  Bars in gas-rich spiral galaxies are short-lived. They drive gas inflows
through their gravity torques, and at the same time self-regulate their
strength. Their robustness has been subject of debate, since it was thought
that only the resulting central mass concentrations (CMCs) were weakening bars,
and only relatively rare massive CMCs were able to completely destroy them.
Through numerical simulations including gas dynamics, we find that with the gas
parameters of normal spiral galaxies, the CMC is not sufficient to fully
dissolve the bar. But another overlooked mechanism, the transfer of angular
momentum from the infalling gas to the stellar bar, can also strongly weaken
the bar. In addition, we show that gravity torques are correctly reproduced in
simulations, and conclude that bars are transient features, with life-time of
1-2 Gyr in typical Sb-Sc galaxies, because of the combined effects of CMCs and
gravity torques, while most existing works had focussed on the CMC effects
alone.
