Astronomy and Astrophysics – Astrophysics
Scientific paper
2003-10-07
Astrophys.J. 601 (2004) 228-241
Astronomy and Astrophysics
Astrophysics
7 figures (some in color). ApJ, in press (tentatively V.601 Jan.20 2004)
Scientific paper
10.1086/380485
We present HST/STIS Ultraviolet spectra of the six known Ofpe/WN9 stars (``slash stars'') in M33. These stars were selected for showing the characteristics of the Ofpe/WN9 class from previous optical ground-based spectroscopy. The UV spectra are rich in wind lines, whose strength and terminal velocity vary greatly among our target sample. We analyse the STIS spectra with non-LTE, line blanketed, spherical models with hydrodynamics, computed with the WM-basic code. We find C to be underabundant and N overabundant, respect to the solar values, with a ratio (by mass) of C/N between 0.02 to 0.9 across the sample. Some stars show very conspicuous wind lines (P Cygni profiles), while two stars have extremely weak winds. The mass-loss rates thus vary greatly across the sample. The mass-loss rates of the hottest stars are lower than typical values of WNL stars, but higher than expected for normal population I massive stars. There is indication that the mass-loss rates may be variable in time. The C/N ratio, and the other physical parameters derived by the spectral modeling (Teff, Lbol, mass), are consistent with evolutionary calculations for objects with moderately high initial masses (approx 30-50 solar masses), evolving towards the WNL stage through an enhanced mass-loss phase.
Bianchi Luciana
Bohlin Ralph
Massey Philip
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