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WR5 was first identified as a WR by Conti & Massey (1981), but their spectrogram also included neighbouring WR4 and WR3; it was thus classified as a WN star. (1990) tentatively classified it as WC based on their narrow-band photometry. 2) is unambiguous: with very strong C lines unlike most typical late-type WN stars. These authors call attention to the strong spectroscopic variability of WR6's emission lines between 19. 4) is virtually identical to the one shown by Terlevich et al., including linewidths and He/H line ratios; this suggests a (temporary?An excellent spectrogram of this star, obtained nine years before ours, is shown by Terlevich et al. ) stagnation in the rapid spectroscopic evolution of this star observed recently.Nearby giant H regions are hosts to an interesting zoo of emission-line objects (Walborn & Fitzpatrick 2000), most of them being post-main-sequence massive stars of notable interest for our understanding of stellar evolution at the top end of the initial mass function.They also offer important test beds to understand the more distant, unresolved starbursts: individual stars can be counted and spectroscopically classified, allowing a direct comparison with the modelization of the spatially integrated properties of their ionizing cluster [see Vacca et al.
For instance, in unresolved clusters or starburst knots of distant galaxies, the equivalent width of the ‘WR bumps’ is a good indicator of the age and upper mass limit of the stellar population (Pindao et al. The small spiral galaxy M33 is host to four giant H regions bright enough to have their own NGC number: NGC 604, the second most luminous starburst cluster in the Local Group; then, in decreasing order of Hα luminosity, NGC 595, 592 and 588.Table 1 lists the names and properties of the imaging data sets that we have used for this research.The data were extracted from the Canadian Astronomy Data Centre's web interface.The seeing during both observing runs ranged from 0.7 to 1.0 arcsec, except when the observations of NGC 592 were obtained (1.5 arcsec at high airmass; see below).
After the spectra were obtained, a superposition of these images with the resulting 2D spectral images ensured a correct identification of the stars.
regions NGC 592, 595 and 604: five of them are known Wolf–Rayet (WR) stars, for which we present a better quality spectrogram, eight are WR candidates based on narrow-band imagery and one is a serendipitous discovery.