High-dispersion spectroscopy of the Of/WN stars R 84 and S 61 of the Large Magellanic Cloud.

Abstract

The Of/WN stars R 84 and S 61 of the LMC have been studied with high-dispersion spectroscopy in the optical and satellite-UV range. The high resolution and high S/N spectra in the optical range are particularly distinguished by strong emission lines of H, He I and [N II]. The comparison of R 84 with S 61 shows that the peculiar emission line spectrum is not caused by the previously found late type companion of R 84. It is found that the UV spectra of both stars closely resemble those of late O-supergiants but all absorption lines are violet-shifted. The absorption lines are stronger than in normal O-type stars. The UV-resonance lines indicate low terminal wind velocities of ≈900 km s^-1 only. Unlike to normal O-type stars the Al III-resonance lines also show pronounced P Cygni profiles with an even lower edge velocity. The mass loss rates are comparable to rates found in normal luminous hot stars. It is suggested that the Of/WN transition type stars are the hotter counterparts of the early B-type P Cygni stars.