution , ( 3 ) tidal energy dissipation via weak-friction equilibrium tides at small eccentricities and via non-radial dynamical oscillations at large eccentricities , and ( 4 ) the larger tidal radius of a pre-MS primary .
Given a 15 % triple star fraction , we simulate a close binary fraction from KL oscillations alone of F _ { close } \approx 0.4 % after \tau = 5 Myr , which increases to F _ { close } \approx 0.8 % by \tau = 5 Gyr .
Dynamical ejections and disruptions of unstable coplanar triples in the disk produce solitary binaries with slightly longer periods P \approx 10 - 100 days .
The remaining \approx 60 % of close binaries with outer tertiaries , particularly those in compact coplanar configurations with log P _ { out } ( days ) \approx 2 - 5 ( a _ { out } < 50 AU ) , can be explained only with substantial extra energy dissipation due to interactions with primordial gas .