Star formation efficiency and flattened gradients in M31

We present a chemical evolution model for M31 based on a pronounced inside-out formation scenario. The model reproduces the three main observational constraints of the M31 disk: the radial distributions of the total baryonic mass, the gas mass, and the O/H abundance. The model shows good agreement with the observed: SFR(r), Mstars(r), the C/H, N/H, Mg/H, Si/H, S/H, Ar/H, Cr/H, Fe/H, and Z gradients. From reproducing the observed gas mass, we find that the star formation efficiency is variable in space, for the whole disk, and is constant in time for most of the evolution (t< 12.8 Gyr). From reproducing the observed SFR, we find that the efficiency decreases almost to zero for 12.8<t(Gyr)< 13.0 and r>12 kpc. All the predicted Xi/H(r) gradients show three different slopes, due to the r-dependence of the the star formation efficiency and the inside-out galactic formation.