Revised stellar temperatures for magellanic cloud O supergiants from far ultraviolet spectroscopic explorer and very large telescope UV-visual echelle spectrograph spectroscopy

We have undertaken quantitative analysis of four LMC and SMC O4-9.7 extreme supergiants using far-ultraviolet FUSE, ultraviolet IUE/Hubble Space Telescope, and optical Very Large Telescope UV-Visual Echelle Spectrograph spectroscopy. Extended, non-LTE model atmospheres that allow for the consistent treatment of line blanketing, developed by Hillier & Miller, are used to analyze wind and photospherics spectral features simultaneously. Using Hα to constrain mass-loss rates, He I-He II photospheric lines reveal stellar temperatures that are systematically (5-7.5 kK) and substantially (15%-20%) lower than previously derived from unblanketed, plane-parallel, non-LTE photospheric studies. We have confidence in these revisions since derived temperatures generally yield consistent fits across the entire 912-7000 Å observed spectral range. In particular, we are able to resolve the UV-optical temperature discrepancy identified for AzV 232 (O7 Iaf⁺) in the SMC by Fullerton and coworkers. The temperature and abundance sensitivity of far-ultraviolet, UV, and optical lines are discussed. "Of" classification criteria are directly linked to (strong) nitrogen enrichment (via N III λ4097) and (weak) carbon depletion (via C III λλ4647-4651), providing evidence for mixing of unprocessed and CNO-processed material at their stellar surfaces. Oxygen abundances are more difficult to constrain, except via O II lines in the O9.7 supergiant, for which it is also found to be somewhat depleted. Unfortunately, He/H is very difficult to determine in individual O supergiants because of uncertainties in microturbulence and the atmospheric scale height. The effect of wind clumping is also investigated, for which P v λλ1118-1128 potentially provides a useful diagnostic in O star winds, unless phosphorus can be independently demonstrated to be underabundant relative to other heavy elements. Revised stellar properties affect existing calibrations of (1) Lyman continuum photons - a factor of 2 lower for the O4 supergiant - and (2) 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, particularly since the stars studied here are among the visually brightest OB stars in external galaxies.