1997A&A...320..500C

We present a detailed, quantitative study of late WN (WNL) stars in the LMC, based on new optical spectroscopy (AAT, MSO) and the Hillier (1990A&A...231..111H) atmospheric model. In a previous paper (Crowther et al. 1995A&A...293..172C), we showed that 4 out of the 10 known LMC Ofpe/WN9 stars should be re-classified WN9-10. We now present observations of the remaining stars (except the LBV R127), and show that they are also WNL (WN9-11) stars, with the exception of R99. Our total sample consists of 17 stars, and represents all but one of the single LMC WN6-11 population and allows a direct comparison with the stellar parameters and chemical abundances of Galactic WNL stars (Crowther et al. 1995A&A...293..403C; Hamann et al. 1995A&A...299..151H). Previously unpublished ultraviolet (HST-FOS, IUE-HIRES) spectroscopy are presented for a subset of our programme stars. We find observational evidence for lower metallicities in LMC WNL stars compared to the Galaxy, though this is not reflected in their stellar properties. For Galactic and LMC stars we find: (i) a similar range in temperature and luminosity, in contrast to evolutionary predictions; (ii) comparable wind performance values ({dot}(M)v_∞/[L/c]) and hydrogen composition, with a broad correlation between increasing helium content and wind performance number; (iii) a general trend to lower wind velocities at lower stellar temperature, with possibly slower winds for LMC WN9-11 stars. Some 30 Dor WNL stars show exceptional properties: Brey 89 (HD38282, WN6h) has the highest luminosity (log (L/L_☉)∼6.25) and mass-loss rate (log({dot}(M)/(M_☉/yr))~-3.6) known for any WR star, while Brey 80 (R135, WN7h) has an enormous wind performance number of 50. The observed physical properties of our sample of LMC WNL stars supports the Crowther et al. (1995A&A...293..427C) evolutionary scheme for Galactic stars, in that the most massive O stars, exclusive to 30 Dor, evolve directly to O3If/WN6 and subsequently WN6-7 stars (e.g. Brey 89), without passing through an intermediate LBV phase. In contrast, lower initial mass stars evolve through a LBV phase, encompassing a WN9-11 stage (e.g. BE 294), with WN8 stars being their immediate successors.