The effective stability parameter for two-component galactic discs: Is
  1/Q ~ 1/Q_stars + 1/Q_gas ?
  The Wang-Silk approximation, 1/Q ~ 1/Q_stars + 1/Q_gas, is frequently used
for estimating the effective Q parameter in two-component discs of stars and
gas. Here we analyse this approximation in detail, and show how its accuracy
depends on the radial velocity dispersions and Toomre parameters of the two
components. We then propose a much more accurate but still simple approximation
for the effective Q parameter, which further takes into account the stabilizing
effect of disc thickness. Our effective Q parameter is a natural generalization
of Toomre's Q, and as such can be used in a wide variety of contexts, e.g. for
predicting star formation thresholds in galaxies or for measuring the stability
level of galactic discs at low and high redshifts.
