Carbon star survey in the local group. V. The outer disk of M31

We employ the CFH12K mosaic to identify carbon stars, using the R, I, CN, and TiO photometric technique, in a 2240 arcmin(2) area, ranging from 17 to 30 kpc of the southwest disk of M31, barely reaching the edge of the observed H I disk. We found 945 C stars with [I-0] = 19.94 and sigma = 0.47. The surface density of C stars along the major axis of M31 follows an exponential pro le with a scale length of 4.85 +/- 0.35 kpc, in agreement with adopted values for the scale length of the disk population. Our survey partially overlaps with the recently discovered G1 density enhancement by Ferguson et al. We confirm that no AGB star excess is detectable in the surveyed part of the clump. The C/M ratio, along the major axis, is derived over a distance range of 7 kpc. The strong C/M gradient seen contrasts with results of previous studies of the C stars in M31.