2.2.1. Large Magellanic Cloud

As part of the Harvard spectral surveys, Anne J. Cannon and Cecilia Payne (later Payne–Gaposchkin) identified 50 Wolf–Rayet stars in the Large Magellanic Cloud (LMC) according to Bengt Westerlund and Alexander Rodgers (1959) [33] quoting an early review article on the stellar content of the LMC by Gerard de Vaucouleurs and collaborators [34]. Westerlund and Rodgers carried out their own search of the LMC, the first systematic search for WR stars in another galaxy, using slitless (objective prism) spectroscopy to identify 50 WRs, 36 of which were in common with the Harvard studies [33]. They note that nine Harvard O-type stars in the 30 Doradus region had been recently reclassified as WN by Michael Feast and coworkers [35] in the previous year. Two decades later, Marc Azzopardi and Jacques Breysacher (1979) completed an even more powerful objective prism survey using an interference filter to further reduce the effects of crowding [36]. This increased the number of known WRs in the the LMC to 100. Accurate spectral types of these 100 LMC WRs were subsequently published by Breysacher in 1981 [37]. In that paper, Breysacher estimated that the LMC likely contained a total of 144 ± 20 LMC WRs, with 44 left to be discovered. He further speculated that the majority of these undiscovered WRs would be found deep within the cores of dense H II regions where slitless spectroscopy often fails. (Indeed, the "final census" catalogue of LMC WRs, discussed below, lists 154 separate WRs [38], well within Breysacher's estimate of 144 ± 20.) These early studies culminated in Breysacher's et al.'s "Fourth Catalog" of LMC WRs [39] (hereafter BAT99), which listed 134 LMC WRs.

The R136 cluster merits separate attention, as investigations of its stellar content led to the recognition that not all luminous stars with WR-like spectra are evolved objects. R136 is of course the central object at the heart of the 30 Doradus nebula in the LMC. Once thought to house a supermassive star, early *Hubble Space Telescope (HST)* images showed it was even more interesting, the core of a super star cluster, with over 3500 stars (120 of which are blue and more luminous than *MV* ∼ −4) most of which lie within 8" (2pc) of the semistellar R136 cluster [40]. Using ground-based spectroscopy in 1985, Jorge Melnick had identified 12 WR stars in or near the central cluster [41]. When Deidre Hunter and collaborators analyzed the first *HST* images of the cluster in 1995, this created a conundrum: the isochrones indicated that the lower mass stars had ages of only 1–2 Myr, but the presence of WR stars implied ages for the massive stars of 3–4 Myr [40]. Why had the formation of high mass stars, with their strong stellar winds, not stopped star formation in the cluster? Melnick had also found early-type O stars in the cluster, possibly as early as O3, although the presence of strong nebulosity made this classification uncertain, and this also seemed to conflict with the ages of the WR stars, as the O3 phase lasts for only a million years. Massey and Hunter obtained *HST* spectroscopy of 65 of the hottest, bluest stars in the cluster, and discovered two amazing facts: (1) the vast majority of these stars were of O3, and that (2) the WR stars were not common, garden-variety WNs [42]. Rather, they were 10× more luminous in the V-band than normal WRs, and their spectra were still rich in hydrogen. Massey and Hunter argued that a similar situation existed in the Galactic giant H II region NGC 3603, where both O3 stars and WRs were known [43]; they examined the archival spectra and concluded that those WR stars were like the H-rich super-bright WR in R136. The obvious conclusion was that these were young (1–2 Myr) objects still burning hydrogen whose high luminosities simply resulted in WR-like emission features, in essence, Of-type stars on steroids [42]. This interpretation built on the important result the previous year by Alex de Koter and collaborators who found that one of the over-luminous, hydrogen-rich WR stars in the core of the R136 cluster had a normal hydrogen abundance, and who had originally suggested that this and similar were still in the hydrogen-burning phase [44].
