*3.2. M33*

Neugent et al. completed the first galaxy-wide survey for WRs using a combination of the image subtraction and photometric method as discussed above [78]. Overall, they discovered 54 new WRs bringing the total number of confirmed WRs in M33 up to 206, a number they believe is complete to ∼5%. A majority of these new discoveries were WNs suggesting that the previous WC/WN ratio had been biased towards the easier to find WCs. The locations of the known WRs across the disk of the galaxy are shown in Figure 6. Notice that the galaxy has been divided up into three regions representing the strong metallicity gradient with the inner region having a higher metallicity than the outer region.

As discussed in the Introduction, the formation of WRs is highly dependent on mass-loss rates, which is, in turn, dependent on the metallicity of the environment. In higher metallicity environments, the mass-loss rates will be higher leading to the creation of more WCs. Thus, we expect the WC/WN ratio to be higher in regions of high metallicity, such as in the center of M33. Indeed, this is what we find. While the full comparison of WC/WN ratios vs. metallicity will be discussed later, Table 1 shows the WC/WN ratio vs. metallicity for the inner, middle, and outer regions of M33. (The cut-offs for these regions are a little different than had been used in the earlier study by [73] shown in Figure 2).

**Figure 6.** Location of known WC and WN stars in M33. WN stars are represented as blue ×s while WC stars are represented as red +s. The green ovals represent distances of *ρ* = 0.25 (1.9 kpc) and *ρ* = 0.50 (3.8 kpc) within the plane of M33. The metallicity gradient extends outward with higher metallicity in the middle and lower in the outer regions; this figure is from [78].

**Table 1.** WC/WN ratio vs. metallicity for the inner, middle, and outer regions of M33.


The metallicity gradient of M33 also allows us to probe the relative number of early and late type WCs vs. metallicity. Smith first discovered that nearly all of the late-type WCs are found in higher metallicity environments than the early-type WCs [85]. Additionally, late-type WCs have CIV *λ*5806 lines that both have smaller equivalent widths and smaller full width half max values than early-type WCs. Thus, plotting these two values against each other vs. metallicity shows that the spectral type becomes earlier as metallicity decreases. This is shown in Figure 7. This proves, independent of any direct metallicity measurements, that the metallicity of M33 increases towards the center of the galaxy.

**Figure 7.** WC line strength vs. line width. The line strength of the CIV *λ*5806 line is plotted against its line width with the spectral subtypes indicated (with "C" used if the subtype has not been well established). Notice how the FWHM increases and subtype decreases as the metallicity decreases (larger values of *ρ*); this figure is from [78].

With this new data discussed in Neugent and Massey, the WC/WN ratio was determined for three regions of medium to high metallicity [78] and the number of WRs was thought to be complete to 5%.
