1994A&A...290..371H

Magnier et al. (1993) recently identified 174 OB associations in M 31 using the PLC method (Battinelli 1991) with the Magnier et al. (1992) catalogue. The objectivity of the PLC method combined with the depth and accuracy of the Magnier et al. BVRI CCD data make these identifications suitable for photometric studies. We have studied the following properties of the 11 richest associations, 9 groups of associations, the eastern and western arms of M 31, and the entire sample of associations: the extinction E_B–V, the slope of the differential luminosity function (dLF) in B and V, and the slope of the initial mass function (IMF). The data are not of sufficient quality to determine the ages of associations more accurately than to show that they are younger than a few 10⁷ years old. The extinction ranges between 0.20 and 0.41 magnitudes (±0.05mag); the western arm is significantly more reddened than the eastern arm. We have also found evidence of a difference between the young stellar populations of the eastern and western arms. The difference is most pronounced in the slopes of the B band dLF of the two arms (east: 0.46±0.02, west: 0.79±0.04, {DELTA}: 0.33±0.045). There is also a marginal difference seen in both the slope of the V band dLF (east: 0.48±0.04, west: 0.64±0.05, {DELTA}: 0.16±0.064) and the slope of the IMF (east: -2.09±0.30, west: -3.02±0.33, {DELTA}: 0.93±0.45). The combined weight of these three pieces of evidence suggests a real difference between the stellar populations of the two arms. It is possible that the difference is not physical, but an observational effect due to, e.g., the different reddening, varying photometric scatter between the two arms or different resolution of the images.