The Ofpe/WN9 Stars in M33

We present HST/STIS Ultraviolet spectra of the six known Ofpe/WN9 stars(``slash stars'') in M33. These stars were selected for showing thecharacteristics of the Ofpe/WN9 class from previous optical ground-basedspectroscopy. The UV spectra are rich in wind lines, whose strength andterminal velocity vary greatly among our target sample. We analyse the STISspectra with non-LTE, line blanketed, spherical models with hydrodynamics,computed with the WM-basic code. We find C to be underabundant and Noverabundant, respect to the solar values, with a ratio (by mass) of C/Nbetween 0.02 to 0.9 across the sample. Some stars show very conspicuous windlines (P Cygni profiles), while two stars have extremely weak winds. Themass-loss rates thus vary greatly across the sample. The mass-loss rates of the hottest stars are lower than typical values of WNLstars, but higher than expected for normal population I massive stars. There isindication that the mass-loss rates may be variable in time. The C/N ratio, andthe other physical parameters derived by the spectral modeling (Teff, Lbol,mass), are consistent with evolutionary calculations for objects withmoderately high initial masses (approx 30-50 solar masses), evolving towardsthe WNL stage through an enhanced mass-loss phase.Comment: 7 figures (some in color). ApJ, in press (tentatively V.601 Jan.20 2004