  Solar-type binaries with short orbital periods ($P_{\rm close}$ $\equiv$ 1 -
10 days; $a$ $\lesssim$ 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 ($F_{\rm close}$ = 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 $F_{\rm close}$ $\approx$
0.4% after $\tau$ = 5 Myr, which increases to $F_{\rm close}$ $\approx$ 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 $P_{\rm out}$ (days) $\approx$ 2 - 5 ($a_{\rm out}$ $<$ 50 AU), can be
explained only with substantial extra energy dissipation due to interactions
with primordial gas.
