The effect of radial gas flows on the chemical evolution of the Milky
  Way and M31
  We present detailed chemical evolution models for the Milky Way and M31 in
presence of radial gas flows. These models follow in detail the evolution of
several chemical elements (H, He, CNO, $\alpha$ elements, Fe-peak elements) in
space and time. The contribution of supernovae of different type to chemical
enrichment is taken into account. We find that an inside-out formation of the
disks coupled with radial gas inflows of variable speed can reproduce very well
the observed abundance gradients in both galaxies. We also discuss the effects
of other parameters, such as a threshold in the gas density for star formation
and efficiency of star formation varying with galactic radius. Moreover, for
the first time we compute the galactic habitable zone in our Galaxy and M31 in
presence of radial gas flows. The main effect is to enhance the number of stars
hosting a habitable planet with respect to the models without radial flow, in
the region of maximum probability for this occurrence. In the Milky Way the
maximum number of stars hosting habitable planets is at 8 kpc from the Galactic
center, and the model with radial gas flows predicts a number of planets which
is 38% larger than that predicted by the classical model.
