1997A&A...317..532C

From detailed atmospheric analysis of the Galactic late O supergiants, HD 151804 (O8Iaf) and HD 152408 (O8:Iafpe) and the morphologically related Wolf-Rayet star, HDE 313846 (WR108; WN9ha) we conclude that little distinguishes the overall structure and composition of the stellar atmosphere of HD 152408 from HDE 313846, such that HD 152408 could be considered to be a W-R star equivalent to HDE 313846. Our quantitative spectroscopic analysis uses ultraviolet (IUE), optical (AAT, Mt. Stromlo) and infrared (UKIRT, CTIO) spectroscopy and is based on model atmospheres obtained with the iterative scheme of Hillier (1990A&A...231..116H). Stellar properties for all three stars are quite similar: log L_*/L_☉≃5.8, R_*/R_☉≃35, -4.9≤log{dot}(M)/M_☉/yr≤-4.5, 955≤v_∞≤1445km/s. Spectroscopic mass-loss rates are in excellent agreement with radio methods and are more representative of late WN (WNL) stars than O-type supergiants. From this comparison we find that the O and WNL spectral classifications are distinguished principally by surface mass flux ({dot}(M)/4πR_*²), with stellar temperature and surface helium content secondary effects. Since surface chemical abundances do not necessarily define the core energy production mechanism - a commonly used distinction between O and WR types - we argue caution when comparing observed O:WNL ratios with theoretical predictions and inferring stellar evolution in various galaxies. The increasing helium content of the programme stars - O8Iaf (y=N(He)/[N(H)+N(He)]=0.20), O8:Iafpe (y=0.40) and WN9ha (y=0.45) - is strongly indicative of a direct evolutionary connection from O8If=>WN9ha without passage through an intermediate Luminous Blue Variable phase, contrary to current evolutionary theory, and suggestive of additional processes (e.g. rotation) bringing processed material to the surface. The high helium content of HD 151804 confirms previous findings that Of stars are helium enriched by core processed material.