  We discuss the mechanism(s) of bar formation in isolated and tidally
interacting disk galaxies using the results of idealized collisionless Nbody
simulations of the galaxies. In order to better understand the mechanism, we
investigate orbital eccentricities (e), epochs of apocenter passages (t_a),
azimuthal angles at t_a (varphi_a), precession rates (Omega_pre), for
individual stars, as well as bar strengths represented by relative m=2 Fourier
amplitude (A_2) and bar pattern speeds (Omega_bar). The main results are as
follows. A significant fraction of stars with initially different varphi_a and
Omega_pre in an isolated disk galaxy can have similar values within several
dynamical timescales. This synchronization of varphi_a and Omega_pre, which is
referred to as apsidal precession synchronization (``APS'') in the present
study, is caused by the enhanced strength of the tangential component of
gravitational force. A weak seed bar (A_2<0.1) is first formed through APS in
local regions of a disk, then the bar grows due to APS. In the bar growth phase
(0.1<A_2<0.4), APS can proceed more efficiently due to stronger tangential
force from the bar so that it can enhance the bar strength further. This
positive feedback loop in APS is the key physical mechanism of bar growth in
isolated stellar disks. Bar formation can be severely suppressed in disks with
lower disk mass fractions and/or higher $Q$ parameters due to much less
efficient APS. APS proceeds more rapidly and more efficiently due to strong
tidal perturbation in the formation of tidal bars compared to spontaneous bar
formation.
