  HST-WFPC2 F555W and F814W photometry were obtained for 16 fields of the
luminous nearby spiral galaxy M31, sampling the stellar content of the disk and
the halo at different distances from the center, from ~ 20 to ~ 150 arcmin
(i.e. ~ 4.5 to 35 kpc), down to limiting V and I magnitudes of ~ 27. The
Color-Magnitude diagrams (CMD) show the presence of complex stellar
populations, including an intermediate age/young population and older
populations with a wide range of metallicity. Those fields superposed on the
disk of M31 generally show a blue plume of stars which we identify with main
sequence members. Accordingly, the star formation rate over the last 0.5 Gyr
appears to have varied dramatically with location in the disk. All the CMDs
show a prominent Red Giant Branch (RGB) with a descending tip in the V band,
characteristic of metallicity higher than 1/10 Solar. A red clump is detected
in all of the fields, and a weak blue horizontal branch is frequently present.
The metallicity distributions (MDs), obtained by comparison of the RGB stars
with globular cluster templates, are basically similar in all the sampled
fields: they all show a long, albeit scantly populated metal-poor tail and a
main component at [Fe/H] ~ -0.6. However, some differences also exist, e.g. in
some fields a very metal-rich ([Fe/H] >= -0.2) component is present. Whereas
the fraction of metal-poor stars seems to be approximately constant in all
fields, the fraction of very-metal-rich stars varies with position and seems to
be more prominent in those fields superposed on the disk and/or with the
presence of streams or substructures. This might indicate and possibly trace
interaction effects with some companion, e.g. M32.
