  We investigate whether the formation mechanism of boxy and peanut-shaped
(B/PS) bulges could depend on the gas content of the galaxy. We have performed
N-body simulations with and without a gaseous component. In the second case
star formation/feedback recipes have also been implemented to create new
stellar populations. As in many previous studies, in our N-body collisionless
simulation, the B/PS is due to the classical break in the z mirror symmetry
lasting roughly 200 Myr. When a gaseous component and star formation recipes
are added to the simulation, the bulge-growing mechanism is quite different.
The young stellar population that is born in the thin gaseous disc rapidly
populates vertical resonant orbits triggered by the combined effects of the
linear horizontal and vertical ILRs. This leads to a B/PS bulge mainly made of
stellar material younger than the surrounding population. The non-linear
analysis of the orbital structure shows that the main orbit family responsible
for the B/PS is not the same in the two cases. The 2:2:1 orbits prevail in the
collisionless simulation whereas additional asymmetrical families contribute to
the B/PS if a dissipative component is present and can form new stars. We found
that 2:3:1 and 2:5:1 orbits trap a significant fraction of the mass. A flat
ringed discy stellar component also appears simultaneously with the thickening
of the young population. It is due to the star formation in a nuclear gaseous
disc located in the central kpc, inside the ILR, and accumulated there by the
torques exerted by the large-scale bar. Remarkably, it remains flat throughout
the simulation although it develops a nuclear bar, leading to a double-barred
galaxy. We predict that two populations of B/PS bulges could exist and even
coexist in the same galaxy.
