2.2.2. The Small Magellanic Cloud

The identification of WRs in the SMC followed a similar pattern, but, thanks to its smaller angular size compared to the LMC, a complete census became possible earlier than for the galaxies discussed above. As summarized in an earlier review [45], four WRs had been found by general spectroscopic studies [46] when Azzopardi and Breysacher used the same technique of objective prism and interference filter photography to find four additional WRs, bringing the total up to eight [47]. A ninth WR was found by spectroscopy from objective prism photography [48]. In 2001, Massey and summer student Alaine Duffy carried out the first CCD survey for WRs in the SMC [49]. They used an on-band, off-band interference filter imaging campaign with the wide-field CCD camera on the CTIO Curtis Schmidt to cover most of the SMC. Photometry of 1.6 million stellar images helped identify a number of candidates, including all of the known SMC WRs, at high significance levels. Two new WNs were then confirmed by follow-up spectroscopy, bringing the total to 11. The survey also found a number of Of-type stars, demonstrating that the survey was sensitive to even the weakest-lined WNs. However, shortly after this, a 12th WR star was discovered in the SMC [50]. This star had been too crowded to have been found in the Massey and Duffy survey. Of these 12 WRs, 11 are of WN-type and only 1 is of WC-type. (Actually, the strength of O VI lines qualifies this as a WO-type star [20].) This low WC/WN ratio is consistent with our expectations based upon the SMC's low metallicity.

Quantitative studies of the strength of He II *λ*4686 emission in SMC WN stars by Peter Conti and collaborators [51] showed that the line was weaker than in WNs of similar types in the Milky Way or LMC, also consistent with the expectation that stellar winds would be weaker in lower-metallicity environments.
