The Ofpe/WN9 stars in M33

Abstract

We present HST/STIS Ultraviolet spectra of the six known Ofpe/WN9 stars (``slash stars'') in M33. These stars were selected for showing the characteristics of the Ofpe/WN9 class from previous optical ground-based spectroscopy. The UV spectra are rich in wind lines, whose strength and terminal velocity vary greatly among our target sample. We analyse the STIS spectra with non-LTE, line blanketed, spherical models with hydrodynamics, computed with the WM-basic code. We find C to be underabundant and N overabundant, respect to the solar values, with a ratio (by mass) of C/N between 0.02 to 0.9 across the sample. Some stars show very conspicuous wind lines (P Cygni profiles), while two stars have extremely weak winds. The mass-loss rates thus vary greatly across the sample. The mass-loss rates of the hottest stars are lower than typical values of WNL stars, but higher than expected for normal population I massive stars. There is indication that the mass-loss rates may be variable in time. The C/N ratio, and the other physical parameters derived by the spectral modeling (Teff, Lbol, mass), are consistent with evolutionary calculations for objects with moderately high initial masses (approx 30-50 solar masses), evolving towards the WNL stage through an enhanced mass-loss phase.