  Galaxy surveys have suggested that rapid and sustained decrease in the
star-formation rate, "quenching", in massive disk galaxies is frequently
related to the presence of a bar. Optical and near-IR observations reveal that
nearly 60% of disk galaxies in the local universe are barred, thus it is
important to understand the relationship between bars and star formation in
disk galaxies. Recent observational results imply that the Milky Way quenched
about 9-10 Gyr ago, at the transition between the cessation of the growth of
the kinematically hot, old, metal-poor thick disk and the kinematically colder,
younger, and more metal-rich thin disk. Although perhaps coincidental, the
quenching episode could also be related to the formation of the bar. To explore
the relation between bar formation and star formation in gas rich galaxies
quantitatively, we simulated gas-rich disk isolated galaxies. Our simulations
include prescriptions for star formation, stellar feedback, and for regulating
the multi-phase interstellar medium. We find that the action of stellar bar
efficiently quenches star formation, reducing the star-formation rate by a
factor of 10 in less than 1 Gyr. Analytical and self-consistent galaxy
simulations with bars suggest that the action of the stellar bar increases the
gas random motions within the co-rotation radius of the bar. Indeed, we detect
an increase in the gas velocity dispersion up to 20-35 km/s at the end of the
bar formation phase. The star-formation efficiency decreases rapidly, and in
all of our models, the bar quenches the star formation in the galaxy. The
star-formation efficiency is much lower in simulated barred compared to
unbarred galaxies and more rapid bar formation implies more rapid quenching.
