The VLT-FLAMES Tarantula Survey XXVI: Properties of the O-dwarf population in 30 Doradus

C. Sabín-Sanjulián(1,2), S. Simón-Díaz(3,4), A. Herrero(3,4), J. Puls(5), F. R. N. Schneider(6), C. J. Evans(7), M. Garcia(8), F. Najarro(8), I. Brott(9), N. Castro(10), P. A. Crowther(11), A. de Koter(12,13), S. E. de Mink(12), G. Gräfener(14), N. J. Grin(14), G. Holgado(3,4), N. Langer(14), D. J. Lennon(15), J. Maíz Apellániz(16), O. H. Ramírez-Agudelo(7), H. Sana(13), W. D. Taylor(7), J. S. Vink(17), N. R. Walborn(18)

1.- Departamento de Física y Astronomía, Universidad de La Serena, Av. Cisternas 1200 Norte, La Serena, Chile
2.- Instituto de Investigación Multidisciplinar en Ciencia y Tecnología, Universidad de La Serena, Raúl Bitrán 1305, La Serena, Chile
3.- Instituto de Astrofísica de Canarias, E-38200 La Laguna, Tenerife, Spain
4.- Departamento de Astrofísica, Universidad de La Laguna, E-38205 La Laguna, Tenerife, Spain
5.- LMU Munich, Universitätssternwarte, Scheinerstrasse 1, 81679 Munchen, Germany
6.- Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, United Kingdom
7.- UK Astronomy Technology Centre, Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ, UK
8.- Centro de Astrobiolog ́ıa (CSIC-INTA), Ctra. de Torrejón a Ajalvir km-4, E-28 850 Torrejón de Ardoz, Madrid, Spain
9.- University of Vienna, Department of Astronomy, Türkenschanzstr. 17, 1180, Vienna, Austria
10.- Department of Astronomy, University of Michigan, 1085 S. University Avenue, Ann Arbor, MI 48109-1107, USA
11.- Department of Physics & Astronomy, Hounsfield Road, University of Sheffield, S3 7RH, UK
12.- Astronomical Institute Anton Pannekoek, Amsterdam University, Science Park 904, 1098 XH, Amsterdam, The Netherlands
13.- Instituut voor Sterrenkunde, Universiteit Leuven, Celestijnenlaan 200D, 3001, Leuven, Belgium
14.- Argelander-Institut für Astronomie der Universit ̈at Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
15.- European Space Astronomy Centre (ESAC), Camino bajo del castillo, s/n Urbanización Villafranca del Castillo, Villanueva de la Cañada, E-28 692 Madrid, Spain
16.- Centro de Astrobiología, CSIC-INTA, Campus ESAC, Camino bajo del castillo s/n, E-28 692 Madrid, Spain
17.- Armagh Observatory, College Hill, Armagh, BT61 9DG, Northern Ireland, UK
18.- Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA

The VLT-FLAMES Tarantula Survey has observed hundreds of O-type stars in the
30 Doradus region of the Large Magellanic Cloud (LMC). We study the properties
of 105 apparently single O-type dwarfs. To determine stellar and wind
parameters, we used the IACOB-GBAT package, an automatic procedure based on a
large grid of atmospheric models calculated with the FASTWIND code. In addition
to classical techniques, we applied the Bayesian BONNSAI tool to estimate
evolutionary masses. We provide a new calibration of effective temperature vs.
spectral type for O-type dwarfs in the LMC, based on our homogeneous analysis
of the largest sample of such objects to date and including all spectral
subtypes. Good agreement with previous results is found, although the sampling
at the earliest subtypes could be improved. Rotation rates and helium
abundances are studied in an evolutionary context. We find that most of the
rapid rotators (vsini higher than 300 km/s ) in our sample have masses below 25
MSun and intermediate rotation-corrected gravities (log gc between 3.9 and
4.1). Such rapid rotators are scarce at higher gravities (i.e. younger ages)
and absent at lower gravities (larger ages). This is not expected from
theoretical evolutionary models, and does not appear to be due to a selection
bias in our sample. We compare the estimated evolutionary and spectroscopic
masses, finding a trend that the former is higher for masses below 20 MSun.
This can be explained as a consequence of limiting our sample to the O-type
stars, and we see no compelling evidence for a systematic mass discrepancy. For
most of the stars in the sample we were unable to estimate the wind-strength
parameter (hence mass-loss rates) reliably, particularly for objects with
luminosity lower than logL/LSun about 5.1. Ultraviolet spectroscopy is needed
to undertake a detailed investigation of the wind properties of these dwarfs.

Reference: arXiv:1702.04773
Status: Manuscript has been accepted