Revised Stellar Temperatures for Magellanic Cloud O Supergiants from FUSE and VLT-UVES Spectroscopy

Abstract

We have undertaken quantitative analysis of four LMC and SMC O4--9.7 extreme supergiants using far-ultraviolet FUSE, ultraviolet IUE/HSTand optical VLT UVES spectroscopy. Extended, non-LTE model atmospheres that allow for the consistent treatment of line blanketing are used to analyse wind & photospheric spectral features simultaneously. Using Halpha to constrain dM/dt, HeI-II photospheric lines reveal T_eff's which are systematically (5-7.5kK) & substantially (15-20%) lower than previously derived from unblanketed, plane-parallel, non-LTE photospheric studies. We have confidence in these revisions, since derived T_eff's generally yield consistent fits across the entire 912-7000Ang observed spectral range. In particular, we are able to resolve previous UV-optical T_eff discrepancies. `Of' classification criteria are directly linked to (strong) nitrogen enrichment (via NIII 4097) & (weak) carbon depletion (via CIII 4647-51), providing evidence for mixing of unprocessed and CNO processed material at their stellar surfaces. Oxygen abundances are more difficult to constrain, except via OII lines in the O9.7 supergiant for which it is also found to be somewhat depleted. The effect of wind clumping is also investigated, for which PV 1118-28 potentially provides a useful diagnostic in O-star winds. Revised stellar properties affect existing calibrations of (i) Lyman continuum photons - a factor of two lower for the O4 supergiant; and (ii) kinetic energy released into the ISM by O supergiants. Our results also have importance for the calibration of the wind momentum-luminosity relationship for OB stars.