*3.3. M31*

The next study was done in M31 by Neugent et al. [77] which has an even higher metallicity than that of the inner region of M33. By using the same detection methods of interference filter imaging, image subtraction and photometry, they discovered 107 new WRs (79 WNs and 28 WCs) bringing the total number of WRs in M31 up to 154, a number they argue is good to within 5%. They additionally found that 86% of the observed WRs were within known OB associations as determined by van den Bergh [86]. The locations of the WRs are shown in Figure 8. Due to the addition of the new WNs, the WC/WN ratio dropped from 2.2 down to 0.67. While this helped bring the observations closer to that of the theoretical model predictions, the full story will be told in Section 7.

**Figure 8.** Locations of all known WR stars in M31. The blue ×s represent WN stars while the red +s represent WC stars. The inner black ellipse is at 9 kpc (*ρ* = 0.43) within the plane of M31, and the outer one is at 15 kpc (*ρ* = 0.71), which represents the location of the majority of OB forming associations within M31; this figure is from [77].

Subsequent to the this study, Mike Shara and collaborators discovered an additional WR star in M31, a WN/C star [87]. Such objects have WN-like spectra but strong C IV *λ*5806,12 line. The star is located in strong nebulosity and is described as heavily reddened (although no specific values are given), and the authors speculate based on this one object that there might be a large population of unfound WRs lying on the "far side" M31's disk, i.e., that only lightly reddened specimens have been found so far. Is this reasonable? First, we note that the width of the "blue plume" (denote OB stars) in the color magnitude diagram of M31 has a similar width to that of the LMC; compare Figures 10 and 12 in [88]. If there were a huge population of highly reddened stars, we would expect the blue plume to be high asymmetric, with a large tail extending to redder magnitudes. Secondly, we can do a crude estimate of what we might expect. We note that the total extinction through the MW's disk is ∼0.4 mag in B [89]. If M31 is similar, then, at an inclination of 77◦ to the line of sight, we expect the total extinction in B from one side to the other to be about 1.8 mag, or in V, about 1.4 mag. This is only 0.6 mag greater than the 0.8 mag in *AV* found for OB stars in some of the handful of well-studied

OB associations [72], and is unsurprising. Thus, although a handful of heavily reddened WRs may certainly have been missed (consistent with the ten that Shara et al. estimate), it seems unlikely that there is an opaque wall obscuring WRs on the far side of M31.
