Volume 579, July 2015
|Number of page(s)||12|
|Section||Stellar structure and evolution|
|Published online||17 July 2015|
The VLT-FLAMES Tarantula survey
XX. The nature of the X-ray bright emission-line star VFTS 399⋆
Department of Physics and AstronomyThe Open University,
2 Astrophysics, Cosmology and Gravity Centre, Department of Astronomy, University of Cape Town, Rondenbosch 7701, Republic of South Africa
3 Department of Astronomy & Astrophysics, 525 Davey Laboratory, Pennsylvania State University, University Park, PA 16802, USA
4 UK Astronomy Technology Centre, Royal Observatory Edinburgh Blackford Hill, Edinburgh, EH9 3HJ, UK
5 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
6 School of Physics & Astronomy, University of Southampton, Highfield, Southampton, S017 1BJ, UK
7 Astronomical Institute Anton Pannekoek, University of Amsterdam, 1098 XH Amsterdam, The Netherlands
8 Carnegie Institution for Science: The Observatories, 813 Santa Barbara St, Pasadena, CA 91101, USA
9 TAPIR institute, California Institute of Technology, Pasadena, CA 91125, USA
10 Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, Belfast BT7 1NN, UK
11 Argelander Institut für Astronomie, Auf den Hügel 71, 53121 Bonn, Germany
12 Departamento de Astrofísica, Centro de Astrobiología (INTA-CSIC), Campus ESA, Apartado Postal 78, 28 691 Villanueva de la Cañada, Madrid, Spain
13 Department of Physics, Denys Wilkinson Building, Keble Road, Oxford, OX1 3RH, UK
14 Warsaw University Observatory, A1. Ujazdowskie 4, 00-478 Warszawa, Poland
Received: 18 June 2014
Accepted: 16 February 2015
Context. The stellar population of the 30 Doradus star-forming region in the Large Magellanic Cloud contains a subset of apparently single, rapidly rotating O-type stars. The physical processes leading to the formation of this cohort are currently uncertain.
Aims. One member of this group, the late O-type star VFTS 399, is found to be unexpectedly X-ray bright for its bolometric luminosity − in this study we aim to determine its physical nature and the cause of this behaviour.
Methods. To accomplish this we performed a time-resolved analysis of optical, infrared and X-ray observations.
Results. We found VFTS 399 to be an aperiodic photometric variable with an apparent near-IR excess. Its optical spectrum demonstrates complex emission profiles in the lower Balmer series and select He i lines − taken together these suggest an OeBe classification. The highly variable X-ray luminosity is too great to be produced by a single star, while the hard, non-thermal nature suggests the presence of an accreting relativistic companion. Finally, the detection of periodic modulation of the X-ray lightcurve is most naturally explained under the assumption that the accretor is a neutron star.
Conclusions. VFTS 399 appears to be the first high-mass X-ray binary identified within 30 Dor, sharing many observational characteristics with classical Be X-ray binaries. Comparison of the current properties of VFTS 399 to binary-evolution models suggests a progenitor mass ≳25 M⊙ for the putative neutron star, which may host a magnetic field comparable in strength to those of magnetars. VFTS 399 is now the second member of the cohort of rapidly rotating “single” O-type stars in 30 Dor to show evidence of binary interaction resulting in spin-up, suggesting that this may be a viable evolutionary pathway for the formation of a subset of this stellar population.
Key words: stars: emission-line, Be / binaries: general / stars: individual: VFTS399
© ESO, 2015
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