O Stars in Transition. II. Fundamental Properties and Evolutionary Status of Ofpe/WN9 Stars from Hubble Space Telescope Ultraviolet Observations

Details O Stars in Transition. II. Fundamental Properties and Evolutionary Status of Ofpe/WN9 Stars from Hubble Space Telescope Ultraviolet Observations O Stars in Transition. II. Fundamental Properties and Evolutionary Status of Ofpe/WN9 Stars from Hubble Space Telescope Ultraviolet Observations

Mar 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997apj...478..340p&link_type=abstract

Astrophysical Journal v.478, p.340

Astronomy and Astrophysics

Astronomy

51

Stars: Early-Type, Ultraviolet: Stars, Stars: Evolution, Stars: Mass Loss, Stars: Wolf-Rayet

Scientific paper

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 x 10-5 M&sun; yr-1. Ofpe/WN9 stellar temperatures and radii are in the range 30,000--39,000 K, and 19--39 R&sun;, respectively. Stellar luminosities are between log (L/L&sun;) = 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&sun;. 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 Xs = 0.5 to 0.3, which still excludes Ofpe/WN9 stars from being core He-burning objects, but is almost identical to the Xs values recently measured in LBVs.

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