1995A&A...293..172C

A detailed study has been carried out for 4 LMC Ofpe/WN9 (`slash') stars, and the sole Galactic WN9 star WR108 (HDE 313846), using new high S/N spectroscopy and archive UV and near-IR spectroscopy. Our observations reveal that photospheric Of features such as HeIIλ4542 are absent from the optical spectra of our sampled stars. All observed optical lines are formed in the stellar wind, and so we prefer WN9 or WN10 classifications based on the relative strengths of NIIλ3995 and NIIIλ4634-41, with BE 381 and Sk-66 40 prototype WN9 and WN10 stars, respectively. HDE 269927c and R84 are given WN9 classifications, with WR108 newly assigned WN9+abs due to the spectral appearance of the upper Balmer series. The distance to WR108 is determined from an analysis of its interstellar spectrum. Previously WR108 has been considered to be a possible member of Sgr OB1 (1.6 kpc, Lundstroem & Stenholm 1984A&AS...58..163L [LS84]) which results in a uniquely low stellar luminosity for this type of object. Using the standard Galactic rotation curve of Fich et al. (1989ApJ...342..272F) we derive 5±1 kpc, resulting in a luminosity very similar to R84. Tailored analyses using the WR standard model, including metals, result in the following stellar parameters for all stars: T_*=29.5±1.0kK, L/L_☉=10^5.65±0.2, {dot}(M)=3.5±1x10^–5M_☉/yr and v_∞=400±100km/s (v_∞=1170km/s for WR108). The stellar parameters determined for R84 compare well with those determined previously by Schmutz et al. (1991ApJ...372..664S)[SCH+91] indicating that the effect of CNO elements is negligible for WN stars. The metallicity of the LMC stars is around Z∼0.008 while Z∼0.035 results for the Galactic WN9 star. Abundances for the LMC stars (H/He=2.5±1, N/He∼0.003, C/N∼0.1) are in reasonable agreement with the results of evolutionary models at low metallicity (Schaerer et al. 1993A&AS...98..523S) although observed luminosities are significantly lower than predictions for stars entering the WR phase. The luminosity and chemistry of R84 are identical to that of the Luminous Blue Variable (LBV) R71 indicating that it is probably associated with an LBV phase rather than a post-red supergiant as suggested by Schmutz et al. (1991ApJ...372..664S). The status of the remaining LMC stars is less clear, although their common spectral characteristics suggest that they are also related to LBVs, with Sk-66 40 the least evolved of the present sample. For WR108, its spectral appearance, derived parameters and abundances (H/He=1.5, C/N∼0.10) suggest an intimate relationship with extreme Galactic Ofpe stars, with the wind density being the principal difference, and evolution probably proceeding directly from Of to Wolf-Rayet.