We have obtained HST-WFPC2 F555W and F814W photometry for 16 fields in the vicinity of the luminous nearby spiral galaxy M31 , sampling the stellar content of the disk and the halo at different distances from the center , from \sim 20 to \sim 150 arcmin ( i.e . \sim 4.5 to 35 kpc ) , down to limiting V and I magnitudes of \sim 27 .

The Color-Magnitude diagrams ( CMD ) obtained for each field 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 .
According to this interpretation , we find that the star formation rate over the last 0.5 Gyr has varied dramatically with location in the disk .

The most evident feature of all the CMDs is 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 clearly detected in all of the fields , and a weak blue horizontal branch is frequently present .

The metallicity distributions , obtained by comparison of the RGB stars with globular cluster templates , all show a long , albeit scantly populated , metal-poor tail and a main component peaking at [ Fe/H ] \sim -0.6 .
The most noteworthy characteristic of the abundance distributions is their overall similarity in all the sampled fields , covering a wide range of environments and galactocentric distances .
Nevertheless , a few interesting differences and trends emerge from the general uniformity of the metallicity distributions .
For example , the median [ Fe / H ] shows a slight decrease with distance along the minor axis ( Y ) up to Y \simeq 20 \arcmin , but the metallicity gradient completely disappears beyond this limit .
Also , in some fields a very metal-rich ( [ Fe/H ] \geq -0.2 ) component is clearly present .

Whereas the fraction of metal-poor stars seems to be approximately constant ( within few percent ) in all fields , the fraction of metal-rich and , especially , 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 ( , e.g . ) ] iba01 .
This might indicate and possibly trace interaction effects with some companion , e.g .
M32 .