The VLT-FLAMES Tarantula Survey: XXIV. Stellar properties of the O-type giants and supergiants in 30 Doradus

• Published in: Astronomy and astrophysics (Berlin) 2017-04, Vol.600, p.A81
• Main Authors: Ramirez-Agudelo, O H, Sana, H, de Koter, A, Tramper, F, Grin, N J, Schneider, F R N, Langer, N, Puls, J, Markova, N, Bestenlehner, J M
• Format: Article
• Language: English
• Subjects: Calibration; Mathematical models; Rotation; Spectra; Stars; Stellar evolution; Stellar winds; Supergiant stars
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361/201628914

Context. The Tarantula region in the Large Magellanic Cloud (LMC) contains the richest population of spatially resolved massive O-type stars known so far. This unmatched sample offers an opportunity to test models describing their main-sequence evolution and mass-loss properties. Aims. Using ground-based optical spectroscopy obtained in the framework of the VLT-FLAMES Tarantula Survey (VFTS), we aim to determine stellar, photospheric and wind properties of 72 presumably single O-type giants, bright giants and supergiants and to confront them with predictions of stellar evolution and of line-driven mass-loss theories. Methods. We apply an automated method for quantitative spectroscopic analysis of O stars combining the non-LTE stellar atmosphere model fastwind with the genetic fitting algorithm pikaia to determine the following stellar properties: effective temperature, surface gravity, mass-loss rate, helium abundance, and projected rotational velocity. The latter has been constrained without taking into account the contribution from macro-turbulent motions to the line broadening. Results. We present empirical effective temperature versus spectral subtype calibrations at LMC-metallicity for giants and supergiants. The calibration for giants shows a +1kK offset compared to similar Galactic calibrations; a shift of the same magnitude has been reported for dwarfs. The supergiant calibrations, though only based on a handful of stars, do not seem to indicate such an offset. The presence of a strong upturn at spectral type O3 and earlier can also not be confirmed by our data. In the spectroscopic and classical Hertzsprung-Russell diagrams, our sample O stars are found to occupy the region predicted to be the core hydrogen-burning phase by state-of-the-art models. For stars initially more massive than approximately 60M sub([middot in circle]), the giant phase already appears relatively early on in the evolution; the supergiant phase develops later. Bright giants, however, are not systematically positioned between giants and supergiants at M sub(init)> or = 25M sub([middot in circle]). At masses below 60M sub([middot in circle]), the dwarf phase clearly precedes the giant and supergiant phases; however this behavior seems to break down at M sub(init)[< or =] 18M sub([middot in circle]). Here, stars classified as late OIII and II stars occupy the region where O9.5-9.7 V stars are expected, but where few such late OV stars are actually seen. Though we can not exclud