The Ofpe/WN9 stars in M33

Abstract

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.

7 figures (some in color). ApJ, in press (tentatively V.601 Jan.20 2004)

References in corpus (5)

• Stellar evolution with rotation X: Wolf-Rayet star populations at solar metallicity
• Revised Stellar Temperatures for Magellanic Cloud O Supergiants from FUSE and VLT-UVES Spectroscopy
• Predictions of variable mass loss for Luminous Blue Variables
• An explanation for the curious mass loss history of massive stars: from OB stars, through Luminous Blue Variables to Wolf-Rayet stars
• Discovery and quantitative spectral analysis of an Ofpe/WN9 (WN11) star in the Sculptor spiral galaxy NGC 300

Cited by in corpus (10)

• On the massive stellar population of the Super Star Cluster Westerlund 1
• The Missing Luminous Blue Variables and the Bistability Jump
• The Wolf-Rayet Content of M33
• SN 2011hw: Helium-Rich Circumstellar Gas and the Luminous Blue Variable to Wolf-Rayet Transition in Supernova Progenitors
• Wolf-Rayet Stars in M33 II: Optical Spectroscopy of emission-line stars in Giant Hii Regions
• A new and unusual LBV-like outburst from a Wolf-Rayet star in the outskirts of M33
• X-ray emission from hydrodynamical simulations in non-LTE wind models
• GCIRS34W: An irregular variable in the Galactic Centre
• Blue luminous stars in nearby galaxies - UIT005: a possible link to the luminous blue variable stage
• Effect of rotational mixing and metallicity on the hot star wind mass-loss rates