A Search for Wolf-Rayet Stars in the Small Magellanic Cloud

We report on a comprehensive search for Wolf-Rayet (W-R) stars in the SMC using interference filter imaging. Photometry of over 1.6 million stellar images on multiple, overlapping fields covering 9.6 deg² found the previously known W-R stars at very high significance levels, two known Of-type stars, plus additional candidates, which we examined with slit spectroscopy. We discovered two new Wolf-Rayet stars, both of type "WN3 + abs," bringing the total number in the SMC to 11. We discuss their spectra, as well as reclassifying the previously known ones with our new data. Our survey also revealed four newly found Of-type stars, including one of the O5f?p type, which is one of the earliest type stars known in the SMC. Another newly identified Of star is AV 398 (O8.5 If), a star often used in extinction studies under the assumption that it is of early B type. We recover S18 (AV 154), a B[e] star whose spectrum currently lacks He II λ4686 emission but which must have had strong emission a year earlier; we compare this star to S Dor, suggesting that it is indeed a luminous blue variable. We also find a previously unknown symbiotic star whose spectrum is nearly identical to the Galactic symbiotic AG Dra. More important, perhaps, than any of these discoveries is the demonstration that there is not a significant number of W-R stars waiting to be discovered in the SMC. The number of W-R stars is a factor of 3 times lower in the SMC (per unit luminosity) than in the LMC. This strongly suggests that at the low metallicity that characterizes the SMC only the most massive stars can evolve to W-R type.