Dynamical Formation of Close Binaries during the Pre-main-sequence Phase

Solar-type binaries with short orbital periods (P_close≡ 1-10 days; a ≲ 0.1 au) cannot 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 (τ ≲ 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 ≈ 0.4% after τ = 5 Myr, which increases to F_close ≈ 0.8% by τ = 5 Gyr. Dynamical ejections and disruptions of unstable coplanar triples in the disk produce solitary binaries with slightly longer periods P ≈ 10-100 days. The remaining ≈ 60% of close binaries with outer tertiaries, particularly those in compact coplanar configurations with log P_out (days) ≈ 2-5 (a_out < 50 au), can be explained only with substantial extra energy dissipation due to interactions with primordial gas.