The VLT-FLAMES Tarantula Survey

XXIV. Stellar properties of the O-type giants and supergiants in 30 Doradus ^⋆,⋆⋆

¹ Astronomical Institute Anton Pannekoek, Amsterdam University, Science Park 904, 1098 Amsterdam, The Netherlands
e-mail: oscar.ramirez@stfc.ac.uk
² Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
³ UK Astronomy Technology Centre, Royal Observatory Edinburgh, Blackford Hill, Edinburgh, EH9 3HJ, UK
⁴ Institute of Astrophysics, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
⁵ European Space Astronomy Centre (ESAC), Camino bajo del Castillo, s/n Urbanizacion Villafranca del Castillo, Villanueva de la Cañada, 28 692 Madrid, Spain
⁶ Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
⁷ LMU Munich, Universitätssternwarte, Scheinerstrasse 1, 81679 München, Germany
⁸ Institute of Astronomy with NAO, Bulgarian Academy of Sciences, PO Box 136, 4700 Smoljan, Bulgaria
⁹ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹⁰ Department of Astronomy, University of Michigan, 1085 S. University Avenue, Ann Arbor, MI 48109-1107, USA
¹¹ Department of Physics & Astronomy, University of Sheffield, Hounsfield Road, Sheffield, S3 7RH, UK
¹² Centro de Astrobiología (CSIC-INTA), Ctra. de Torrejón a Ajalvir km-4, 28850 Torrejón de Ardoz, Madrid, Spain
¹³ Departamento de Astrofísica, Universidad de La Laguna, Avda. Astrofísico Francisco Sánchez s/n, 38071 La Laguna, Tenerife, Spain
¹⁴ Instituto de Astrofísica de Canarias, C/Vía Láctea s/n, 38200 La Laguna, Tenerife, Spain
¹⁵ Centro de Astrobiología (CSIC-INTA), ESAC campus, Camino bajo del castillo s/n, Villanueva de la Cañada, 28 692 Madrid, Spain
¹⁶ Instituto de Investigación Multidisciplinar en Ciencia y Tecnología, Universidad de La Serena, 1305 Raúl Bitrán, La Serena, Chile
¹⁷ Armagh Observatory, College Hill, Armagh, BT61 9DG, Northern Ireland, UK

Received: 12 May 2016
Accepted: 27 December 2016

Abstract

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 60 M_⊙, 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_init ≳ 25 M_⊙. At masses below 60 M_⊙, the dwarf phase clearly precedes the giant and supergiant phases; however this behavior seems to break down at M_init ≲ 18 M_⊙. Here, stars classified as late O III and II stars occupy the region where O9.5-9.7 V stars are expected, but where few such late O V stars are actually seen. Though we can not exclude that these stars represent a physically distinct group, this behavior may reflect an intricacy in the luminosity classification at late O spectral subtype. Indeed, on the basis of a secondary classification criterion, the relative strength of Si iv to He i absorption lines, these stars would have been assigned a luminosity class IV or V. Except for five stars, the helium abundance of our sample stars is in agreement with the initial LMC composition. This outcome is independent of their projected spin rates. The aforementioned five stars present moderate projected rotational velocities (i.e., ν_esini < 200kms^-1) and hence do not agree with current predictions of rotational mixing in main-sequence stars. They may potentially reveal other physics not included in the models such as binary-interaction effects. Adopting theoretical results for the wind velocity law, we find modified wind momenta for LMC stars that are ~0.3 dex higher than earlier results. For stars brighter than 10⁵ L_⊙, that is, in the regime of strong stellar winds, the measured (unclumped) mass-loss rates could be considered to be in agreement with line-driven wind predictions if the clump volume filling factors were f_V ~ 1/8 to 1/6.