SECULAR EVOLUTION OF ISOLATED BARRED GALAXIES .2. COUPLING BETWEEN STARS AND INTERSTELLAR-MEDIUM VIA STAR-FORMATION

The secular evolution of isolated barred galaxies is studied by means of fully self-consistent 3D numerical simulations with stars, gas, sta r formation, and radiative cooling. The formation of a strong bar in a typical Sc disc produces a starburst of intermediate power inside the bar and in the centre. Very young bars (less than or similar to 500 M yr) are characterised by intense star formation along their major axis (essentially observed in SBc) whereas star formation in older bars pr edominates either at the centre or along a nuclear ring and along an i nner ring (mainly observed in SBb or SBa). Newly formed stars in the c entral regions are very easily swept out of the plane by vertical reso nances which results in the formation of a small young bulge. These ar e clear evidences of an evolution from late-type to early-type galaxie s. Our method to simulate star formation, based on Toomre's criterion, naturally reproduces in the disc the observations of (i) the threshol d of star formation at low gas surface densities (less than or similar to 7 M. pc(-2)), (ii) th, mild power-law dependence of the star forma tion rate (SFR) with the gas surface density (SFR similar to Sigma(g)( approximate to 1.3)) at higher gas densities (less than or similar to, 13 Mo pc(-2)), and, (iii) the highly non-linear behaviour in-between if energy release from supernovae is allowed. In the bar region, energ y release leads to a significant alteration of the power-law relation between SFR and gas surface density, i.e. the ''Schmidt law'' shows a deep trough. The model parameters which mainly influence the rate of s tar formation are the gas mass fraction, the amount of mechanical ener gy released by supernovae and winds, the presence or absence of a bar, and the effectiveness of radiative cooling. The model parameters whic h mainly influence the space distribution of star formation are the st ar formation efficiency, the amount mechanical energy released, and th e presence or absence of a bar. In general, the radiative cooling is s o efficient that the mechanical energy injected is more important than the thermal part. Gaseous discs including star formation are supporte d by the turbulent pressure generated by mechanical energy release. Ov er most of the galaxy, the slopes of pre-existing abundance gradients in both gas and stars are considerably reduced over a few dynamical ti mescales by the formation of a bar. As observed, the stronger the bar, the shallower the chemical composition gradient. However, the intense central star formation steepens the abundance gradients in the corres ponding region (less than or similar to 1 kpc). In some models, the fu rious star formation along the bar is able to steepen the abundance gr adient inside the bar. At a fixed radius, scatter around the mean gase ous abundance comes from a large-scale variation between arm and inter arm regions (about 0.6 - 0.8 dex), and an intrinsic, small-scale scatt er (about 0.3 - 0.4 dex). The azimuthal variation is damped with time unless spiral density waves are regenerated.