X iv : a st ro - p h / 04 02 20 7 v 1 9 Fe b 20 04 The Effective Temperatures of Hot Stars II . The Early - O Types . 1

We derived the stellar parameters of a sample of Galactic early-O type stars by analysing their UV and Far-UV spectra from FUSE (905-1187Å), IUE, HSTSTIS and ORFEUS (1200-2000Å). The data have been modeled with spherical, hydrodynamic, line-blanketed, non-LTE synthetic spectra computed with the WM-basic code. We obtain effective temperatures ranging from Teff= 41,000 K to 39,000 K for the O3-O4 dwarf stars, and Teff= 37,500 K for the only supergiant of the sample (O4 If). Our values are lower than those from previous empirical calibrations for early-O types by up to 20%. The derived luminosities of the dwarf stars are also lower by 6 to 12%; however, the luminosity of the supergiant is in agreement with previous calibrations within the error bars. Our results extend the trend found for later-O types in a previous work by Bianchi & Garcia. Subject headings: Stars: fundamental parameters — stars: mass loss — stars: early-type — stars: winds, outflows — ultraviolet: stars Departamento de Astrof́ısica, Universidad de La Laguna, Avda. Astrof́ısico Francisco Sánchez s/n, 38206 La Laguna (Tenerife), Spain. Based on observations with the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer (FUSE) , which is operated by The Johns Hopkins University under NASA contract NAS5-32985, on International Ultraviolet Explorer (IUE ) observations from the MAST and INES archives and on MAST archival data from the Hubble Space Telescope (HST ) and the Orbiting Retrievable Far and Extreme Ultraviolet Spectrometers (ORFEUS ) mission.