Revised Stellar Temperatures for Magellanic Cloud O Supergiants fromFar Ultraviolet Spectroscopic Explorerand Very Large Telescope UV‐Visual Echelle Spectrograph Spectroscopy

Abstract

We have undertaken quantitative analysis of four LMC and SMC O4-9.7 extreme supergiants using far-ultraviolet FUSE , ultraviolet IUE /HST and optical VLT UVES spectroscopy. Extended, non-LTE model atmospheres that allow for the consistent treatment of line blanketing (Hillier & Miller 1998) are used to analyse wind and photospheric spectral features simultaneously. Using Hα to constrain Ṁ , He i-ii photospheric lines reveal stellar temperatures which are systematically (5-7.5kK) 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 λλ912-7000 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 et al. (2000).The temperature and abundance sensitivity of far-UV, UV and optical lines is discussed. 'Of' classification criteria are directly linked to (strong) nitrogen enrichment (via N iii λ4097) and (weak) carbon depletion (via C iii λλ4647-51), 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, due to uncertainties in microturbulence and the atmospheric scale height. The effect of wind clumping is also investigated, for which P v λλ1118-28 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 (i) Lyman continuum

Bright OB stars in the Galaxy

“…Furthermore, we suggest a similar correction for mid and late subtypes. When strong winds are present, this reduction can become a factor of two larger, similar to the situation encountered in O-SGs (Crowther et al 2002). 4.…”

“…During the last years, the quantitative analyses of spectra in the far-UV/UV and optical domains (e.g., Herrero et al 2002;Bianchi & Garcia 2002;Crowther et al 2002;Bouret et al 2003;Repolust et al 2004;Massey et al 2004;Heap et al 2006) have unambiguously shown that the inclusion of line-blocking and blanketing and wind effects (if present) significantly modifies the temperature scale of O-stars (for a recent calibration at solar metallicity, see Markova et al 2004;Martins et al 2005). Regarding B-SGs, particularly of later subtype, this issue has not been addressed so far, mostly due to lacking T eff estimates.…”

Gemini GMOS spectroscopy of HeII nebulae in M 33

“…Alternatively, could MC8 be an Of supergiant, since they possess much weaker He II λ4686 emission than WN stars plus brighter absolute magnitudes of M V = −7 mag. We are able to discriminate between these two possibilies since Of supergiants possess narrow λ4686 emission, with FWHM ∼ 2-7 Å (Crowther et al 2002b) versus FWHM ∼= 25-60 Å for early-type WN stars (Crowther & Hadfield 2006). From our MC8 Gemini GMOS spectrum, He II λ4686 emission is broad (FWHM ∼ 30 Å), allowing us to conclude that MC8 is a probable cluster hosting an early-type WN star.…”

Structure in the winds of O-Type stars: observations and inferences

“…Fullerton: Very similar discrepancies are seen in samples of O-type stars observed with FUSE in both the LMC (Crowther et al 2002, Massa et al 2003 and SMC (Hillier et al 2003). Although these samples are limited, they don't provide any evidence to suggest that the degree of clumping depends strongly on the overall metallicity of the star.…”

“…5 shows that measurements ofṀ from ρ 2 diagnostics for a "blue-ribbon" sample of 40 Galactic O-type stars are always greater than measurements from the P v resonance lines by a factor of at least 20 (Fullerton, Massa, & Prinja 2006). Detailed model atmosphere calculations indicate similar levels of disagreement between Hα and the P v doublet for O-type supergiants in the Galaxy (Bouret et al 2005), Large Magellanic Cloud (Crowther et al 2002;Massa et al 2003) and Small Magellanic Cloud (Hillier et al 2003).…”