O stars in transition. II. Fundamental properties and evolutionary status of Ofpe/WN9 stars from HST ultraviolet observations

We present new HST/FOS ultraviolet spectroscopic observations of seven LMC Ofpe/WN9 stars. We find that Ofpe/WN9 stars have slow winds with terminal velocities of about 400 km s^-1 and high mass-loss rates of the order of 2-5 × 10^-5 M_⊙ yr^-1. Ofpe/WN9 stellar temperatures and radii are in the range 30,000-39,000 K, and 19-39 R_⊙, respectively. Stellar luminosities are between log (L/L_⊙) = 5.6 and 6.3. We study the Ofpe/WN9 stars winds and examine their evolutionary status. We find that Ofpe/WN9 stars are intermediate between O and W-R stars in terms of the wind momentum flux. We also find that the stellar properties and wind momentum of the Ofpe/WN9 sample place them in the evolutionary sequence: O → Of → H-rich WNL → Ofpe/WN9, for initial stellar masses less than ∼ 100 M_⊙. In view of persisting discrepancies of standard massive star models with observations, we compute massive main-sequence models according to three different evolutionary scenarios. We find that both higher mass-loss rate and enhanced mixing between core and envelope are required in order to yield models compatible with the derived stellar and wind properties of Ofpe/WN9 stars. The emerging picture may be consistent with earlier evidence of Ofpe/WN9 stars being quiescent luminous blue variables (LBVs). This idea is further strengthened by the highly reduced surface H mass fractions of the Ofpe/WN9 stars. We derive X_s = 0.5 to 0.3, which still excludes Ofpe/WN9 stars from being core He-burning objects, but is almost identical to the X_s values recently measured in LBVs.