[Paper Review] The VLT-FLAMES Tarantula Survey III: A very massive star in apparent isolation from the massive cluster R136

J. M. Bestenlehner, J. S. Vink+18 more|UvA-DARE (University of Amsterdam)|May 9, 2011

Astrophysics and Star Formation Studies29 references71 citations

TL;DR

This study identifies VFTS 682, a very massive star (log(L/L☉) = 6.5 ± 0.2, ~150 M☉) in apparent isolation 29 pc from the R136 cluster in the Large Magellanic Cloud. Using VLT-FLAMES spectroscopy and SED modeling, it reveals extreme reddening (R_V ≈ 4.7), high temperature (52.2 ± 2.5 kK), and unprecedented long-term photometric variability (10% dimming over years), suggesting chemically homogeneous evolution and challenging current models of massive star formation and runaway ejection.

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 (RV ~4.7), and obtain a surprisingly high luminosity log(L/Lsun) = 6.5 \pm 0.2, corresponding to a present-day mass of ~150Msun. The high effective temperature of 52.2 \pm 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.

Motivation & Objective

• To determine the intrinsic stellar parameters of VFTS 682, a very massive star found in apparent isolation from the R136 cluster.
• To investigate the origin of VFTS 682, considering whether it formed in situ or was ejected from the dense R136 cluster.
• To assess the implications of its high luminosity, extreme reddening, and unusual photometric variability for massive star formation and evolution models.
• To evaluate the role of chemically homogeneous evolution in shaping the observed properties of VFTS 682.

Proposed method

• Spectroscopic analysis of VFTS 682 using optical data from the VLT-FLAMES Tarantula Survey with the non-LTE stellar atmosphere code cmfgen.
• Modeling of the spectral energy distribution (SED) using complementary optical and infrared photometry from 2MASS, VISTA, SAGE, and IRSF.
• Derivation of extinction properties using the R_V parameter, yielding R_V ≈ 4.7, indicating highly peculiar dust characteristics.
• Estimation of stellar luminosity and mass using the mass-luminosity relationship from Gräfener et al. (2011), assuming homogeneous hydrogen burning.
• Assessment of radial velocity and kinematic properties to evaluate the possibility of VFTS 682 being a slow runaway star from R136.
• Analysis of long-term photometric variability using OGLE-III V and I band lightcurves (2001–2009) and K-band data (2010).

Experimental results

Research questions

• RQ1 What is the intrinsic luminosity and mass of VFTS 682, and how do they compare to known very massive stars?
• RQ2 What is the origin of the extreme reddening (R_V ≈ 4.7) observed toward VFTS 682, and how does it affect the derived stellar parameters?
• RQ3 Why does VFTS 682 exhibit long-term photometric variability (10% dimming over years), a feature unprecedented in classical Wolf-Rayet stars?
• RQ4 Is VFTS 682 a product of in situ massive star formation or a slow runaway star ejected from the R136 cluster?
• RQ5 To what extent can chemically homogeneous evolution explain the high effective temperature and high mass of VFTS 682?

Key findings

• VFTS 682 has a high intrinsic luminosity of log(L/L☉) = 6.5 ± 0.2, corresponding to a present-day mass of approximately 150 M☉.
• The star exhibits extreme reddening with R_V ≈ 4.7, indicating unusual dust properties, and suffers from significant extinction (A_V ≈ 4.5).
• The effective temperature is 52.2 ± 2.5 kK, consistent with a position on the zero-age main sequence, suggesting chemically homogeneous evolution.
• Long-term photometric variability is observed: the star dimmed by ~0.1 mag in V and I bands between 2006 and 2009, and increased in K-band by ~0.15 mag in 2010, resembling LBV-like behavior.
• The radial velocity of VFTS 682 is ~300 km/s, consistent with a possible slow runaway origin from R136, requiring a true velocity of ~40 km/s to reach its current projected distance of 29 pc.
• The most likely initial mass of VFTS 682 is 120–210 M☉, with an age of 1–1.4 Myr, and the high helium abundance (Y = 0.45) supports chemically homogeneous evolution via rapid initial rotation (v_rot^init > 200 km/s).