  We reassess the applicability of the Toomre criterion in galactic disks and
we study the local star formation law in 16 disk galaxies for which abundance
gradients are published. The data we use consists of stellar light profiles,
atomic and molecular gas (deduced from CO with a metallicity-dependent
conversion factor), star formation rates (from H-alpha emissivities),
metallicities, dispersion velocities and rotation curves. We show that the
Toomre criterion applies successfully to the case of the Milky Way disk, but it
has limited success with the data of our sample; depending on whether the
stellar component is included or not in the stability analysis, we find average
values for the threshold ratio of the gas surface density to the critical
surface density in the range 0.5 to 0.7. We also test various star formation
laws proposed in the literature, i.e. either the simple Schmidt law or
modifications of it, that take into account dynamical factors. We find only
small differences among them as far as the overall fit to our data is
concerned; in particular, we find that all three SF laws (with parameters
derived from the fits to our data) match particularly well observations in the
Milky Way disk. In all cases we find that the exponent n of our best fit SFR
has slightly higher values than in other recent works and we suggest several
reasons that may cause that discrepancy.
