The VLT-FLAMES Tarantula survey

XX. The nature of the X-ray bright emission-line star VFTS 399^⋆

¹ Department of Physics and AstronomyThe Open University, Walton Hall, Milton Keynes MK7 6AA, UK
e-mail: s.clark@open.ac.uk
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

Abstract

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.