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

Anna Pasquali, Norbert Langer, Werner Schmutz, Claus Leitherer, Antonella Nota, Ivan Hubeny, Anthony F.J. Moffat

Research output: Contribution to journalArticle

38 Scopus citations

Abstract

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 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.

Original languageEnglish (US)
Pages (from-to)340-357
Number of pages18
JournalAstrophysical Journal
Volume478
Issue number1 PART I
DOIs
StatePublished - Jan 1 1997

Keywords

  • Stars: Wolf-Rayet
  • Stars: early-type
  • Stars: evolution
  • Stars: mass loss
  • Ultraviolet

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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