Letter to the Editor

The VLT-FLAMES Tarantula Survey

III. A very massive star in apparent isolation from the massive cluster R136^⋆

¹ Armagh Observatory, College Hill, Armagh BT61 9DG, UK
e-mail: jbl@arm.ac.uk
² Centro de Astrobiología (CSIC-INTA), Ctra. de Torrejón a Ajalvir km-4, 28850 Torrejón de Ardoz, Madrid, Spain
³ UK Astronomy Technology Centre, Royal Observatory Edinburgh, Blackford Hill, Edinburgh, EH9 3HJ, UK
⁴ Excellence Cluster Universe, Boltzmannstr. 2, 85748 Garching, Germany
⁵ School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL, UK
⁶ Institute of Astronomy & Astrophysics, National Observatory of Athens, I. Metaxa & Vas. Pavlou Street, P. Penteli 15236, Greece
⁷ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 87548 Garching bei München, Germany
⁸ Harvard-Smithsonian CfA, 60 Garden Street, Cambridge, MA 02138, USA
⁹ Dept. of Physics & Astronomy, Hounsfield Road, University of Sheffield, S3 7RH, UK
¹⁰ Argelander-Institut für Astronomie der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
¹¹ SUPA, IfA, University of Edinburgh, Royal Observatory Edinburgh, Blackford Hill, Edinburgh, EH9 3HJ, UK
¹² Departamento de Astrofísica, Universidad de La Laguna, 38205 La Laguna, Tenerife, Spain
¹³ European Southern Observatory, Alonso de Cordova 1307, Casilla 19001, Santiago 19, Chile
¹⁴ Astronomical Institute Anton Pannekoek, University of Amsterdam, Kruislaan 403, 1098 SJ, Amsterdam, The Netherlands
¹⁵ Astronomical Institute, Utrecht University, Princetonplein 5, 3584CC, Utrecht, The Netherlands
¹⁶ ESA, Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹⁷ Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
¹⁸ Warsaw University Observatory, Aleje Ujazdowskie 4, 00-478 Warsaw, Poland

Received: 6 April 2011
Accepted: 2 May 2011

Abstract

VFTS 682 is located in an active star-forming region, at a projected distance of 29 pc from the young massive cluster R136 in the Tarantula Nebula of the Large Magellanic Cloud. It was previously reported as a candidate young stellar object, and more recently spectroscopically revealed as a hydrogen-rich Wolf-Rayet (WN5h) star. Our aim is to obtain the stellar properties, such as its intrinsic luminosity, and to investigate the origin of VFTS 682. To this purpose, we model optical spectra from the VLT-FLAMES Tarantula Survey with the non-LTE stellar atmosphere code cmfgen, as well as the spectral energy distribution from complementary optical and infrared photometry. We find the extinction properties to be highly peculiar (R_V ~ 4.7), and obtain a surprisingly high luminosity log (L/L_⊙) = 6.5 ± 0.2, corresponding to a present-day mass of  ~150 M_⊙. The high effective temperature of 52.2 ± 2.5 kK might be explained by chemically homogeneous evolution – suggested to be the key process in the path towards long gamma-ray bursts. Lightcurves of the object show variability at the 10% level on a timescale of years. Such changes are unprecedented for classical Wolf-Rayet stars, and are more reminiscent of Luminous Blue Variables. Finally, we discuss two possibilities for the origin of VFTS 682: (i) the star either formed in situ, which would have profound implications for the formation mechanism of massive stars, or (ii) VFTS 682 is a slow runaway star that originated from the dense cluster R136, which would make it the mostmassive runaway known to date.