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arXiv · 1503.00930

The VLT-FLAMES Tarantula survey XX. The nature of the X-ray bright emission line star VFTS 399

J. S. Clark·E. S. Bartlett·P. S. Broos·L. K. Townsley·W. D. Taylor·N. R. Walborn·A. J. Bird·H. Sana·S. E. de Mink·P. L. Dufton·C. J. Evans·N. Langer·J. Maíz Apellániz·F. R. N. Schneider·I. Soszyński
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Abstract

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. 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. We find 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 HeI 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. VFTS 399 appears to be the first high-mass X-ray binary identified within 30 Dor. Comparison of the current properties of VFTS 399 to binary-evolution models suggests a progenitor mass in excess of 25Msun 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.

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J. S. Clark, E. S. Bartlett, P. S. Broos, L. K. Townsley, W. D. Taylor, N. R. Walborn, A. J. Bird, H. Sana, S. E. de Mink, P. L. Dufton, C. J. Evans, N. Langer, J. Maíz Apellániz, F. R. N. Schneider, I. Soszyński. 2015-03-03. The VLT-FLAMES Tarantula survey XX. The nature of the X-ray bright emission line star VFTS 399. https://doi.org/10.1051/0004-6361/201424427

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