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O Stars in Transition. II. Fundamental Properties and Evolutionary Status of Ofpe/WN9 Stars from Hubble Space Telescope Ultraviolet Observations
We present new HST/FOS ultraviolet spectroscopic observations of sevenLMC Ofpe/WN9 stars. We find that Ofpe/WN9 stars have slow winds withterminal velocities of about 400 km s-1 and high mass-loss rates of theorder of 2--5 x 10-5 Mȯ yr-1. Ofpe/WN9 stellar temperatures andradii are in the range 30,000--39,000 K, and 19--39 Rȯ,respectively. Stellar luminosities are between log (L/Lȯ) = 5.6 and6.3. We study the Ofpe/WN9 stars winds and examine their evolutionarystatus. We find that Ofpe/WN9 stars are intermediate between O and W-Rstars in terms of the wind momentum flux. We also find that the stellarproperties and wind momentum of the Ofpe/WN9 sample place them in theevolutionary sequence: O --> Of --> H-rich WNL --> Ofpe/WN9,for initial stellar masses less than ~100 Mȯ. In view of persistingdiscrepancies of standard massive star models with observations, wecompute massive main-sequence models according to three differentevolutionary scenarios. We find that both higher mass-loss rate andenhanced mixing between core and envelope are required in order to yieldmodels compatible with the derived stellar and wind properties ofOfpe/WN9 stars. The emerging picture may be consistent with earlierevidence of Ofpe/WN9 stars being quiescent luminous blue variables(LBVs). This idea is further strengthened by the highly reduced surfaceH mass fractions of the Ofpe/WN9 stars. We derive Xs = 0.5 to 0.3, whichstill excludes Ofpe/WN9 stars from being core He-burning objects, but isalmost identical to the Xs values recently measured in LBVs.
| Carbon Stars with Strong C^{13} and Lithium Spectral Features
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1971PASP...83..667G&db_key=AST
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