Program: GN-2021B-Q-143

We have recently discovered a new type of Wolf-Rayet (WR) star in the LMC. These "WN3/O3" stars have physical properties similar to other WN-type WRs, showing enhanced He and N abundances, but have mass-loss rates more like that of O-type stars. Are these stars the product of binary evolution, or are they the "missing link" between O-type stars and Wolf-Rayet stars? The key to answering this lies with determining the metallicity range over which WN3/O3s form. So far, all the known WN3/O3s are found in the LMC, where the metallicity is about 1/2 solar. If they are found at all metallicities, then the binary explanation is still viable, but if they occur only over a small metalicity range, then they are more likely the product of single star evolution, and may be a hitherto unrecognized normal stage in the lives of most massive stars at some sub-solar metallicites. We have conducted a deep search for these objects in the nearby spiral galaxy M33, which has a strong metallicity gradient, and found 10 potential candidates. Thanks to Gemini/GMOS time in 2018B and 2020B we have now observed 60% of this sample, confirming several WN3/O3s, but sadly, none of the four WN3/O3 candidates in the high metallcity central region of M33 have been observed yet. We now need the spectroscopy that only the large aperture of Gemini (and dark skies and good seeing of Mauna Kea) can provide to determine the metallicity range over which these stars form.