Ramírez-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.; Castro, N.; Crowther, P. A.; Evans, C. J.; García, M.; Gräfener, G.; Herrero, A.; van Kempen, B.; Lennon, D. J.; Maíz Apellániz, J.; Najarro, F.; Sabín-Sanjulián, C.; Simón-Díaz, S.; Taylor, W. D.; Vink, J. S.
Bibliographical reference
Astronomy and Astrophysics, Volume 600, id.A81, 82 pp.
Advertised on:
4
2017
Journal
Citations
73
Refereed citations
66
Description
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 Minit ≳ 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 Minit ≲ 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.,
νesini < 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 fV 1/8 to 1/6.
Based on observations collected at the European Southern Observatory
under program ID 182.D-0222.Tables C.1-C.5 are also available at the CDS
via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/600/A81
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