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Post Info TOPIC: VFTS 682
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Title: Runaway massive stars from R136: VFTS 682 is very likely a "slow runaway"
Authors: Sambaran Banerjee, Pavel Kroupa, Seungkyung Oh

We conduct a theoretical study on the ejection of runaway massive stars from R136 --- the central massive, star-burst cluster in the 30 Doradus complex of the Large Magellanic Cloud. Specifically, we investigate the possibility of the very massive star (VMS) VFTS 682 being a runaway member of R136. Recent observations of the above VMS, by virtue of its isolated location and its moderate peculiar motion, have raised the fundamental question whether isolated massive star formation is indeed possible. We perform the first realistic N-body computations of fully mass-segregated R136-type star clusters in which all the massive stars are in primordial binary systems. These calculations confirm that the dynamical ejection of a VMS from a R136-like cluster, with kinematic properties similar to those of VFTS 682, is common. Hence the conjecture of isolated massive star formation is unnecessary to account for this VMS. Our results are also quite consistent with the ejection of 30 Dor 016, another suspected runaway VMS from R136. We further note that during the clusters' evolution, mergers of massive binaries produce a few single stars per cluster with masses significantly exceeding the canonical upper-limit of 150 solar mass. The observations of such single super-canonical stars in R136, therefore, do not imply an IMF with an upper limit greatly exceeding the accepted canonical 150 solar mass limit, as has been suggested recently, and they are consistent with the canonical upper limit.

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VFTS 682



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Google earth file: VFTS 682.kmz (2kb, kmz)

Position (2000): RA 05 38 55.51, Dec -69 04 26.72



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ESO's VLT Finds a Brilliant but Solitary Superstar

eso1117a.jpg
Credit ESO

An extraordinarily bright isolated star has been found in a nearby galaxy - the star is three million times brighter than the Sun. All previous similar "superstars" were found in star clusters, but this brilliant beacon shines in solitary splendour. The origin of this star is mysterious: did it form in isolation or was it ejected from a cluster? Either option challenges astronomers' understanding of star formation.
An international team of astronomers has used ESO's Very Large Telescope to carefully study the star VFTS 682  in the Large Magellanic Cloud, a small neighbouring galaxy to the Milky Way. By analysing the star's light, using the FLAMES instrument on the VLT, they have found that it is about 150 times the mass of the Sun. Stars like these have so far only been found in the crowded centres of star clusters, but VFTS 682 lies on its own.

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Title: The VLT-FLAMES Tarantula Survey III: A very massive star in apparent isolation from the massive cluster R136
Authors: Joachim M. Bestenlehner, Jorick S. Vink, G. Gräfener, F. Najarro, C. J. Evans, N. Bastian, A. Z. Bonanos, E. Bressert, P. A. Crowther, E. Doran, K. Friedrich, V. Hénault-Brunet, A. Herrero, A. de Koter, N. Langer, D. J. Lennon, J. Maíz Apellániz, H. Sana, I. Soszynski, W. D. Taylor

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 (RV ~4.7), and obtain a surprisingly high luminosity log(L/Lsun) = 6.5 ±0.2, corresponding to a present-day mass of ~150Msun. The high effective temperature of 52.2 ±2.5kK 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 most massive runaway known to date.

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