The VLT-FLAMES Tarantula Survey XXXII. Low-luminosity late O-type stars -- classification, main physical parameters, and silicon abundances

Abstract

Analysis of late O-type stars observed in the Large Magellanic Cloud (LMC) by the VLT-FLAMES Tarantula Survey (VFTS) revealed a discrepancy between the physical properties estimated from model-atmosphere analysis and those expected from their morphological classifications. Here we revisit the analysis of 32 of these puzzling objects using new hydrogen-helium-silicon FASTWIND models and a different fitting approach to re-evaluate their physical properties. Our new analysis confirms that these stars indeed have properties that are typical of late O-type dwarfs. We also present the first estimates of silicon abundances for O-type stars in the 30 Dor clusters NGC 2060 and NGC 2070, with a weighted mean abundance for our sample of 7.05 +/- 0.03. Our values are about 0.20 dex lower than those previously derived for B-type stars in the LMC clusters N 11 and NGC 2004 using TLUSTY models. Various possibilities (e.g. differences in the analysis methods, effects of microturbulence, and real differences between stars in different clusters) were considered to account for these results. We also used our grid of FASTWIND models to reassess the impact of using the Galactic classification criteria for late O-type stars in the LMC by scrutinising their sensitivity to different stellar properties. At the cool edge of the O star regime the HeII 4686/HeI 4713 ratio used to assign luminosity class for Galactic stars can mimic giants or bright giants in the LMC, even for objects with high gravities (log_g > 4.0 dex). We argue that this line ratio is not a reliable luminosity diagnostic for late O-type stars in the LMC, and that the SiIV 4989/HeI4026 ratio is more robust for these types.

18 pages, 10 figures accepted for publication in A&A

References in corpus (15)

• The R136 star cluster hosts several stars whose individual masses greatly exceed the accepted 150 Msun stellar mass limit
• An excess of massive stars in the local 30 Doradus starburst
• The VLT-FLAMES Survey of Massive Stars: Observations centered on the Magellanic Cloud clusters NGC 330, NGC 346, NGC 2004, and the N11 region
• The VLT-FLAMES survey of massive stars: Evolution of surface N abundances and effective temperature scales in the Galaxy and Magellanic Clouds
• Bright OB stars in the Galaxy IV. Stellar and wind parameters of early to late B supergiants
• The VLT-FLAMES survey of massive stars: Wind properties and evolution of hot massive stars in the LMC
• The Tarantula Massive Binary Monitoring: I. Observational campaign and OB-type spectroscopic binaries
• The IACOB project: III. New observational clues to understand macroturbulent broadening in massive O- and B-type stars
• The VLT-FLAMES Tarantula Survey XVI. The optical+NIR extinction laws in 30 Doradus and the photometric determination of the effective temperatures of OB stars
• The VLT-FLAMES Tarantula Survey III: A very massive star in apparent isolation from the massive cluster R136
• The MiMeS Survey of Magnetism in Massive Stars: CNO surface abundances of Galactic O stars
• The Physical Properties and Effective Temperature Scale of O-type Stars as a Function of Metallicity. III. More Results from the Magellanic Clouds
• The VLT-FLAMES Tarantula Survey XIX. B-type Supergiants - Atmospheric parameters and nitrogen abundances to investigate the role of binarity and the width of the main sequence
• A spectroscopic investigation of the O-type star population in four Cygnus OB associations. II. Determination of the fundamental parameters
• Metallicity estimates of young clusters in the Magellanic Clouds from Stromgren photometry of supergiant stars

Cited by in corpus (4)

• Empirical mass-loss rates and clumping properties of Galactic early-type O supergiants
• METAL: The Metal Evolution, Transport, and Abundance in the Large Magellanic Cloud Hubble program. II. Variations of interstellar depletions and dust-to-gas ratio within the LMC
• Empirical mass-loss rates and clumping properties of O-type stars in the LMC
• Micro-turbulence across the Hertzsprung-Russell diagram. Observational constrains for stars in the MW